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<div class="www_sectiontitle">Open LLVM Projects</div>

<ul>
  <li>Google Summer of Code Ideas & Projects
    <ul>
      <li>
        <a href="#gsoc22">Google Summer of Code 2022</a>
        <ul>
          <li>
            <b>LLVM Core</b>
            <ul>
              <li><a href="#llvm_shared_jitlink">Implement a shared-memory based JITLinkMemoryManager for out-of-process JITting</a></li>
              <li><a href="#llvm_build_jit_tutorial">Modernize the LLVM "Building A JIT" tutorial series</a></li>
              <li><a href="#llvm_jit_new_format">Write JITLink support for a new format/architecture</a></li>
              <li><a href="#llvm_instrumentaion_for_compile_time">Instrumentation of Clang/LLVM for Compile Time</a></li>
              <li><a href="#llvm_lto_dependency_info">Richer symbol dependency information for LTO</a></li>
              <li><a href="#llvm_mlgo_passes">Machine Learning Guided Ordering of Compiler Optimization Passes</a></li>
              <li><a href="#llvm_mlgo_loop">Learning Loop Transformation Heuristics</a></li>
              <li><a href="#llvm_module_inliner">Evaluate and Expand the Module-Level Inliner</a></li>
              <li><a href="#llvm_undef_load">Remove undef: move uninitialized memory to poison</a></li>
              <li><a href="#llvm_abi_export">Add ABI/API export annotations to the LLVM build</a></li>
            </ul>
          </li>

          <li><a href="http://clang.llvm.org/"><b>Clang</b></a>
            <ul>
              <li><a href="#clang-template-instantiation-sugar">Extend clang AST to
                provide information for the type as written in template
                instantiations</a>
              </li>
              <li><a href="#clang-sa-structured-bindings">Implement support for
                C++17 structured bindings in the Clang Static Analyzer</a>
              </li>
	      <li><a href="#clang-improve-diagnostics">Improve Clang Diagnostics</a>
	      </li>
            </ul>
          </li>

          <li>
            <a href="https://polly.llvm.org"><b>Polly</b></a>
            <ul>
              <li><a href="#polly_npm">Completely switch to new pass manager</a></li>
            </ul>
          </li>
          <li>
            <b><a href="https://enzyme.mit.edu">Enzyme</a></b>
            <ul>
              <li><a href="#enzyme_tblgen">Move Enzyme Instruction Transformation Rules to Tablegen</a></li>
              <li><a href="#enzyme_vector">Vector Reverse-Mode Automatic Differentiation</a></li>
              <li><a href="#enzyme_pm">Enable The New Pass Manager</a></li>
            </ul>
          </li>

        </ul>
      </li>
      <li>
        <a href="#gsoc21">Google Summer of Code 2021</a>
        <ul>
          <li>
            <b>LLVM Core</b>
            <ul>
              <li><a href="#llvm_distributing_lit">Distributed lit testing</a></li>
              <li><a href="#llvm_loop_heuristics">Learning Loop Transformation Heuristics</a></li>
              <li><a href="#llvm_ir_fuzzing">Fuzzing LLVM-IR Passes</a></li>
              <li><a href="#llvm_ir_assume"><tt>llvm.assume</tt> the missing pieces</a></li>
              <li><a href="#llvm_shared_jitlink">Implement a shared-memory based JITLinkMemoryManager for out-of-process JITting</a></li>
              <li><a href="#llvm_build_jit_tutorial">Modernize the LLVM "Building A JIT" tutorial series</a></li>
              <li><a href="#llvm_jit_new_format">Write JITLink support for a new format/architecture</a></li>
              <li><a href="#llvm_ir_issues">Fix fundamental issues in LLVM's IR</a></li>
              <li><a href="#llvm_utilize_loopnest">Utilize LoopNest Pass</a></li>
            </ul>
          </li>
          <li><a href="http://clang.llvm.org/"><b>Clang</b></a>
            <ul>
              <li><a href="#clang-template-instantiation-sugar">Extend clang AST to
                provide information for the type as written in template
                instantiations</a>
              </li>
            </ul>
          </li>
          <li>
            <b>OpenMP</b>
            <ul>
              <li><a href="#openmp_gpu_jit">JIT-ing OpenMP GPU kernels transparently</a></li>
            </ul>
          </li>
          <li>
            <b>OpenACC</b>
            <ul>
              <li><a href="#openacc_rt_diagnostics">OpenACC Diagnostics from the OpenMP Runtime</a></li>
            </ul>
          </li>
          <li>
            <b><a href="https://polly.llvm.org">Polly</a></b>
            <ul>
              <li><a href="#polly_isl_bindings">Use official isl C++ bindings</a></li>
            </ul>
          </li>
          <li>
            <b><a href="https://enzyme.mit.edu">Enzyme</a></b>
            <ul>
              <li><a href="#enzyme_blas">Integrate custom derivatives of BLAS, Eigen, and similar routines into Enzyme</a></li>
              <li><a href="#enzyme_swift">Integrate Enzyme into Swift to provide high-performance differentiation in Swift</a></li>
              <li><a href="#enzyme_fixed">Differentiation of Fixed-Point Arithmetic</a></li>
              <li><a href="#enzyme_rust">Integrate Enzyme into Rust to provide high-performance differentiation in Rust</a></li>
            </ul>
          </li>
          <li>
            <b>Clang Static Analyzer</b>
            <ul>
              <li><a href="#static_analyzer_profling">Clang Static Analyzer performance profiling</a></li>
              <li><a href="#static_analyzer_constraint_solver">Clang Static Analyzer constraint solver improvements</a></li>
            </ul>
          </li>
          <li>
            <b>LLDB</b>
            <ul>
              <li><a href="#lldb_diagnostics">A structured approach to diagnostics in LLDB</a></li>
            </ul>
          </li>
        </ul>
      </li>
      <li>
        <a href="#gsoc20">Google Summer of Code 2020</a>
      <ul>
        <li>
          <b>LLVM Core</b>
          <ul>
            <li><a href="#llvm_optimized_debugging">Improve debugging of optimized code</a></li>
            <li><a href="#llvm_ipo">Improve inter-procedural analyses and optimizations</a></li>
            <li><a href="#llvm_par">Improve parallelism-aware analyses and optimizations</a></li>
            <li><a href="#llvm_dbg_invariant">Make LLVM passes debug info invariant</a></li>
            <li><a href="#llvm_mergesim">Improve MergeFunctions to incorporate MergeSimilarFunction patches and ThinLTO Support</a></li>
            <li><a href="#llvm_dwarf_yaml2obj">Add DWARF support to yaml2obj</a></li>
            <li><a href="#llvm_hotcold">Improve hot cold splitting to aggressively outline small blocks</a></li>
            <li><a href="#llvm_pass_order">Advanced Heuristics for Ordering Compiler Optimization Passes</a></li>
            <li><a href="#llvm_ml_scc">Machine learning and compiler optimizations: using inter-procedural analysis to select optimizations</a></li>
            <li><a href="#llvm_postdominators">Add PostDominatorTree in LoopStandardAnalysisResults</a></li>
            <li><a href="#llvm_loopnest">Create loop nest pass</a></li>
            <li><a href="#llvm_instdump">Instruction properties dumper and checker</a></li>
            <li><a href="#llvm_movecode">Unify ways to move code or check if code is safe to be moved</a></li>
          </ul>
          <li><a href="http://clang.llvm.org/"><b>Clang</b></a>
            <ul>
              <li><a href="#clang-template-instantiation-sugar">Extend clang AST to
                provide information for the type as written in template
                instantiations</a>
              </li>
              <li><a href="#clang-sa-cplusplus-checkers">Find null smart pointer dereferences
                                                         with the Static Analyzer</a>
              </li>
            </ul>
          </li>
         <li><a href="http://lldb.llvm.org/"><b>LLDB</b></a></li>
          <ul>
            <li><a href="#lldb-autosuggestions">Support autosuggestions in LLDB's command line</a></li>
            <li><a href="#lldb-more-completions">Implement the missing tab completions for LLDB's command line</a></li>
            <li><a href="#lldb-reimplement-lldb-cmdline">Reimplement LLDB's command-line commands using the public SB API.</a></li>
            <li><a href="#lldb-batch-testing">Add support for batch-testing to the LLDB testsuite.</a></li>
          </ul>
          <li>
            <b>MLIR</b>
            <ul>
              <li>See the <a href="https://mlir.llvm.org/getting_started/openprojects/">MLIR open project list</a></li>
            </ul>
        </li>
      </ul>

      </li>
      <li><a href="#gsoc19">Google Summer of Code 2019</a>
      <ul>
        <li>
          <b>LLVM Core</b>
          <ul>
            <li><a href="#debuginfo_codegen_mismatch">Debug Info should have no
              effect on codegen</a></li>
            <li><a href="#llvm_function_attributes">Improve (function) attribute
              inference</a></li>
            <li><a href="#improve_binary_utilities">Improve LLVM binary utilities
            </a></li>
          </ul>
        </li>
        <li><a href="http://clang.llvm.org/"><b>Clang</b></a>
          <ul>
            <li><a href="#clang-astimporter-fuzzer">Implement an ASTImporter
              fuzzer</a>
            </li>
            <li><a href="#improve-autocompletion">Improve shell autocompletion
              for Clang</a>
            </li>
            <li><a href="#analyze-llvm">Apply the Clang Static Analyzer to LLVM-based
              Projects</a>
            </li>
            <li><a href="#header-generation">Generate annotated sources based on
              LLVM-IR analyses</a>
            </li>
          </ul>
        </li>
      </ul>
    </li>
      <li><a href="#gsoc18">Google Summer of Code 2018</a></li>
      <li><a href="#gsoc17">Google Summer of Code 2017</a></li>
    </ul></li>
  <li><a href="#what">What is this?</a></li>
  <li><a href="#subprojects">LLVM Subprojects: Clang and more</a></li>
  <li><a href="#improving">Improving the current system</a>
  <ol>
    <li><a href="#target-desc">Factor out target descriptions</a></li>
    <li><a href="#code-cleanups">Implementing Code Cleanup bugs</a></li>
    <li><a href="#programs">Compile programs with the LLVM Compiler</a></li>
    <li><a href="#llvmtest">Add programs to the llvm-test suite</a></li>
    <li><a href="#benchmark">Benchmark the LLVM compiler</a></li>
    <li><a href="#statistics">Benchmark Statistics and Warning System</a></li>
    <li><a href="#coverage">Improving Coverage Reports</a></li>
    <li><a href="#misc_imp">Miscellaneous Improvements</a></li>
  </ol></li>

  <li><a href="#new">Adding new capabilities to LLVM</a>
  <ol>
    <li><a href="#llvm_ir">Extend the LLVM intermediate representation</a></li>
    <li><a href="#pointeranalysis">Pointer and Alias Analysis</a></li>
    <li><a href="#profileguided">Profile-Guided Optimization</a></li>
    <li><a href="#compaction">Code Compaction</a></li>
    <li><a href="#xforms">New Transformations and Analyses</a></li>
    <li><a href="#codegen">Code Generator Improvements</a></li>
    <li><a href="#misc_new">Miscellaneous Additions</a></li>
  </ol></li>

  <li><a href="#using">Project using LLVM</a>
  <ol>
    <li><a href="#machinemodulepass">Add a MachineModulePass</a></li>
    <li><a href="#encodeanalysis">Encode Analysis Results in MachineInstr IR</a></li>
    <li><a href="#codelayoutjit">Code Layout in the LLVM JIT</a></li>
    <li><a href="#fieldlayout">Improved Structure Splitting and Field Reordering</a></li>
    <li><a href="#slimmer">Finish the Slimmer Project</a></li>
  </ol></li>
</ul>

<div class="doc_author">
  <p>Written by the <a href="/">LLVM Team</a></p>
</div>

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<div class="www_sectiontitle">
  <a name="gsoc22">Google Summer of Code 2022</a>
</div>
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<div class="www_text">
  <p>
    Welcome prospective Google Summer of Code 2022 Students! This document is
    your starting point to finding interesting and important projects for LLVM,
    Clang, and other related sub-projects. This list of projects is not only
    developed for Google Summer of Code, but open projects that really need
    developers to work on and are very beneficial for the LLVM community.
  </p>

  <p>We encourage you to look through this list and see which projects excite
    you and match well with your skill set. We also invite proposals not on this
    list. More information and discussion about GSoC can be found in
    <a href="https://discourse.llvm.org/c/community/gsoc" target="_blank">
      discourse
    </a>. If you have questions about a particular project please find the
    relevant entry in discourse, check previous discussion and ask. If there is
    no such entry or you would like to propose an idea please create a new
    entry. Feedback from the community is a requirement for your proposal to be
    considered and hopefully accepted.
  </p>

  <p>The LLVM project has participated in Google Summer of Code for several years
    and has had some very successful projects. We hope that this year is no
    different and look forward to hearing your proposals. For information on how
    to submit a proposal, please visit the Google Summer of Code main
    <a href="https://summerofcode.withgoogle.com/">website.</a>
  </p>
</div>

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<div class="www_subsection">
  <a>LLVM</a>
</div>
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<div class="www_subsubsection">
  <a name="llvm_shared_jitlink">Implement a shared-memory based JITLinkMemoryManager for out-of-process JITting</a>
</div>
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<div class="www_text">
  <p><b>Description of the project:</b>
    Write a shared-memory based JITLinkMemoryManager.
    <br />
    LLVM’s JIT uses the JITLinkMemoryManager interface to allocate both working
    memory (where the JIT fixes up the relocatable objects produced by the
    compiler) and target memory (where the JIT’d code will reside in the target).
    JITLinkMemoryManager instances are also responsible for transporting
    fixed-up code from working memory to target memory. LLVM has an existing
    cross-process allocator that uses remote procedure calls (RPC) to allocate
    and copy bytes to the target process, however a more attractive solution
    (when the JIT and target process share the same physical memory) would be to
    use shared memory pages to avoid copies between processes.

  </p>

  <p><b>Expected results:</b>
    <ul>Implement a shared-memory based JITLinkMemoryManager:
      <li>Write generic LLVM APIs for shared memory allocation.</li>
      <li>
        Write a JITLinkMemoryManager that uses these generic APIs to allocate
        shared working-and-target memory.
      </li>
      <li>Make an extensive performance study of the approach.</li>
    </ul>

  <p><b>Confirmed Mentor:</b>
    <a href=https://github.com/vgvassilev>Vassil Vassilev</a>,
    <a href=https://github.com/lhames>Lang Hames</a></p>

  <p><b>Desirable skills:</b> Intermediate C++; Understanding of LLVM and the
    LLVM JIT in particular; Understanding of virtual memory management APIs.
  </p>

  <p><b>Project type:</b> Large</p>

  <p><b>Discourse</b>
    <a href="https://discourse.llvm.org/t/implement-a-shared-memory-based-jitlinkmemorymanager-for-out-of-process-jitting">
      URL</a>
  </p>
</div>

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<div class="www_subsubsection">
  <a name="llvm_build_jit_tutorial">Modernize the LLVM "Building A JIT" tutorial series</a>
</div>
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<div class="www_text">
  <p><b>Description of the project:</b>
    The LLVM BuildingAJIT tutorial series teaches readers to build their own JIT
    class from scratch using LLVM’s ORC APIs, however the tutorial chapters have
    not kept pace with recent API improvements. Bring the existing tutorial
    chapters up to speed, write up a new chapter on lazy compilation (chapter
    code already available) or write a new chapter from scratch.
  </p>

  <p><b>Expected results:</b>
    <ul>
      <li>
        Update chapter text for Chapters 1-3 -- Easy, but offers a chance to get
        up-to-speed on the APIs.
      </li>
      <li>
        Write chapter text for Chapter 4 -- Chapter code is already available,
        but no chapter text exists yet.
      </li>
      <li>
        Write a new chapter from scratch -- E.g. How to write an out-of-process
        JIT, or how to directly manipulate the JIT'd instruction stream using
        the ObjectLinkingLayer::Plugin API.
      </li>
    </ul>

  <p><b>Confirmed Mentor:</b>
    <a href=https://github.com/vgvassilev>Vassil Vassilev</a>,
    <a href=https://github.com/lhames>Lang Hames</a></p>
  <p><b>Desirable skills:</b> Intermediate C++; Understanding of LLVM and the
    LLVM JIT in particular; Familiarity with RST (reStructed Text); Technical
    writing skills.
  </p>
  <p><b>Project type:</b> Medium</p>

  <p><b>Discourse</b>
    <a href="https://discourse.llvm.org/t/modernize-the-llvm-building-a-jit-tutorial-series">
      URL</a>
  </p>
</div>

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<div class="www_subsubsection">
  <a name="llvm_jit_new_format">Write JITLink support for a new format/architecture</a>
</div>
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<div class="www_text">
  <p><b>Description of the project:</b>
    JITLink is LLVM’s new JIT linker API -- the low-level API that transforms
    compiler output (relocatable object files) into ready-to-execute bytes in
    memory. To do this JITLink’s generic linker algorithm needs to be
    specialized to support the target object format (COFF, ELF, MachO), and
    architecture (arm, arm64, i386, x86-64). LLVM already has mature
    implementations of JITLink for MachO/arm64 and MachO/x86-64, and a
    relatively new implementation for ELF/x86-64. Write a JITLink implementation
    for a missing target that interests you. If you choose to implement support
    for a new architecture using the ELF or MachO formats then you will be able
    to re-use the existing generic code for these formats. If you want to
    implement support for a new target using the COFF format then you will need
    to write both the generic COFF support code and the architecture support
    code for your chosen architecture.
  </p>

  <p><b>Expected results:</b>
    Write a JITLink specialization for a not-yet-supported format/architecture.
  </p>
  <p><b>Confirmed Mentor:</b>
    <a href=https://github.com/vgvassilev>Vassil Vassilev</a>,
    <a href=https://github.com/lhames>Lang Hames</a></p>
  <p><b>Desirable skills:</b> Intermediate C++; Understanding of LLVM and the
    LLVM JIT in particular; familiarity with your chosen format/architecture,
    and basic linker concepts (e.g. sections, symbols, and relocations).
  </p>
  <p><b>Project type:</b> Large</p>

  <p><b>Discourse</b>
    <a href="https://discourse.llvm.org/t/write-jitlink-support-for-a-new-format-architecture">
      URL</a>
  </p>
</div>

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<div class="www_subsubsection">
  <a name="llvm_instrumentaion_for_compile_time">Instrumentation of Clang/LLVM for Compile Time</a>
</div>
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<div class="www_text">
  <p><b>Description of the project:</b>
    Every developer, at some point (usually while waiting for their program to
    compile), has asked "Why is it taking so long?"  This project is to seek an
    answer to this question.  There exists within LLVM, and by extension CLANG,
    a timing infrastructure that records events within the compiler.  However,
    its utilization is inconsistent and insufficient.  This can be improved by
    adding more instrumentation throughout LLVM and CLANG but one must be careful.
    Too much instrumentation, or instrumenting the wrong things, can be confusing
    and overwhelming, thus making it no more useful than not enough information.
    The trick is to find the right places to instrument and controlling the
    instrumentation.  Seeking out these key spots will take you through the
    entire compilation process, from preprocessing through to final code
    generation, and all phases between.  As you instrument the code, you will
    look at the data as you evolve it, which will further direct your search.
    You will develop new ways to control and filter the information to allow a
    better understanding of where the compiler is spending its time.  You will
    seek out and develop example test inputs that illustrate where the compiler
    can be improved, which will in turn, help direct your instrumenting and search.
    You will consider and develop ways of controlling the instrumentation to
    allow better understanding and detailed examination of phases of compilation.
    Through all of this, you will gain an understanding of how a compiler works,
    from front end processing, through the LLVM optimization pipeline, through
    to code generation.  You will see, and understand, the big picture of what
    is required to compile and optimize a C/C++ program, and in particular, how
    CLANG, LLVM and LLC accomplish these tasks.  Your mentors have a combined
    experience of approximately 25 years of compiler development and around 8
    years of experience with LLVM itself to help you on your quest.
  </p>

  <p><b>Expected results:</b>
    <ul>
      <li>Targetted expansion of the use of the existing timing infrastructure</li>
      <li>Identification of appropriate test inputs for improving compile time</li>
      <li>Identification of compile time hotspots</li>
      <li>New and improved methods of controlling the timing infrastructure</li>
    </ul>
  </p>
  <p><b>Confirmed Mentor:</b> Jamie Schmeiser, Whitney Tsang</p>
  <p><b>Desirable skills:</b> C++ programming skills; CLANG/LLVM knowledge an asset but not necessary; self motivated; curiosity; desire to learn</p>
  <p><b>Project type:</b>175 or 350 hour</p>
  <p><b>Difficulty Rating:</b>Easy - Medium</p>
  <p><b>Discourse</b>
    <a href="https://discourse.llvm.org/t/instrumentation-of-clang-llvm-for-compile-time">
      URL</a>
  </p>
</div>

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<div class="www_subsubsection">
  <a id="llvm_mlgo_passes">Machine Learning Guided Ordering of Compiler Optimization Passes</a>
</div>
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<div class="www_text">
  <p><b>Description of the project</b>
  This continues the work of GSoC 2020 and <a href="https://summerofcode.withgoogle.com/archive/2021/projects/6411038932598784">2021</a>.

  Developers generally use standard optimization pipelines like -O2 and -O3 to
  optimize their code. Manually crafted heuristics are used to determine which
  optimization passes to select and how to order the execution of those passes.
  However, this process is not tailored for a particular application, or kind
  of application, as it is designed to perform “reasonably well” for any input.

  We want to improve the existing heuristics or replace the heuristics with
  machine learning-based models so that the LLVM compiler can provide a superior
  order of the passes customized per application.

  The last milestone enabled feature extraction, and started investigating training
  a policy for selecting a more appropriate pass pipeline.
  </p>
  <p><b>Project size:</b> either 175 or 350 hr.</p>
  <p><b>Difficulty:</b> Medium</p>
  <p><b>Skills:</b> C/C++, some compiler experience. ML experience is a bonus.</p>
  <p><b>Expected outcomes</b>: Pre-trained model selecting the most economical
    optimization pipeline, with no loss in performance; hook-up of model in LLVM;
    (re-)training tool.</p>
  <p><b>Mentors</b>
    Tarindu Jayatilaka, Mircea Trofin, Johannes Doerfert
  </p>
  <p>
  <b>Discourse</b>
  <a href="https://discourse.llvm.org/t/machine-learning-guided-ordering-of-compiler-optimization-passes/60415">URL</a>
  </p>
</div>


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<div class="www_subsubsection">
  <a id="llvm_mlgo_loop">Learning Loop Transformation Policies</a>
</div>
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<div class="www_text">
  <p><b>Description of the project</b>
  This project is a continuation of last <a href="https://summerofcode.withgoogle.com/archive/2021/projects/5732097817313280">year’s</a>.
  In 2021, the project achieved its first milestone - separating correctness
  decisions from policy decisions. This opens up the possibility of replacing
  the latter with machine-learned ones.

  Rough milestones: 1) select an initial set of features and use the existing ML
  Guided Optimizations (MLGO) infra to generate training logs; 2) define a reward
  signal, computable at compile time, to guide a reinforcement learning training loop;
  3) iterate through training and refine reward/feature set
  </p>
  <p><b>Project size:</b> either 175 or 350 hr, ideally 350 hr</p>
  <p><b>Difficulty:</b> Medium/Hard</p>
  <p><b>Skills:</b> C/C++, some compiler experience. ML experience is a bonus.</p>
  <p><b>Expected outcomes</b>: policy ('advisor') interface for loop unrolling,
    with current heuristic as default implementation; set up feature extraction
    for reinforcement learning training; set up a reward metric; set up training
    algorithm, and iterate over policy training</p>

  <p><b>Mentors</b>
    Johannes Doerfert, Mircea Trofin
  </p>
  <p>
  <b>Discourse</b>
  <a href="https://discourse.llvm.org/t/learning-loop-transformation-policies/60413">URL</a>
  </p>
</div>

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<div class="www_subsubsection">
  <a id="llvm_module_inliner">Evaluate and Expand the Module-Level Inliner</a>
</div>
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<div class="www_text">
  <p><b>Description of the project</b>
    LLVM's inliner is a bottom-up, strongly-connected component-level pass. This
    places limits on the order in which call sites are evaluated, which impacts
    the effectiveness of inlining.
    
    We now have a functional Module Inliner, as result of <a href="https://summerofcode.withgoogle.com/archive/2021/projects/5195658885070848">GSoC2021 work</a>.
    We want to call site priority schemes, effectiveness/frequency of running
    function passes after successful inlinings, interplay with the ML inline
    advisor, to name a few areas of exploration.
  </p>
  <p><b>Project size:</b> either 175 or 350 hr, ideally 350 hr, milestones allow
    for 175hr scoping</p>
  <p><b>Difficulty:</b> Medium/Hard</p>
  <p><b>Skills:</b> C/C++, some compiler experience.</p>
  <p><b>Expected outcomes</b>: Proposal and Evaluation of alternative traversal
    orders; evaluation of 'clustering' inlining decisions (inline more than one
    call site at a time); evaluation of effectiveness/frequency of function
    optimization passes after inlining
  </p>
  <p><b>Mentors</b>
    Kazu Hirata, Liqiang Tao, Mircea Trofin
  </p>
  <p>
  <b>Discourse</b>
  <a href="https://discourse.llvm.org/t/evaluate-and-expand-the-module-level-inliner/60525">URL</a>
  </p>
</div>

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<div class="www_subsubsection">
  <a id="llvm_lto_dependency_info">Richer symbol dependency information for LTO</a>
</div>
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<div class="www_text">
  <p><b>Description of the project:</b>
    C and C++ programs are often composed of various object files produced from
    separately-compiled source files that are then linked together.
    When compiling one source file, knowledge that can be derived from the logic
    contained within the other source files would normally not be available.
    Link-time optimization, also known as LTO, is a way for optimization to be
    done using information from more than one source file.
  </p>
  <p>In LLVM, LTO is achieved by using LLVM bitcode objects as the output from
    the "compile" step and feeding those objects into the link step.
    LLVM's LTO operates in conjunction with the linker.
    The linker is invoked by the user and the linker in turn drives LLVM's LTO
    when it encounters LLVM bitcode files, getting information from LTO about
    what symbols a bitcode object defines or references.
    Information about what symbols are defined in or referenced from an object
    is necessary for the linker to perform symbol resolution, and a linker is
    normally able to extract such information from regular (non-bitcode) object
    files.
  </p>
  <p>The implied consequences of LLVM's LTO implementation
    with respect to linker GC
    (linker garbage collection) can be improved, especially for aggressive forms
    of linker GC with lazy inclusion of objects and sections.
    In particular, the symbols referenced but undefined by an LTO module are,
    to the linker, monolithic at the module level.
    At the same time, the symbols referenced but undefined by regular
    (non-LTO) objects are monolithic to LTO.
    Together, this means that the inclusion of an LTO module
    into the overall process potentially leads, in the linker's initial symbol
    resolution, to all the undefined symbols in that module being considered as
    referenced; in turn, additional artifacts (e.g., archive members) may be
    added into the resolution, which further leads to references that may
    resolve to symbols defined in LTO modules and a premature conclusion that
    the definition of these symbols are needed.
    This at least means potentially unnecessary codegen is being done for
    functions that will be garbage-collected in the end (waste of electricity
    and time).
  </p>
  <p>We acknowledge that an ideal implementation probably involves a "coroutine"
    like interaction between the linker and LTO codegen where information flows
    back and forth; however, such an endeavour is invasive to both linkers and
    to LLVM.
  </p>
  <p>We believe that by</p>
  <ul>
    <li>having the linker register, via an API to LTO, symbol reference "nodes"
    modelling the relationship between a symbol and the symbols that are
    referenced in turn from (the object file section containing) its
    linker-selected definition, and
    </li>
    <li>using that information in LTO processing,</li>
  </ul>
  <p>the LTO processing will be able to effectively identify a more accurate set
    of LTO symbols that are visible outside of the LTO unit.
    The linker merely needs to identify only exported symbols and entry points
    (such as the entry point for an executable and functions involved in
    initialization and finalization).
  </p>
  <p>Having the LLVM opt/codegen understand the dependency implications from the
    "outside world" is strictly better than the other direction: the symbols
    referred to by relocations in non-LTO code are pretty much fixed as compiled
    (whereas some references in LTO code may disappear with optimization).
  </p>

  <p><b>Expected results:</b></p>
  <ol>
    <li>Modification of the C++ LTO interface used by LLD to implement an
      interface to record the symbol reference dependency data (incorporating
      awareness of sections and comdats). This may additionally include a
      method to add LTO objects provisionally, simulating behaviours where
      linkers only add objects as needed.
    </li>
    <li>
      Modification of LTO to use new symbol reference information
      for definitions in regular objects when visiting definitions
      in the IR prior to the
      internalization pass to discover (transitive) symbol references and
      record the so-referenced symbols as being visible to regular objects.
      This may additionally include the "late" incorporation of LTO objects
      added provisionally into the merged LTO module.
    </li>
    <li>Modification of LLD (for ELF) to modify initial resolution to use the
      new interface as a replacement for setting
      <code>VisibleToRegularObj</code>
      except for entry point functions (including C++ dynamic initialization and
      finalization).
    </li>
  </ol>

  <p><b>Confirmed Mentors:</b>
    Sean Fertile,
    Hubert Tong,
    Wael Yehia
  </p>

  <p><b>Desirable skills:</b>
    Intermediate C++;
    basic linker concepts (e.g., symbols, sections, and relocations)
  </p>

  <p><b>Project size:</b> 350 hours</p>

  <p><b>Difficultly:</b> Medium/Hard</p>

  <p><b>Discourse</b>
    <a href="https://discourse.llvm.org/t/richer-symbol-dependency-information-for-lto/60335">
      URL</a>
  </p>
</div>


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<div class="www_subsubsection">
  <a id="llvm_undef_load">Remove undef: move uninitialized memory to poison</a>
</div>
<!-- *********************************************************************** -->
<div class="www_text">
  <p><b>Description of the project</b>
    The existence of the undef value in LLVM prevents several optimizations,
    even in programs where it is not used. Therefore, we have been trying to
    move all uses of undef to poison so we can eventually remove undef from
    LLVM.<br/>
    This project focuses on uninitialized memory: right now the semantics of
    LLVM is that loading a value from uninitilized memory yields an undef value.
    This prevents, for example, SROA/mem2reg from optimizing conditional loads
    as phi(undef, %x) cannot be replaced with x, as %x might be poison.<br/>
    This project consists in devising a consistent semantics for uninitialized
    (based on existing proposals), an upgrade plan for LLVM, and implementing
    the changes in LLVM and clang.
    In clang the changes should be specific to bit-fields.<br/>
    For more information see the following
    <a href="https://github.com/llvm/llvm-project/issues/52930">discussion</a>
    and/or contact the mentor.<br/>
    Further reading:
    <a href="https://web.ist.utl.pt/nuno.lopes/pubs/llvmmem-oopsla18.pdf">introduction to LLVM's memory model</a>.
  </p>
  <p><b>Project size:</b> 350 hr</p>
  <p><b>Difficulty:</b> Medium/Hard</p>
  <p><b>Skills:</b> Intermediate C++</p>
  <p><b>Expected outcomes</b>:
    <ul>
      <li>Semantics for memory operations that removes the need for undef
        values</li>
      <li>Upgrade plan for LLVM and frontends</li>
      <li>Implementation of the proposed semantics in LLVM</li>
      <li>Implementation of auto-upgrade path for old LLVM IR files</li>
      <li>Implementation of fixes in clang to use the new IR features</li>
      <li>Benchmarking to check for regressions and/or perf improvements</li>
    </ul>
  </p>
  <p><b>Mentors:</b>
    <a href="https://web.ist.utl.pt/nuno.lopes/">Nuno Lopes</a>
  </p>
</div>


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<div class="www_subsubsection">
  <a id="llvm_abi_export">Add API/ABI export annotations to the LLVM build</a>
</div>
<!-- *********************************************************************** -->
<div class="www_text">
  <p><b>Description of the project</b>
<p>Currently, all libraries inside LLVM export all their symbols publicly. When
linking statically against them, the linker will remove unused symbols and this
is not a problem.</p>

<p>When the libraries are built as shared libraries however, the number of exported
symbols is very large and symbols that are meant to be internal spill into the
public ABI of the shared libLLVM.so.</p>

<p>In this project, we’d like to change the default visibility of library symbols
to “hidden”, add an annotation macro to LLVM and use the macro to gradually move
the entire library in this direction. This will eventually enable building the
shared libLLVM.so on Windows as well.<p>

<p>In practice, this means adding -fvisibility=hidden to individual libraries and
annotating exported symbols with the LLVM export annotation.</p>

<p>We would like this work to be as unintrusive into other developer’s workflow as
possible, so starting with a small internal library would be beneficial,
e.g. one of the LLVM targets or IR passes.</p>

<p>For further reading, there is a Discourse thread avaiable that discusses the
idea behind this proposal:
<a href="https://discourse.llvm.org/t/supporting-llvm-build-llvm-dylib-on-windows/58891">
  Supporting LLVM_BUILD_LLVM_DYLIB on Windows</a>
as well as the linked Phabricator review with a patch implementing the functionality:
<a href="https://reviews.llvm.org/D109192">⚙ D109192 [WIP/DNM] Support:
  introduce public API annotation support</a>
None of this work has been committed yet but can be used as a starting point
for this proposal.</p>
  </p>
  <p><b>Project size:</b> Medium</p>
  <p><b>Difficulty:</b> Easy</p>
  <p><b>Skills:</b> Build systems, CMake, LLVM</p>
  <p><b>Expected outcomes</b>:
    <ul>
      <li>Export macro implemented and commited to LLVM</li>
      <li>At least one internal target ported to the new export scheme</li>
    </ul>
  </p>
  <p><b>Mentors:</b>
    Timm Bäder, Tom Stellard
  </p>
</div>


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<div class="www_subsection">
  <a>Clang</a>
</div>
<!-- *********************************************************************** -->

<!-- *********************************************************************** -->
<div class="www_subsubsection">
  <a name="clang-template-instantiation-sugar">Extend clang AST to provide
    information for the type as written in template instantiations.</a>
</div>
<!-- *********************************************************************** -->

<div class="www_text">
  <p><b>Description of the project: </b>
    When instantiating a template, the template arguments are canonicalized
    before being substituted into the template pattern. Clang does not preserve
    type sugar when subsequently accessing members of the instantiation.

    <pre>
    std::vector&lt;std::string&gt; vs;
    int n = vs.front(); // bad diagnostic: [...] aka 'std::basic_string&lt;char&gt;' [...]

    template&lt;typename T&gt; struct Id { typedef T type; };
    Id&lt;size_t&gt;::type // just 'unsigned long', 'size_t' sugar has been lost
    </pre>

    Clang should "re-sugar" the type when performing member access on a class
    template specialization, based on the type sugar of the accessed
    specialization. The type of vs.front() should be std::string, not
    std::basic_string&lt;char, [...]&gt;.
    <br /> <br />
    Suggested design approach: add a new type node to represent template
    argument sugar, and implicitly create an instance of this node whenever a
    member of a class template specialization is accessed. When performing a
    single-step desugar of this node, lazily create the desugared representation
    by propagating the sugared template arguments onto inner type nodes (and in
    particular, replacing Subst*Parm nodes with the corresponding sugar). When
    printing the type for diagnostic purposes, use the annotated type sugar to
    print the type as originally written.
    <br /> <br />
    For good results, template argument deduction will also need to be able to
    deduce type sugar (and reconcile cases where the same type is deduced twice
    with different sugar).
  </p>

  <p><b>Expected results: </b>
    Diagnostics preserve type sugar even when accessing members of a template
    specialization. T&lt;unsigned long&gt; and T&lt;size_t&gt; are still the
    same type and the same template instantiation, but
    T&lt;unsigned long&gt;::type single-step desugars to 'unsigned long' and
    T&lt;size_t&gt;::type single-step desugars to 'size_t'.</p>

  <p><b>Confirmed Mentor:</b>
    <a href=https://github.com/vgvassilev>Vassil Vassilev</a>,
    <a href=https://github.com/zygoloid>Richard Smith</a></p>

  <p><b>Desirable skills:</b>
    Good knowledge of clang API, clang's AST, intermediate knowledge of C++.
  </p>
  <p><b>Project type:</b> Large</p>

  <p><b>Discourse</b>
    <a href="https://discourse.llvm.org/t/clang-extend-clang-ast-to-provide-information-for-the-type-as-written-in-template-instantiations">
      URL</a>
  </p>
</div>

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<div class="www_subsubsection">
  <a name="clang-sa-structured-bindings">Implement support for
    C++17 structured bindings in the Clang Static Analyzer</a>
</div>
<!-- *********************************************************************** -->

<div class="www_text">
  <p><b>Description of the project: </b>
    Even though a lot of new C++ features are supported by the static analyzer
    automatically by the virtue of clang AST doing all the work under the hood,
    the C++17 "structured binding" syntax
    <pre>    auto [x, y] = ...;</pre>
    requires some extra work on the Static Analyzer side. The analyzer's transfer functions
    need to be taught about the new AST nodes, <a href="https://clang.llvm.org/doxygen/classclang_1_1BindingDecl.html">BindingDecl</a>
    and <a href="https://clang.llvm.org/doxygen/classclang_1_1DecompositionDecl.html">DecompositionDecl</a>,
    to work correctly in all <a href="https://en.cppreference.com/w/cpp/language/structured_binding">three interpretations</a>
    described by the Standard.
    <br /><br />
    Incomplete support for structured bindings is a common source of
    false positives in the uninitialized variable checker on modern C++ code,
    such as <a href="https://github.com/llvm/llvm-project/issues/42387">#42387</a>.
    <br /><br />
    It is likely that the Clang CFG also needs to be updated. Such changes in
    the CFG may improve quality of clang warnings outside of
    the Static Analyzer.
  </p>

  <p><b>Expected results: </b>
    The Static Analyzer correctly models structured binding and decomposition
    declarations. In particular, binding variables no longer appear
    uninitialized to the Static Analyzer's uninitialized variable checker.</p>

  <p><b>Confirmed Mentor:</b>
    <a href=https://github.com/haoNoQ>Artem Dergachev</a>,
    <a href=https://github.com/t-rasmud>Rashmi Mudduluru</a>,
    <a href=https://github.com/xazax-hun>Gábor Horváth</a>,
    <a href=https://github.com/Szelethus>Kristóf Umann</a>
  </p>

  <p><b>Desirable skills:</b>
    Intermediate knowledge of C++. Some familiarity with Clang AST and/or
    some static analysis background.
  </p>
  <p><b>Project size:</b> 350 hr</p>
  <p><b>Difficulty:</b> Medium/Hard</p>
  <p><b>Discourse</b>
    <a href="https://discourse.llvm.org/t/implement-support-for-c-17-structured-bindings-in-the-clang-static-analyzer/60588">
      URL</a>
  </p>
</div>

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<div class="www_subsubsection">
  <a name="clang-improve-diagnostics">Improve Clang Diagnostics.</a>
</div>
<!-- *********************************************************************** -->

<div class="www_text">
  <p><b>Decription:</b>
  Clang Diagnostics, which issues Warnings and Errors to the programmer, are a critical
  feature of the compiler. Great diagnostics can have a significant impact on the
  user experience of the compiler and increase their productivity.
  </p>

  <p>Recent improvements in GCC
  <a href="https://developers.redhat.com/blog/2018/03/15/gcc-8-usability-improvements"> [1] </a>
  <a href="https://developers.redhat.com/blog/2019/03/08/usability-improvements-in-gcc-9/"> [2] </a>
  shows that there is significant headroom to improve diagnostics
  (and user interactions in general). It would be a very impactful project
  to survey and identify all the possible improvements to clang on this
  topic and start redesigning the next generation of our diagnostics.
  </p>

  <p>
  In addition, we will also make conclusions on issues reported on the LLVM Github Issue page labeled
  with <a href="https://github.com/llvm/llvm-project/labels/clang%3Adiagnostics"> clang-diagnostics</a>
  and if they need fixing, we will prepare patches otherwise simply close them.
  </p>

  <p><b>Expected outcomes</b>:
  Diagnostics will be improved:
    <ul>
      <li>Improve diagnostic aesthetics</li>
      <li>Cover missing diagnostics</li>
      <li>Reduce false positive rate</li>
      <li>Reword diagnostics</li>
    </ul>
  </p>

  <p><b>Confirmed Mentor:</b>
    <a href=https://github.com/AaronBallman>Aaron Ballman</a>,
    <a href=https://github.com/erichkeane>Erich Keane</a>,
    <a href=https://github.com/xgupta>Shivam Gupta</a></p></p>

  <p><b>Desirable skills:</b> C++ coding experience</p>
  <p><b>Project type:</b> Large/350 hr</p>

  <p><b>Discourse</b>
    <a href="https://discourse.llvm.org/t/improve-clang-diagnostics/61521">
      URL</a>
  </p>
</div>

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<div class="www_subsection">
  <a>Polly</a>
</div>
<!-- *********************************************************************** -->

<!-- *********************************************************************** -->
<div class="www_subsubsection">
  <a name="polly_npm">Complete switch to new pass manager</a>
</div>
<!-- *********************************************************************** -->

<div class="www_text">
  <p><b>Description of the Project:</b>
    While the standard Polly-enabled -O1/-O2/-O3 optimization pass pipelines work fine with the <a href="https://blog.llvm.org/posts/2021-03-26-the-new-pass-manager/">New Pass Manager</a> (NPM), some parts of Polly still only works with the legacy pass manager.
    This includes some passes such as -polly-export-jscop/-polly-export-jscop, regression testing, Polly-ACC, command line options such as -polly-show, the PassInstrumentation mechanism used by e.g. -print-after-all.
    LLVM (and Clang) have moved to NPM being the default and support for the legacy pass manager is deprecated, slowly degenerates and features getting removed.
    That is, all of Polly's functionality should eventually work with the NPM as well, and be prepared for the complete removal of the legacy pass manager.
    More details about the two pass managers found <a href="https://github.com/banach-space/llvm-tutor#about-pass-managers-in-llvm">here</a>.
  </p>
  <p><b>Expected results:</b>
    The goal is to make Polly more usable with using only the NPM. Milestones, not necessarily all to be reached in this GSoC, are:
    <br/>
    1. Make all of Polly's functionality available in the NPM (or decide to deprecate/remove it)
    <br/>
    2. Better integration into the NPM (such as supporting PassInstrumentation); If the NPM turns out to be inadequate, use only a monolothic function pass.
    <br/>
    3. Replace the legacy pass manager in regression tests.
    <br/>
    4. Be ready for complete removal of the legacy pass manager in LLVM.
  </p>
  <p><b>Confirmed mentor:</b>
    <a href="https://github.com/Meinersbur">Michael Kruse</a>
  </p>
  <p><b>Desirable skills:</b>
    Understanding of the C++ template pattern used by the new pass manager (<a href="https://en.wikipedia.org/wiki/Curiously_recurring_template_pattern">CRTP</a>, <a href="https://en.wikipedia.org/wiki/Mixin">Mixins</a>, etc).
    Familarity with how LLVM can be <a href="https://www.lurklurk.org/linkers/linkers.html">linked</a> (static, BUILD_SHARED_LIBS, and SHLIB/DYLIB) and its <a href="https://www.llvm.org/docs/WritingAnLLVMPass.html#building-pass-plugins">plugin loading machanisms</a> (static, -load and -load-pass-plugin).
    Ideally, already worked with LLVM's new pass manager.
  </p>
  <p><b>Project size:</b> Medium</p>
  <p><b>Difficulty:</b> Medium/Hard</p>
  <p><b>Discourse</b> <a href="https://discourse.llvm.org/t/61174">URL</a></p>
</div>

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<div class="www_subsection">
  <a>Enzyme</a>
</div>
<!-- *********************************************************************** -->

<!-- *********************************************************************** -->
<div class="www_subsubsection">
  <a name="enzyme_tblgen">Move Enzyme Instruction Transformation Rules to Tablegen</a>
</div>
<!-- *********************************************************************** -->

<div class="www_text">
  <p><b>Description of the project:</b>
    Enzyme performs automatic differentiation (in the calculus sense) of LLVM programs. This enables users to use Enzyme to perform various algorithms such as back-propagation in ML or scientific simulation on existing code for any language that lowers to LLVM. The support for an increasing number of LLVM Versions (7-main), AD modes (Reverse, Forward, Forward-Vector, Reverse-Vector, Jacobian), and libraries (BLAS, OpenMP, MPI, CUDA, ROCm, ...) leads to a steadily increasing code base. In order to limit complexity and help new contributors we would like to express our core logic using LLVM Tablegen. The applicant is free to decide how to best map the program transformation abstractions within Enzyme to Tablegen.
  </p>
  <p><b>Expected results:</b>
     1. A working tablegen rule generation system within Enzyme
     <br/>
     2. Moving several existing rules to the new autogenerated system (e.g. LLVM instructions, LLVM intrinsics, BLAS calls, MPI calls, ...
     <br/>
   </p>

  <p><b>Confirmed mentor:</b>
    <a href="mailto:wmoses@mit.edu">William Moses</a>, Valentin Churavy
  </p>
  <p><b>Desirable skills:</b>
    Good knowledge of C++, calculus, and LLVM and/or Clang, and/or MLIR internals. Experience with Tablegen, Enzyme or automatic differentiation would be nice, but can also be learned in the project.
  </p>
  <p><b>Project size:</b> Large</p>
  <p><b>Difficulty:</b> Medium</p>
  <p><b>Discourse</b> <a href="https://discourse.llvm.org/t/enzyme-moving-instruction-rules-to-tablegen/61176">URL</a></p>
</div>

<!-- *********************************************************************** -->
<div class="www_subsubsection">
  <a name="enzyme_vector">Vector Reverse-Mode Automatic Differentiation</a>
</div>
<!-- *********************************************************************** -->

<div class="www_text">
  <p><b>Description of the project:</b>
    Enzyme performs automatic differentiation (in the calculus sense) of LLVM programs. This enables users to use Enzyme to perform various algorithms such as back-propagation in ML or scientific simulation on existing code for any language that lowers to LLVM. Enzyme already implements forward and reverse mode automatic differentiation. Enzyme also implements vector forward mode automatic differentiation, which allows Enzyme to batch the derivative computation of several objects in a single call. The goal of this project is too extend this capability in order to perform vector reverse mode. In doing so, multiple sweeps of reverse mode automatic differentiation can be performed at the same time, reducing memory, time, and otherwise generally enabling further optimization.
  </p>
  <p><b>Expected results:</b>
     Vectorized version of reverse mode automatic differentiation
   </p>

  <p><b>Confirmed mentor:</b>
    <a href="mailto:wmoses@mit.edu">William Moses</a>, Tim Gymnich
  </p>
  <p><b>Desirable skills:</b>
     Good knowledge of C++ and some experience with LLVM API's. Experience with Enzyme or automatic differentiation would be nice, but can also be learned in the project.
  </p>
  <p><b>Project size:</b> Medium</p>
  <p><b>Difficulty:</b> Medium</p>
  <p><b>Discourse</b> <a href="https://discourse.llvm.org/t/enzyme-vector-reverse-mode-automatic-differentiation/61177">URL</a></p>
</div>


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<div class="www_subsubsection">
  <a name="enzyme_pm">Enable The New Pass Manager</a>
</div>
<!-- *********************************************************************** -->

<div class="www_text">
  <p><b>Description of the project:</b>
    Enzyme is a compiler plugin for LLVM that performs automatic differentiation (in the calculus sense) of LLVM programs. This enables users to use Enzyme to perform various algorithms such as back-propagation in ML or scientific simulation on existing code for any language that lowers to LLVM.
    <br/>
    Enzyme integrates into frontends through the use of an LLVM plugin that can be loaded into Clang, LLVM (opt), the linker (lld), libraries (HIPRtc), directly loaded (Julia), among others (Flang, Rust, etc).
    <br/>
    While using various pieces of machinery from the new pass manager internally, Enzyme does not currently automatically register its transformation passes when using the new pass manager. This creates problems for users on LLVM 13 or above, where the new pass manager is run by default and may not understand why they get linker errors from their code not being differentiated (currently they must add a flag to specify the old pass manager).
    <br/>
    The goal of this project is to enable Enzyme to be called by the new pass manager in LLVM and generally create a coherent user experience.
    <br/>
  </p>

  </p>
  <p><b>Expected results:</b>
      1. Enzyme can be called by the new pass manager
      <br/>
      2. [Optional] Additional syntactic sugar that makes it easier to use Enzyme.
      <br/>
   </p>

  <p><b>Confirmed mentor:</b>
    <a href="mailto:wmoses@mit.edu">William Moses</a>, Valentin Churavy
  </p>
  <p><b>Desirable skills:</b>
     Good knowledge of C++, and LLVM. Experience with Enzyme would be nice, but can also be learned in the project.
  </p>
  <p><b>Project size:</b> Small</p>
  <p><b>Difficulty:</b> Medium</p>
  <p><b>Discourse</b> <a href="https://discourse.llvm.org/t/enzyme-enable-the-new-pass-manager/61178">URL</a></p>
</div>

<!-- *********************************************************************** -->
<div class="www_sectiontitle">
  <a name="gsoc21">Google Summer of Code 2021</a>
</div>
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<div class="www_text">
  <p>
    Welcome prospective Google Summer of Code 2021 Students! This document is your
    starting point to finding interesting and important projects for LLVM, Clang,
    and other related sub-projects. This list of projects is not only developed for
    Google Summer of Code, but open projects that really need developers to work on
    and are very beneficial for the LLVM community. </p>

  <p>We encourage you to look through this list and see which projects excite you
    and match well with your skill set. We also invite proposals not on this
    list. You must propose your idea to the LLVM community through our
    developers' mailing list (llvm-dev@lists.llvm.org or specific subproject mailing
    list). Feedback from the community is a requirement for your proposal to be
    considered and hopefully accepted.
  </p>

  <p>The LLVM project has participated in Google Summer of Code for several years
    and has had some very successful projects. We hope that this year is no
    different and look forward to hearing your proposals. For information on how to
    submit a proposal, please visit the Google Summer of Code
    main <a href="https://developers.google.com/open-source/gsoc/">website.</a></p>
</div>

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<div class="www_subsection">
  <a>LLVM</a>
</div>
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<div class="www_subsubsection">
  <a name="llvm_distributing_lit">Distributed lit testing</a>
</div>
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<div class="www_text">
  <p><b>Description of the project:</b>
    The LLVM lit test suites consist of thousands of small independent tests.
    Due to the number of tests, it can take a long time to run the full suite,
    even on a high-spec computer. Builds are already distributable across
    multiple computers available on the same network, using software such as
    distcc or icecream, so running tests on a single machine becomes a potential
    bottleneck. One way to speed up running of the tests could be to distribute
    test execution across many computers too. Lit provides a test sharding
    mechanism, which allows multiple computers to run parts of the same
    testsuite in tandem, but this currently assumes access to a single common
    filesystem, which may not be possible in all cases and a knowledge of which
    machines the suite can currently be run on.

    This project’s goal is to update the existing lit harness (or write a
    wrapper around it) to allow distribution of the tests in this way, with the
    idea that developers can write their own interface between the harness and
    the distribution system of their choice. This harness may need to be able to
    identify test dependencies such as input files and executables, send the
    tests to the distribution system (possibly in batches), and receive, collate
    and report the results to the user, in a similar manner to how lit already
    does.
  </p>

  <p><b>Expected results:</b> An easy to use harness as described above. Some
    evidence that given a distributed system, a user can expect to see test
    suite execution to speed up if they are using that harness.</p>

  <p><b>Confirmed mentor:</b> James Henderson</p>
  <p><b>Desirable skills:</b> Good knowledge of Python. Familiarity with LLVM
    lit testing. Some knowledge of distribution systems would also be
    beneficial.</p>
</div>

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<div class="www_subsubsection">
  <a name="llvm_loop_heuristics">Learning Loop Transformation Heuristics</a>
</div>
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<div class="www_text">
  <p><b>Description of the project:</b>
    This is a short description, please reach out to Johannes (jdoerfert on
    IRC) and Mircea Trofin if it sounds interesting.

    We successfully introduced an ML framework for inliner decisions, now we want
    to expand the scope. In this project we will look at loop transformation
    heuristics, such as the unroll factor. As a motivational example we can look
    at a small trip count <a href="https://godbolt.org/z/Eeqcvs">dgemm</a> which
    we optimize pretty poorly. With the nounroll pragmas we do a better job but
    still not close to gcc.

    The project is open-ended and we could look at various passes/heuristics
    concurrently.
  </p>

  <p><b>Preparation resources:</b> The ML inliner framework in the LLVM code
  base as well as the <a href="https://arxiv.org/abs/2101.04808">paper</a>. LLVM
  transform passes (that are based on heuristics), e.g., loop unroll.</p>

  <p><b>Expected results:</b> Measurable better performance with a learned
  predictor, potentially a set of "classical" heuristics derived from the ML
  model.</p>

  <p><b>Confirmed Mentor:</b> Johannes Doerfert, Mircea Trofin</p>
  <p><b>Desirable skills:</b> Intermediate knowledge of ML, C++, self motivation.</p>
</div>

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<div class="www_subsubsection">
  <a name="llvm_ir_fuzzing">Fuzzing LLVM-IR Passes</a>
</div>
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<div class="www_text">
  <p><b>Description of the project:</b>
    This is a short description, please reach out to Johannes (jdoerfert on
    IRC) if it sounds interesting.

    Fuzzing often reveals a myriad of bugs. CSmith (and others) showed how to do
    this with C-like languages and we have used <a
    href="https://www.youtube.com/watch?v=UBbQ_s6hNgg">LLVM-IR fuzzing</a> in
    the past successfully. In this project we will apply fuzzing to new passes
    that are in development, e.g., the Attributor pass. We want to find and fix
    crashes but also other bugs, including compile time performance problems.
  </p>

  <p><b>Preparation resources:</b> The <a
    href="https://llvm.org/docs/FuzzingLLVM.html#llvm-opt-fuzzer">LLVM fuzzer
    infrastructure</a>. LLVM passes that we might want to fuzz, e.g. the
  Attributor pass. Prior IR-Fuzzing work
  (https://www.youtube.com/watch?v=UBbQ_s6hNgg)</p>


  <p><b>Expected results:</b> Crashes, maybe also a way to catch non-crash
  bugs, including performance problems.</p>

  <p><b>Confirmed Mentor:</b> Johannes Doerfert</p>
  <p><b>Desirable skills:</b> Intermediate knowledge C++, self motivation.</p>
</div>

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<div class="www_subsubsection">
  <a name="llvm_ir_assume"><tt>llvm.assume</tt> the missing pieces</a>
</div>
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<div class="www_text">
  <p><b>Description of the project:</b>
    This is a short description, please reach out to Johannes (jdoerfert on
    IRC) if it sounds interesting.

    <tt>llvm.assume</tt> is a powerful mechanism to retain knowledge. Since it
    inception it was improved already multiple times but there are major
    extensions still outstanding which we want to tackled in this project.
    An incomplete list of topics includes:
    <ul>
      <li> range-based assumptions, design idea 3) in the <a href="https://lists.llvm.org/pipermail/llvm-dev/2019-December/137632.html">RFC</a>. </li>
      <li> outline arbitrary assumption/assertion code, design idea 2) in the <a href="https://lists.llvm.org/pipermail/llvm-dev/2019-December/137632.html">RFC</a>. </li>
      <li> side-effect free assumptions, see <a href="https://reviews.llvm.org/D89054">this review</a>. </li>
      <li> more knowledge retention usages </li>
      <li> less interference with optimizations </li>
    </ul>

  </p>

  <p><b>Preparation resources:</b> The llvm.assumption usage, the assumption
  cache, the "enable-knowledge-retention" option, the <a
  href="https://lists.llvm.org/pipermail/llvm-dev/2019-December/137632.html">RFC</a>
  and <a href="https://reviews.llvm.org/D89054">this review</a>.
  </p>


  <p><b>Expected results:</b> New llvm.assume use cases, improved performance through knowledge retention, optimization based on assertions.</p>

  <p><b>Confirmed Mentor:</b> Johannes Doerfert</p>
  <p><b>Desirable skills:</b> Intermediate knowledge C++, self motivation.</p>
</div>

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<div class="www_subsubsection">
  <a name="llvm_ir_issues">Fix fundamental issues in LLVM's IR</a>
</div>
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<div class="www_text">
  <p><b>Description of the project:</b>
    LLVM's IR has fundamental, long-standing issues. Many are related with
    undefined behaviors. Others are simply a fallout from underspecification
    and different interpretations by diffferent people.
    <a href="https://github.com/AliveToolkit/alive2">Alive2</a> is a tool that
    detects bugs in LLVM's optimizations automatically. Using Alive2, we track
    bugs exposed by the unit tests on a
    <a href="https://web.ist.utl.pt/nuno.lopes/alive2/">dashboard</a>.
  </p>

  <p><b>Expected results:</b>
    1) Report and fix bugs detected by Alive2.
    2) Pick one fundamental IR issue and
    make progress towards fixing it, including proposing fixes for the
    <a href="https://llvm.org/docs/LangRef.html">semantics</a>, testing
    fixes to the semantics by running Alive2 over the LLVM unit tests and
    medium-sized programs, test performance of semantic fixes and fix
    performance regressions.
  </p>
  <p><b>Confirmed Mentor:</b> Nuno Lopes, Juneyoung Lee</p>
  <p><b>Desirable skills:</b> Intermediate C++; willingness to learn about LLVM
    IR semantics; experience reading papers (preferred).
  </p>
</div>

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<div class="www_subsubsection">
  <a name="llvm_utilize_loopnest">Utilize LoopNest Pass</a>
</div>
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<div class="www_text">
  <p><b>Description of the project:</b>
    The idea of LoopNest pass is recently added, and there are no existing
    passes utilizing it. Before having LoopNest pass, if you want to write a
    pass that works on a loop nest, you have to pick from either a function
    pass or a loop pass. If you chose to write it as a function pass, then you
    lose the ability to add loops dynamically back to the pipeline. If you
    decide to write it as a loop pass, then you are wasting compile time to
    traverse to your pass and return right away when the given loop is not the
    outermost loop. In this project, we want to utilize the recently introduced
    LoopNest pass for passes intended for loop nest and have the same ability
    as the LoopPass to dynamically add loops to the pipeline. In addition,
    improve the current implementation of LoopNestPass when necessary.
  </p>
  <p><b>Expected results (possibilities):</b>
    Utilize LoopNest Pass for some existing transformations/analyses.
  </p>
  <p><b>Confirmed Mentors:</b>
    Whitney Tsang, Ettore Tiotto
  </p>
  <p><b>Desirable skills:</b>
    Intermediate knowledge of C++, self-motivation.
  </p>
</div>

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<div class="www_subsubsection">
  <a name="openmp_gpu_jit">JIT-ing OpenMP GPU kernels transparently</a>
</div>
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<div class="www_text">
  <p><b>Description of the project:</b>
    This is a short description, please reach out to Johannes (jdoerfert on
    IRC) if it sounds interesting.

    OpenMP GPU kernels are usually lowered to native binaries, e.g., cubin, and
    embedded into the host object. At runtime, OpenMP "plugins" will connect with
    the device driver, e.g., CUDA, to load and run such embedded binary images.
    In this project we want to develop a new plugin that takes LLVM-IR code, optimizes
    the IR with kernel parameters known only at runtime, and then generates the GPU
    binary for consumption by other plugins. Similar to the <a
      href=https://openmp.llvm.org/docs/design/Runtimes.html#remote-offloading-plugin>remote
      offload plugin</a> we can do this transparently to the user. In addition to the JIT
    infrastructure setup in the plugin we will need to embed the IR into the host object.

  </p>

  <p><b>Preparation resources:</b>OpenMP target offloading infrastructure, LLVM JIT infrastructure</a>.
  </p>


  <p><b>Expected results:</b> A JIT-capable offload plugin which can achieve superior performance when kernel specialization is enabling optimizations.</p>

  <p><b>Confirmed Mentor:</b> Johannes Doerfert</p>
  <p><b>Desirable skills:</b> Intermediate knowledge C++, JIT compilation, self motivation.</p>
</div>

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<div class="www_subsection">
  <a>OpenACC</a>
</div>
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<div class="www_subsubsection">
  <a name="openacc_rt_diagnostics">OpenACC Diagnostics from the OpenMP Runtime</a>
</div>
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<div class="www_text">
  <p><b>Description of the project:</b>
    Clacc and Flacc are projects to introduce OpenACC support to Clang and
    Flang.  For that purpose, OpenACC runtime support is being developed on top
    of LLVM's OpenMP runtime.  However, diagnostics emitted by LLVM's OpenMP
    runtime are expressed in terms of OpenMP concepts, and so those diagnostics
    are not always meaningful to OpenACC users.  This project should address
    this issue in two steps:
    <ol>
      <li>
        Develop a mechanism that selects OpenACC versions of diagnostics that
        are emitted as a result of OpenACC-related calls into the runtime.  This
        mechanism should be general enough that it could be used for programming
        languages besides OpenMP and OpenACC.  One possible approach is to
        extend internationalization mechanisms already present in some
        components of the OpenMP runtime.
      </li>
      <li>
        Provide OpenACC translations for existing OpenMP diagnostics.  This step
        requires an understanding of the relationship between OpenACC and OpenMP
        as implemented in Clacc and Flacc.
      </li>
    </ol>
    Many components of OpenACC support that will depend upon this project have
    not yet been upstreamed and are under development.  A high-level
    understanding of those efforts is helpful for this project and can be
    provided by the mentors.  Nevertheless, this project can be completed in
    upstream LLVM's OpenMP runtime now independently of those efforts.
  </p>

  <p>
    <b>Expected results:</b> A version of upstream LLVM's OpenMP runtime that
    can emit OpenACC diagnostics as needed.
  </p>

  <p><b>Confirmed Mentors:</b> Valentin Clement, Joel E. Denny</p>

  <p>
    <b>Desirable skills:</b> Intermediate C++; Experience with OpenACC or
    OpenMP
  </p>
</div>

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<div class="www_subsection">
  <a>Polly</a>
</div>
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<div class="www_subsubsection">
  <a name="polly_isl_bindings">Use official isl C++ bindings</a>
</div>
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<div class="www_text">
  <p><b>Description of the project:</b>
    Polly use algorithms from the
    <a href="http://isl.gforge.inria.fr/">Integer Set Library (isl)</a>, which is a
    library written in C. It uses reference-counting for memory management.
    Getting reference counting correct is much easier in C++ using RAII,
    therefore we created a C++ binding for isl:
    <a href="https://github.com/llvm/llvm-project/blob/main/polly/lib/External/isl/include/isl/isl-noexceptions.h">isl-noexceptions.h</a>.
    Since then, isl also gained two official C++ bindings,
    <a href="https://github.com/llvm/llvm-project/blob/main/polly/lib/External/isl/include/isl/cpp.h">cpp.h</a>
    and
    <a href="https://github.com/llvm/llvm-project/blob/main/polly/lib/External/isl/include/isl/cpp-checked.h">cpp-checked.h</a>.

    We would like to replace the Polly-maintained C++ bindings with the upstream
    bindings. Unfortunately, this is not an in-place replacement. Differences
    include how errors are checked, method names, which functions are
    considered as operator/constructor overloads and the set of exported functions.
    This will require changing Polly's uses of the C++ bindings and submitting
    patches to isl to export additional functionality needed by Polly.
  </p>

  <p><b>Expected results:</b>
    Reduce the differences between the Polly-maintained isl-noexceptions.h
    bindings and one of the two C++ bindings that isl supports. Due to
    isl-noexceptions.h exporting more functions and classes than the upstream
    bindings do, a complete replacement will probably be out of reach, but
    even reducing the differences will reduce the maintenance cost of Polly's
    isl-noexceptions.h.
  </p>

  <p><b>Confirmed mentor:</b> Michael Kruse</p>
  <p><b>Desirable skills:</b>
    Deep knowledge of C++, in particular RAII and move-semantics. Interest in API design. Ideally, you already wrote some library's header file.
    Experience with the isl library would be nice, but can also be learned in the project.
  </p>
</div>

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<div class="www_subsection">
  <a>Enzyme</a>
</div>
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<div class="www_subsubsection">
  <a name="enzyme_blas">Integrate custom derivatives of BLAS, Eigen, and similar routines into Enzyme</a>
</div>
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<div class="www_text">
  <p><b>Description of the project:</b>
    <a href="https://enzyme.mit.edu/">Enzyme</a> performs automatic differentiation
    (in the calculus sense) of LLVM programs. This enables users to use Enzyme to
    perform various algorithms such as back-propagation in ML
    or scientific simulation on existing code for any language that lowers to LLVM.

    Enzyme does so by applying the chain rule to every instruction in every
    function called by the original function to be differentiated. While functional,
    this is not necessarily optimal for high-level matrix operations which may
    have algebraic properties for faster derivative computation.

    Enzyme also has a mechanism for specifying a custom gradient for a given function.
    If a custom derivative is available, Enzyme will use that rather than fallback
    to implementing its own.

    Many programs use BLAS libraries to efficiently compute matrix and tensor
    operations. This project would enable high-performance automatic differentiation
    of BLAS and similar libraries (such as Eigen) by specifying custom derivative
    rules for their operations.
  </p>

  <p><b>Expected results:</b>
    Efficient differentiation of BLAS and Eigen codes by writing custom
    derivative rules for matrix and tensor operations.
  </p>

  <p><b>Confirmed mentor:</b> <a href="mailto:wmoses@mit.edu">William Moses</a>, Johannes Doerfert</p>
  <p><b>Desirable skills:</b>
    Good knowledge of C++, calculus, and linear algebra. Experience with BLAS, Eigen,
    or Enzyme would be nice, but can also be learned in the project.
  </p>
</div>

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<div class="www_subsubsection">
  <a name="enzyme_swift">Integrate Enzyme into Swift to provide high-performance differentiation in Swift</a>
</div>
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<div class="www_text">
  <p><b>Description of the project:</b>
    <a href="https://enzyme.mit.edu/">Enzyme</a> performs automatic differentiation
    (in the calculus sense) of LLVM programs. This enables users to use Enzyme to
    perform various algorithms such as back-propagation in ML
    or scientific simulation on existing code for any language that lowers to LLVM.

    While this functions for any frontend that emits LLVM IR, it may be desirable
    to have closer integration between Enzyme and the frontend for the sake of
    passing additional information and creating a better user experience.

    Swift provides automatic differentiation through the use of specifying custom
    derivative rules in the front-end. Enzyme could be integrated directly with
    Swift, differentiating the eventual LLVM, but it would lose out on all this
    additional information about custom derivatives. Moreover, calling into
    Enzyme naiively would be without Type checking, fine AD-specific debug information,
    or various other nice tools that Swift provides users of AD.

    This project would seek to integrate Enzyme and the Swift front end to provide
    both a nice user-experience for swift programmers who want to use Enzyme
    to enable high-performance automatic differentiation, and also to allow Enzyme
    to take advantage of derivative-specific metadata already available in swift.
  </p>

  <p><b>Expected results:</b>
    Creation of a custom type-checked linguistic construct in Swift for calling Enzyme.
    Mechanisms for passing Swift's differentiation-specific metadata for use by Enzyme.
  </p>

  <p><b>Confirmed mentor:</b> <a href="mailto:wmoses@mit.edu">William Moses</a>, Vassil Vassilev</p>
  <p><b>Desirable skills:</b>
    Good knowledge of C++ and Swift. Experience with Enzyme or automatic differentiation
    would be nice, but can also be learned in the project.
  </p>
</div>

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<div class="www_subsubsection">
  <a name="enzyme_fixed">Differentiation of Fixed-Point Arithmetic</a>
</div>
<!-- *********************************************************************** -->

<div class="www_text">
  <p><b>Description of the project:</b>
    <a href="https://enzyme.mit.edu/">Enzyme</a> performs automatic differentiation
    (in the calculus sense) of LLVM programs. This enables users to use Enzyme to
    perform various algorithms such as back-propagation in ML
    or scientific simulation on existing code for any language that lowers to LLVM.

    In a variety of fields, it is desirable to compute on fixed-point values
    (e.g. integers) rather than floating point values. This avoid certain truncation
    errors that may be critical to a given application. Moreover, particular pieces
    of hardware may simply be more efficient on fixed point rather than floating
    point values.

    This project would seek to extend Enzyme to support differentiation of not only
    floating point base values, but also fixed point base values..
  </p>

  <p><b>Expected results:</b>
    Implementation of adjoints for LLVM fixed point intrinsics, requisite type analysis
    rules, and integration into a front-end for an end-to-end test.
  </p>

  <p><b>Confirmed mentor:</b> <a href="mailto:wmoses@mit.edu">William Moses</a></p>
  <p><b>Desirable skills:</b>
    Good knowledge of C++, caclulus, and LLVM internals. Experience with Enzyme or
    automatic differentiation would be nice, but can also be learned in the project.
  </p>
</div>

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<div class="www_subsubsection">
  <a name="enzyme_rust">Integrate Enzyme into Rust to provide high-performance differentiation in Rust</a>
</div>
<!-- *********************************************************************** -->

<div class="www_text">
  <p><b>Description of the project:</b>
    <a href="https://enzyme.mit.edu/">Enzyme</a> performs automatic differentiation
    (in the calculus sense) of LLVM programs. This enables users to use Enzyme to
    perform various algorithms such as back-propagation in ML
    or scientific simulation on existing code for any language that lowers to LLVM.

    While this functions for any frontend that emits LLVM IR, it may be desirable
    to have closer integration between Enzyme and the frontend for the sake of
    passing additional information and creating a better user experience.

    This project would seek to integrate Enzyme and the Rust front end to provide
    a nice user-experience for Rust programmers who want to use Enzyme
    to enable high-performance automatic differentiation. This also potentially
    involves integration of LLVM plugin support/custom codegen into rustc.
  </p>

  <p><b>Expected results:</b>
    Creation of a custom type-checked linguistic construct in Rust for calling Enzyme.
    Mechanisms for parsing Rust's Type information (represented as debug LLVM debug
    info) directly into type analysis.
  </p>

  <p><b>Confirmed mentor:</b> <a href="mailto:wmoses@mit.edu">William Moses</a></p>
  <p><b>Desirable skills:</b>
    Good knowledge of C++ and Rust. Experience with Enzyme or automatic differentiation
    would be nice, but can also be learned in the project.
  </p>
</div>

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<div class="www_subsubsection">
  <a name="static_analyzer_profling">Clang Static Analyzer performance profiling</a>
</div>
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<div class="www_text">
  <p><b>Description of the project:</b>
    <ul>
     <li>
       Chart how much time is spent in transfer functions – including (but not
       limited to!) checker callbacks.
     </li>
     <li>
       Add llvm Statistics and Timers for quickly obtaining precise and concise
       dumps without external profilers. Statistics on state splits might be
       particularly interesting!
     </li>
     <li>
       Measure time spent analyzing specific stack frames. Say, how much time
       do we spend inlining <tt>std::string</tt> methods? This time could be
       saved if we add custom  models for these methods instead.
     </li>
    </ul>
  </p>

  <p><b>Confirmed mentor:</b>  Artem Dergachev</p>
</div>

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<div class="www_subsubsection">
  <a name="static_analyzer_constraint_solver">Clang Static Analyzer constraint solver improvements</a>
</div>
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<div class="www_text">
  <p><b>Description of the project:</b>
    CSA has a small in-house constraint solver, it is pretty trivial, but super
    fast.  The goal is to support range-based logic for some of the symbolic
    operators, while keeping it linear.  Additionally, a unit-test framework
    can be designed specifically for testing constraint solvers (right now it’s
    tested rather awkwardly). This project has a couple of interesting
    properties.  It can be segmented into small chunks, and each of these
    chunks has a non-trivial solution.  It might introduce you to a world of
    solvers (it is a good idea to check your ideas with some heavy-weight
    solver such as z3). And because the existing solver is simple, there is a
    myriad of possible extensions to try.
  </p>

  <p><b>Confirmed mentor:</b>  Valeriy Savchenko</p>
</div>

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<div class="www_subsubsection">
  <a name="lldb_diagnostics">A structured approach to diagnostics in LLDB</a>
</div>
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<div class="www_text">
  <p><b>Description of the project:</b>
    <ul>
     <li>
       Design and integrate a new diagnostic abstraction (similar to
       clang::Diagnostic) to report errors, warnings and notes in a structured
       way.
     </li>
     <li>
       Allow us to differentiate between bugs (unexpected errors) and things
       the debugger simply doesn’t know (expected errors). Be smart about
       printing global errors only once. Have the ability of being verbose and
       have additional metadata (source location, DWARF unit, object file, etc,
       depending on the type of error and where it originated). </li>
     <li>
       Should be compatible and tightly integrated with the existing classes,
       such as the Status and CommandReturnObject.
     </li>
    </ul>
  </p>

  <p><b>Confirmed mentor:</b>
    <a href="mailto:teemperor@gmail.com,jonas@devlieghere.com?subject=[GSoC]%20LLDB%20Diagnostics">Jonas Devlieghere and Raphael Isemann</a></p>
</div>

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<div class="www_sectiontitle">
  <a name="gsoc20">Google Summer of Code 2020</a>
</div>
<!-- *********************************************************************** -->

<div class="www_text">
  <p>
    LLVM participation in Google Summer of Code 2020 was very successful and resulted
    in many interesting projects contributed to LLVM. For the list of accepted and
    completed projects, please take a look into Google Summer of Code
    <a href="https://summerofcode.withgoogle.com/archive/2020/organizations/5902726635978752/">
      website.</a>
  </p>
</div>

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<div class="www_subsection">
  <a>LLVM</a>
</div>
<!-- *********************************************************************** -->

<!-- *********************************************************************** -->
<div class="www_subsubsection">
  <a name="llvm_ipo">Improve inter-procedural analyses and optimizations</a>
</div>
<!-- *********************************************************************** -->

<div class="www_text">
  <p><b>Description of the project:</b>
    This is a short description, please reach out to Johannes (jdoerfert on IRC)
    if it sounds interesting.

    During the GSoC'19 we build the Attributor framework to improve the
    inter-procedural capabilities of LLVM. This is useful on its own but
    especially in situations where inlining is impossible or undesirable.

    In this GSoC project we will look at capabilities not yet available in the
    Attributor and for the potential to connect the Attributor with existing
    intra- and inter-procedural optimizations.

    In this project there is a lot of freedom to determine the actual tasks but
    we will provide a pool of smaller and medium sized tasks that can be chosen
    from as well.
  </p>

  <p><b>Preparation resources:</b> The Attributor YouTube videos from the
  LLVM Developers Meeting 2019 and the recording of the IPO panel from the same
  meeting. The Attributor framework as well as other existing inter-procedural
  analyses and optimizations in LLVM.</p>

  <p><b>Expected results:</b> Measurable better IPO, especially visible in cases
                              where inlining is not an option or undesirable.</p>

  <p><b>Confirmed Mentor:</b> Johannes Doerfert</p>
  <p><b>Desirable skills:</b> Intermediate knowledge of C++, self motivation.</p>
</div>

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<div class="www_subsubsection">
  <a name="llvm_par">Improve parallelism-aware analyses and optimizations</a>
</div>
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<div class="www_text">
  <p><b>Description of the project:</b>
    This is a short description, please reach out to Johannes (jdoerfert on IRC)
    if it sounds interesting.

    With the OpenMPOpt pass (<a href='https://reviews.llvm.org/D69930'>under
      review</a>) we started to teach the LLVM optimization pipeline about
    OpenMP parallelism encoded as OpenMP runtime calls.

    In this GSoC project we will look at capabilities not yet available in the
    OpenMPOpt pass and for the potential to connect existing intra- and
    inter-procedural optimizations, e.g. the Attributor.

    In this project there is a lot of freedom to determine the actual tasks but
    we will provide a pool of smaller and medium sized tasks that can be chosen
    from as well.
  </p>

  <p><b>Preparation resources:</b> The "Optimizing Indirections, using
  abstractions without remorse" video on YouTube from the LLVM Developers
  Meeting 2018. The paper "Compiler Optimizations for OpenMP" and "Compiler
  Optimizations For Parallel Programs" both by J. Doerfert and H. Finkel (the
  slides for these are potentially even more useful).</p>

  <p><b>Expected results:</b> Measurable better performance or program analysis
  results for parallel programs with a focus on OpenMP.</p>

  <p><b>Confirmed Mentor:</b> Johannes Doerfert</p>
  <p><b>Desirable skills:</b> Intermediate knowledge of C++, self motivation.</p>
</div>

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<div class="www_subsubsection">
  <a name="llvm_dbg_invariant">Make LLVM passes debug info invariant</a>
</div>
<!-- *********************************************************************** -->

<div class="www_text">
  <p><b>Description of the project:</b>
    Generating debug information is one of the fundamental tasks a compiler
    typically fulfills. It is clear that executable generated code should not
    depend on the presence of debug information.
    <br><br>
    Unfortunately there are known cases in LLVM were code generation differs
    depending on whether debug information is enabled (`-g`) or not. These kind
    of bugs can lead to bad debug experience ranging from unexpected execution
    behaviour to the point of programs running fine in debug mode while crashing
    without debug information.
    <br><br>
    The issue has likely not a single cause but is triggered during different
    passes on different architectures. One such reason is the insertion of Call
    Frame Information (CFI) in the compiler backend during frame lowering and
    other later passes. The presence of CFI instructions seems to change
    instruction scheduling which therefore leads to different generated code.
  </p>

  <p><b>Preparation resources:</b>
    <ul>
      <li>
        <a href="https://llvm.org/PR37728">PR37728</a> is a
        meta-bug that collects several related issues of differing codegen.
      </li>
      <li>
        <a href="https://llvm.org/PR37240">PR37240</a> is a
        bug discussing the CFI issue mentioned above.
      </li>
      <li>
        The following
        <a href="http://lists.llvm.org/pipermail/llvm-dev/2019-September/135433.html">
        RFC</a> discusses some possible mitigation strategies and gives some
        background information on the CFI issue.
      </li>
    </ul>
  </p>
  <p><b>Expected results:</b>
    <ul>
    <li>
      Write some tooling based on existing scripts to automatically generate
      examples of differing codegen. This is intended as a starting task to get
      to know the existing LLVM tools, learn to read LLVM's internal outputs etc.
    </li>
    <li>
      Choose one or more (depending on the difficulty) bugs that cause codegen
      differences and try to provide patches to fix them. We would be
      particularly interested in the mentioned CFI issue but working on some of
      the other related bugs is also absolutely fine.
    </li>
    </ul>
  </p>

  <p><b>Confirmed Mentors:</b> Paul Robinson and David Tellenbach</p>

  <p><b>Desirable skills:</b>
    Intermediate knowledge of C++, some familarity with general computer
    architecture, some familarity with the x86 or Arm/AArch64 instruction set.
  </p>
</div>

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<div class="www_subsubsection">
  <a name="llvm_mergesim">Improve MergeFunctions to incorporate MergeSimilarFunction patches and ThinLTO Support</a>
</div>
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 <div class="www_text">
  <p><b>Description of the project:</b> MergeSimilarFunctions pass is able to
    merge not just identical functions, but also functions with small differences in
    their instructions to reduce code size. It does this by inserting control flow
    and an additional argument in the merged function to account for the
    differences.

    This work was presented at
    the <a href="http://llvm.org/devmtg/2013-11/#talk3">LLVM Dev Meeting in
    2013</a> A more detailed description was published in a paper at
    <a href="http://dl.acm.org/citation.cfm?id=2597811">LCTES 2014</a>. The code
    was released to the community at the time. Meanwhile, the pass has been in
    production use at QuIC for the past few years and has been actively
    maintained internally. In order to magnify the impact of
    MergeSimilarFunctions, it has been ported to ThinLTO and the patches have
    been upstreamed (see stack of 5 patches mentioned below). But instead of
    replacing the existing MergeFunctions pass in LLVM-upstream the community
    suggested we improve the existing one with the ideas from
    MergeSimilarFunctions.  And then leverage the ThinLTO on top of that. The
    MergeSimilarFunction used in ThinLTO gives impressive code size reduction
    across a wide range of workloads and the work was presented at
    <a href="https://llvm.org/devmtg/2018-10/talk-abstracts.html#talk2">LLVM-dev
    2018</a>. The LLVM project would greatly benefit from this code size
    optimization as most embedded systems (think SmartPhones) applications are
    constrained on code-size.
  </p>
  <p><b>Preparation resources:</b>
  <ul>
    <li>
      Stack of patches:
      <ul>
        <li>
          <a href="https://reviews.llvm.org/D52896">MergeSimilarFunctions 1/n: a code size pass to merge functions with small differences</a>
        </li>
        <li>
          <a href="https://reviews.llvm.org/D52898">[Porting MergeSimilarFunctions 2/n] Changes to DataLayout</a>
        </li>
        <li>
          <a href="https://reviews.llvm.org/D52966">[Merge SImilar Function ThinLTO 3/n] Add hash code to function summary</a>
        </li>
        <li>
          <a href="https://reviews.llvm.org/D53253">[Merge SImilar Function ThinLTO 4/n] Make merge function decisions before the thin-lto stage</a>
        </li>
        <li>
          <a href="https://reviews.llvm.org/D53254">[Merge SImilar Function ThinLTO 5/n] Set up similar function to be imported</a>
        </li>
      </ul>
      The paches can be easily applied to LLVM-trunk and would give a developer a decent head start ;).
    </li>
    <li>List of llvm-dev mailing list posts on previous discussions around Merge Functions
      <ul>
        <li><a href="http://lists.llvm.org/pipermail/llvm-dev/2019-January/129835.html">Link1</li>
        <li><a href="http://lists.llvm.org/pipermail/llvm-dev/2019-March/131066.html">Link2</li>
        <li><a href="http://lists.llvm.org/pipermail/llvm-dev/2019-February/129863.html">Link3</li>
        <li><a href="http://lists.llvm.org/pipermail/llvm-dev/2019-January/129832.html">Link4</li>
      </ul>
    </li>
    <li>
      <a href="http://dl.acm.org/citation.cfm?id=2597811">The original paper: LCTES 2014</a>
    </li>
    <li>
      <a href="https://llvm.org/devmtg/2018-10/talk-abstracts.html#talk2">Video and slides of the presentation</a>
    </li>
  </ul>
  </p>
  <p><b>Expected results:</b>
    <ul>
      <li>
    Improve MergeFunctions to have feature parity with MergeSimilarFunctions.
      </li>
      <li>
    Enable MergeFunctions to ThinLTO.
      </li>
    </ul>
  </p>

  <p><b>Confirmed Mentors:</b>Aditya Kumar (hiraditya on IRC and phabricator), JF Bastien (jfb on phabricator)</p>

  <p><b>Desirable skills:</b>
    Course on compiler design, SSA Representation,
    Intermediate knowledge of C++, Familiarity with LLVM Core.
  </p>
</div>

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<div class="www_subsubsection">
  <a name="llvm_dwarf_yaml2obj">Add DWARF support to yaml2obj</a>
</div>
<!-- *********************************************************************** -->

<div class="www_text">
  <p><b>Description of the project:</b>
    LLVM provides a tool called yaml2obj which coverts a YAML document into an
    object file, for various different file formats such as ELF, COFF and
    Mach-O, along with obj2yaml which does the inverse. The tool is commonly
    used to test parts of LLVM, as YAML is often easier to use to describe an
    object file than raw assembly and more maintainable than a pre-built binary.
    DWARF is a debugging file format commonly used by LLVM. Many of the tests
    for LLVM’s DWARF emission are written in assembly, but it would be nicer to
    write them in YAML. However, yaml2obj does not properly support emission of
    DWARF sections. This project is to add functionality to yaml2obj to make
    writing test inputs for DWARF tests simpler, particularly for ELF objects.
  </p>

  <p><b>Preparation resources:</b>
    Reading up on the DWARF file format will be useful, in particular the
    standards available at http://dwarfstd.org/Download.php. Also, familiarising
    yourself with the basics of the ELF file format, as described here
    https://www.sco.com/developers/gabi/latest/contents.html, may be beneficial.
  </p>
  <p><b>Expected results:</b>
    The ability to use yaml2obj to generate DWARF sections for object files.
    Particularly important is ensuring the input YAML can be more easily
    understood than the equivalent assembly.
  </p>

  <p><b>Confirmed Mentors:</b> James Henderson</p>

  <p><b>Desirable skills:</b>
    Intermediate knowledge of C++.
  </p>
</div>

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<div class="www_subsubsection">
  <a name="llvm_hotcold">Improve hot cold splitting to aggressively outline small blocks</a>
</div>
<!-- *********************************************************************** -->

<div class="www_text">
 <p><b>Description of the project:</b> Hot Cold Splitting in LLVM is an IR level
   function splitting transformation. The goal of hot/cold splitting is to improve
   the memory locality of code and helps reduce startup working set. The splitting pass
   does this by identifying cold blocks and moving them into separate functions. Because it
   is implemented at the IR level all the back end target benefit from it.

   It is a relatively new optimization and it was recently presented at
   the <a href="https://llvm.org/devmtg/2019-10/talk-abstracts.html#tech8">LLVM Dev Meeting in
   2019</a> and the slides are <a href="https://llvm.org/devmtg/2019-10/slides/Kumar-HotColdSplitting.pdf">here</a>
   Because most of the benefit comes from outlining small blocks e.g., __assert_rtn. The goal of this project
   is to identify potential blocks via static analysis e.g., exception handling code, optimizing personality functions.

   Use cost-model to ensure outlining reduces the code size of the caller, use tail call whenever appropriate to save
   instructions.

 </p>
 <p><b>Preparation resources:</b>
 <ul>
   <li>
     <a href="http://lists.llvm.org/pipermail/llvm-dev/2019-January/129606.html">Update on hot cold splitting</a>
   </li>
   <li>
     The following two papers provide earlier work on hot cold splitting. While these papers are a good start, LLVM's
     HCS has completely different implementation in two aspects a) It is implemented at IR level and outlines basic
     blocks as function rather than naked branches. b) It is based on regions and outlines a set of basic blocks.
     <ul>
       <li>
         <a href="http://pages.cs.wisc.edu/~fischer/cs701.f05/code.positioning.pdf">Original paper on hot cold splitting by
           Pettis and Hansen.</a>Section 5 on procedure splitting is interesting one. It has nice examples ;) to help
         understand why HCS works.
       </li>
       <li>
         <a href="https://www.cs.cmu.edu/afs/cs/academic/class/15745-s07/www/papers/p80-cohn.pdf">Paper on hot cold
           splitting</a> The paper provides some details on one approach to split functions. This is helpful to get a
         different perspective and may help get new ideas.
       </li>
     </ul>
   </li>
   <li>
     <a href="https://llvm.org/devmtg/2019-10/talk-abstracts.html#tech8">Video and slides of the presentation</a>
   </li>
 </ul>
 </p>
 <p><b>Expected results:</b>
   <ul>
     <li>
       Improve Hot Cold Splitting to detect and outline cold blocks from program via static analysis or profile
       information. Use appropriate cost model to weigh benefit of HCS.
       In case compile time overhead becomes quadratic, come up with a cost model to detect when quadratic behavior
       gets triggered and bail out based on a compiler flag.
     </li>
   </ul>
 </p>

 <p><b>Confirmed Mentors:</b>Aditya Kumar (hiraditya on IRC and phabricator)</p>

 <p><b>Desirable skills:</b>
   Course on compiler design, SSA Representation,
   Intermediate knowledge of C++, Familiarity with LLVM Core.
 </p>
</div>

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<div class="www_subsubsection">
  <a name="llvm_pass_order">Advanced Heuristics for Ordering Compiler Optimization Passes</a>
</div>
<!-- *********************************************************************** -->

<div class="www_text">
 <p><b>Description of the project:</b>
Selecting optimization passes for given application is very important but
non-trivial problem because of the huge size of the compiler transformation
space (incl. pass ordering). While the existing heuristics can provide high
performance code for certain applications, they cannot easily benefit a wide
range of application codes. The goal of the project is to learn the interplay
between LLVM transformation passes and code structures, then improve the
existing heuristics (or replace the heuristics with machine learning-based
models) so that the LLVM compiler can provide a superior order of the passes
customized per application.
 </p>
 <p><b>Expected results (possibilities):</b>
 <ul>
   <li>
Insights about (implicit) dependences between existing passes.
   </li>
   <li>
New pass pipelines (think -O3a, -O3b, ...) selectable by the user that tend to perform substantially better on certain kinds of programs.
   </li>
   <li>
An improved LLVM pass heuristic or new machine learning-based models that can select
the best order for LLVM transformation passes based on code structures.
   </li>
  </ul>
 </p>

 <p><b>Preparation resources:</b>
 <ul>
   <li>
HERCULES: Strong Patterns towards More Intelligent Predictive Modeling, Eunjung Park; Christos Kartsaklis; John Cavazos, IEEE ICPP’14
https://ieeexplore.ieee.org/abstract/document/6957226
   </li>

   <li>
Predictive Modeling in a Polyhedral Optimization Space, Eunjung Park, John Cavazos, Louis-Noël Pouchet, Cédric Bastoul, Albert Cohen & P. Sadayappan, IJPP’13
https://link.springer.com/article/10.1007/s10766-013-0241-1
   </li>

   <li>
Machine Learning in Compiler Optimization, Zheng Wang and Michael O’Boyle, IEEE Magazine 2018.
https://ieeexplore.ieee.org/document/8357388
   </li>
 </ul>
 </p>

 <p><b>Confirmed Mentors:</b>EJ Park, Giorgis Georgakoudis, Johannes Doerfert</p>

 <p><b>Desirable skills:</b>
    C++, Python, experience with LLVM and learning-based prediction preferable.
 </p>
</div>

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<div class="www_subsubsection">
  <a name="llvm_ml_scc">Machine learning and compiler optimizations: using inter-procedural analysis to select optimizations</a>
</div>
<!-- *********************************************************************** -->

<div class="www_text">
 <p><b>Description of the project:</b>
Current machine learning models for compiler optimization select the best
optimization strategies for functions based on isolated per function analysis.
In this approach, the constructed models are not aware of any relationships
with other functions around it (callers or callees) which can be helpful to
decide the best optimization strategies for each function. In this project, we
want to explore the SCC (Strongly Connected Components) call graph to add
inter-procedural features in constructing machine learning-based models to find
the best optimization strategies per function. Moreover, we want to explore the
case that it is helpful to group strongly related functions together and
optimize them as a group, instead of per function.
 </p>
 <p><b>Expected results (possibilities):</b>
 <ul>
   <li>
Improved heuristics for existing (inter-procedural) passes, e.g. to weight inlining versus function cloning based on code features.
   </li>
   <li>
Machine learning models to select the best optimizations using code features
and inter-procedural analysis. This model can be used for functions in
isolation or groups of functions, e.g., CGSCCs.
   </li>
 </ul>
 </p>

 <p><b>Preparation resources:</b>
 <ul>
   <li>
HERCULES: Strong Patterns towards More Intelligent Predictive Modeling, Eunjung Park; Christos Kartsaklis; John Cavazos, IEEE ICPP’14
https://ieeexplore.ieee.org/abstract/document/6957226
   </li>

   <li>
Predictive Modeling in a Polyhedral Optimization Space, Eunjung Park, John Cavazos, Louis-Noël Pouchet, Cédric Bastoul, Albert Cohen & P. Sadayappan, IJPP’13
https://link.springer.com/article/10.1007/s10766-013-0241-1
   </li>

   <li>
Machine Learning in Compiler Optimization, Zheng Wang and Michael O’Boyle, IEEE Magazine 2018.
https://ieeexplore.ieee.org/document/8357388
   </li>
 </ul>
 </p>

 <p><b>Confirmed Mentors:</b>EJ Park, Giorgis Georgakoudis, Johannes Doerfert</p>

 <p><b>Desirable skills:</b>
    C++, Python, experience with LLVM and learning-based prediction preferable.
 </p>
</div>

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<div class="www_subsubsection">
  <a name="llvm_postdominators"></a>
</div>
<!-- *********************************************************************** -->

<div class="www_text">
  <p><b>Description of the project:</b>
    There is currently no easy way to use the result of
    PostDominatorTreeAnalysis in a loop pass, as PostDominatorTreeAnalysis is a
    function analysis, and it is not included in LoopStandardAnalysisResults. If one adds
    PostDominatorTreeAnalysis in LoopStandardAnalysisResults, then all loop passes
    need to preserve it, meaning that all loop passes need to make sure the result is up to
    date. In this project, we want to modify some commonly used utilities to generate a
    list of updates, which can be consume by different updaters, e.g. DomTreeUpdater to
    update DominatorTree and PostDominatorTree, and MSSAU to update MemorySSA,
    etc, instead of only updating the DominatorTree. In additional, we want to change
    existing loop passes to preserve the PostDominatorTree. Finally, adding
    PostDominatorTree in LoopStandardAnalysisResults.
  </p>
  <p><b>Expected results (possibilities):</b>
    PostDominatorTree added in LoopStandardAnalysisResults, and
    can be used by loop passes. More common utilities change to generate list of updates
    to be easily obtained by different updaters.
  </p>
  <p><b>Confirmed Mentors:</b>
    Whitney Tsang, Ettore Tiotto, Bardia Mahjour
  </p>
  <p><b>Desirable skills:</b>
    Intermediate knowledge of C++, self-motivation.
  </p>
  <p><b>Preparation resources:</b>
    <a href="https://reviews.llvm.org/rL336163"></a>
    <a href="http://llvm.org/doxygen/classllvm_1_1DomTreeUpdater.html"></a>
    <a href="https://llvm.org/doxygen/classllvm_1_1PostDominatorTreeAnalysis.html"></a>
    <a href="http://llvm.org/doxygen/structllvm_1_1LoopStandardAnalysisResults.html"></a>
</div>

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<div class="www_subsubsection">
  <a name="llvm_loopnest">Create LoopNest Pass</a>
</div>
<!-- *********************************************************************** -->

<div class="www_text">
  <p><b>Description of the project:</b>
    Currently if you want to write a pass that works on a loop
    nest, you have to pick from either a function pass or a loop pass. If you chose to write
    it as a function pass, then you lose the ability to add loops dynamically back to the
    pipeline. If you decide to write it as a loop pass, then you are wasting compile time to
    traverse to your pass and return right away when the given loop is not the outermost
    loop. In this project, we want to create a LoopNestPass, where transformations
    intended for loop nest can inherit from it, and have the same ability as the LoopPass to
    dynamically add loops to the pipeline. In addition, create all the adaptors requires to
    add loop nest passes at different points of the pass builder.
  </p>
  <p><b>Expected results (possibilities):</b>
    Transformations/Analyses can be written as LoopNestPass,
    without compromising compile time or usability.
  </p>
  <p><b>Confirmed Mentors:</b>
    Whitney Tsang, Ettore Tiotto
  </p>
  <p><b>Desirable skills:</b>
    Intermediate knowledge of C++, self-motivation.
  </p>
  <p><b>Preparation resources:</b>
    <a href="https://reviews.llvm.org/D68789"</a>
    <a href="https://llvm.org/doxygen/classllvm_1_1PassBuilder.html"</a>
  </p>
</div>

<!-- *********************************************************************** -->

<div class="www_subsubsection">
  <a name="llvm_instdump">Instruction properties dumper and checker</a>
</div>
<!-- *********************************************************************** -->

<div class="www_text">
  <p><b>Description of the project:</b>
    TableGen is flexible and allow the end-user to define and set common properties of
    records (instructions). Every target has dozens or hundreds of such instruction
    properties. As target code evolve, the td files become more and more complicated,
    it become harder to see whether the setting of some properties is necessary, even
    correct or not. eg: whether hasSideEffects property is correctly set on all
    instructions?

    One can manually search through the TableGen-generated files; or write some
    script to run TableGen and matching the output for some specific properties, but a
    standalone utility that can dump and check instruction properties
    systematically (eg: also allow target to define some verification rules) might be
    better from a build-process-management standpoint. This can help to find quite
    some hidden bugs and hence improve the overall codegen code quality. In
    addition, the utility can be used to write regression tests for instruction
    properties, which will increase the quality and precision of LLVM's
    regression tests.
  </p>
  <p><b>Expected results (possibilities):</b>
    A standalone llvm tool or utility that can dump and check instruction properties systematically
  </p>
  <p><b>Confirmed Mentors:</b>
    Hal Finkel, Jinsong Ji , Qingshan Zhang
  </p>
  <p><b>Desirable skills:</b>
    Intermediate knowledge of C++, self-motivation.
  </p>
</div>

<!-- *********************************************************************** -->

<div class="www_subsubsection">
  <a name="llvm_movecode">Unify ways to move code or check if code is safe to be moved</a>
</div>
<!-- *********************************************************************** -->

<div class="www_text">
  <p><b>Description of the project:</b>
    Determining whether it is safe to move code around is
    implemented in several transformations in LLVM (e.g. canSinkOrHoistInst in LICM,
    or makeLoopInvariant in Loop). Each of these implementations may return different
    results for a given query, making code motion safety checks inconsistent and
    duplicated. On the other hand, the mechanism for doing the actual code motion is also
    different in each transformation. Code duplication causes maintenance problems and
    increases the time taken to write new transformation. In this project, we want to first
    identify all the existing ways in loop transformations (could be function or loop pass)
    to check if code is safe to move, and to move code, and create a standardize way to do
    so.
  </p>
  <p><b>Expected results (possibilities):</b>
    A standardize/superset of all the existing ways in loop
    transformations of checking if code is safe to be moved and to move <code class=""></code>
  </p>

  <p><b>Confirmed Mentors:</b>
    Whitney Tsang, Ettore Tiotto, Bardia Mahjour
  </p>
  <p><b>Desirable skills:</b>
    Intermediate knowledge of C++, self-motivation.
  </p>
  <p><b>Preparation resources:</b>
    <a href="https://github.com/llvm/llvm-project/blob/master/llvm/include/llvm/Transforms/Utils/CodeMoverUtils.h"></a>
    <a href="https://llvm.org/doxygen/LICM_8cpp_source.html"></a>
    <a href="https://llvm.org/doxygen/classllvm_1_1Loop.html"></a>
  </p>
</div>

<!-- *********************************************************************** -->
<div class="www_subsection">
  <a>MLIR</a>
</div>
<!-- *********************************************************************** -->

<div class="www_text">
<p>All the items in the list of
<a href="https://mlir.llvm.org/getting_started/openprojects/">open projects</a>
are opened to GSOC. Feel free to propose your own ideas as well on
<a href="https://llvm.discourse.group/c/llvm-project/mlir">Discourse</a>.
</p></div>

<!-- *********************************************************************** -->
<div class="www_subsubsection">
  <a name="clang-sa-cplusplus-checkers">Find null smart pointer dereferences
                                        with the Static Analyzer</a>
</div>
<!-- *********************************************************************** -->

<div class="www_text">
  <p><b>Description of the project: </b>
    The Clang Static Analyzer already knows how to prevent crashes caused by
    null pointer dereference in arbitrary code, however it often "gives up"
    when the code is too complicated. In particular, implementation details
    of C++ standard classes, even simple ones such as smart pointers
    or optionals, may be too convoluted for the Analyzer to fully understand.
    Moreover, the exact behavior depends on which implementation of
    the Standard Library is used (e.g., GNU libstdc++ or LLVM's own libc++).
  </p>
  <p>
    We can enable the Analyzer to find more bugs in modern C++ code
    by teaching it explicitly about the behavior of C++ standard classes,
    and therefore skipping the whole process in which the Analyzer
    tries to understand all the implementation details on its own.
    For example, we could teach it that a default-constructed smart pointer
    is null, and any attempt to dereference it would result in a crash.
    The project would therefore consist in manually providing implementations
    for various methods of standard classes.
  </p>

  <p><b>Expected results: </b>
    We want the Static Analyzer to emit warnings when a null smart pointer
    dereference would occur in the code. For example:
    <pre>
    #include &lt;memory&gt;

    int foo(bool flag) {
      std::unique_ptr&lt;int&gt; x;  <i>// note: Default constructor produces a null unique pointer;</i>

      if (flag)                <i>// note: Assuming 'flag' is false;</i>
        return 0;              <i>// note: Taking false branch</i>

      return *x;               <i>// warning: Dereferenced smart pointer 'x' is null.</i>
    }
    </pre>
    We should be able to cover at least one class fully, for example, <tt>std::unique_ptr</tt>,
    and then see if we can generalize our results to other classes, such as <tt>std::shared_ptr</tt>
    or the C++17 <tt>std::optional</tt>.
  </p>


  <p><b>Confirmed Mentor:</b> Artem Dergachev, Gábor Horváth</p>

  <p><b>Desirable skills:</b>
    Intermediate knowledge of C++.
  </p>
</div>


<!-- *********************************************************************** -->
<div class="www_subsection">
  <a>LLDB</a>
</div>
<!-- *********************************************************************** -->

<!-- *********************************************************************** -->
<div class="www_subsubsection">
  <a name="lldb-autosuggestions">Support autosuggestions in LLDB's command line</a>
</div>
<!-- *********************************************************************** -->

<div class="www_text">
  <p><b>Description of the project: </b> LLDB's command line offers several convenience
    features that are inspired by features of UNIX shells such as tab completions or a command history.
    One feature that is not implemented yet are 'autosuggestions'. These are suggestions
    for possible commands that the user might want to type, but unlike tab completions they
    are displayed directly behind the cursor while the user is typing a command. A good demonstration
    how this could look like are the autosuggestions implemented in <a href="https://fishshell.com">fish shell</a>.
  </p>
  <p>
    This project is about implementing autosuggestions in LLDB's editline-based command shell.
  </p>
  <p><b>Confirmed Mentor:</b>
    <a href="mailto:teemperor@gmail.com,jonas@devlieghere.com?subject=[GSoC]%20Autosuggestions">Jonas Devlieghere and Raphael Isemann</a></p>
  <p><b>Desirable skills:</b>
    Intermediate knowledge of C++.
  </p>
</div>

<!-- *********************************************************************** -->
<div class="www_subsubsection">
  <a name="lldb-more-completions">Implement the missing tab completions for LLDB's command line</a>
</div>
<!-- *********************************************************************** -->

<div class="www_text">
  <p><b>Description of the project: </b> LLDB's command line offers several convenience
    features that are inspired by features of UNIX shells such as tab completions for commands.
    These tab completions are implemented by a completion engine that is not only used by the
    command line interface of LLDB, but also by graphical interfaces for LLDB such as IDEs.

    While the tab completions in LLDB are really useful, they are currently not implemented for
    all commands and their respective arguments. This project is about implementing the remaining
    completions for the commands in LLDB which will greatly improve the user experience of LLDB.
    Improving existing completions is also part of the project.

    Note that the completions are not static list of strings but often require inspecting and
    understanding the internal state of LLDB. As LLDB commands and their tab completions cover
    all aspects of LLDB, this project offers a great way to get an overview of all the functionality
    in LLDB.
  </p>
  <p><b>Confirmed Mentor:</b><a href="mailto:teemperor@gmail.com?subject=[GSoC]%20Completions">Raphael Isemann</a></p>

  <p><b>Desirable skills:</b>
    Intermediate knowledge of C++.
  </p>
</div>


<!-- *********************************************************************** -->
<div class="www_subsubsection">
  <a name="lldb-reimplement-lldb-cmdline">Reimplement LLDB's command-line commands
  using the public SB API.</a>
</div>
<!-- *********************************************************************** -->

<div class="www_text">
  <p><b>Description of the project: </b> Just as LLVM is a library to
    build compilers, LLDB is a library to build debuggers. LLDB vends
    a stable, public SB API. Due to historic reasons the LLDB command
    line interface is currently implemented on top of LLDB's private
    API and it duplicates a lot of functionality that is already
    implemented in the public API. Rewriting LLDB's command line
    interface on top of the public API would simplify the
    implementation, eliminate duplicate code, and most importantly
    reduce the testing surface.
  </p>
  <p>
    This work will also provide an opportunity to clean up the SB API
    of commands that have accrued too many overloads over time and
    convert them to make use of option classes to both gather up all
    the variants and also future-proof the APIs.
  </p>
  <p><b>Confirmed Mentor:</b>Adrian Prantl and Jim Ingham</p>

  <p><b>Desirable skills:</b>
    Intermediate knowledge of C++.
  </p>
</div>

<!-- *********************************************************************** -->
<div class="www_subsubsection">
  <a name="lldb-batch-testing">Add support for batch-testing to the LLDB
    testsuite.</a>
</div>
<!-- *********************************************************************** -->

<div class="www_text">
  <p><b>Description of the project: </b>One of the tensions in the
    testsuite is that spinning up a process and getting it to some
    point is not a cheap operation, so you'd like to do a bunch of
    tests when you get there.  But the current testsuite bails at the
    first failure, so you don't want to do many tests since the
    failure of one fails all the others. On the other hand, there are
    some individual test assertions where the failure of the assertion
    <em>should</em> cause the whole test to fail.  For example, if you
    fail to stop at a breakpoint where you want to check some variable
    values, then the whole test should fail.  But if your test then
    wants to check the value of five independent locals, it should be
    able to do all five, and then report how many of the five variable
    assertions failed. We could do this by adding <em>Start</em>
    and <em>End</em> markers for a batch of tests, do all the tests in
    the batch without failing the whole test, and then report the
    error and fail the whole test if appropriate. There might also be
    a nice way to do this in Python using scoped objects for the test
    sections.
  </p>
  <p><b>Confirmed Mentor:</b> Jim Ingham</p>

  <p><b>Desirable skills:</b>
    Intermediate knowledge of Python.
  </p>
</div>

<!-- *********************************************************************** -->
<div class="www_sectiontitle">
  <a name="gsoc19">Google Summer of Code 2019</a>
</div>
<!-- *********************************************************************** -->

<div class="www_text">
  <p>Google Summer of Code 2019 contributed a lot to the LLVM project. For the list of
    accepted and completed projects, please take a look into Google Summer of Code
    <a href="https://summerofcode.withgoogle.com/archive/2019/organizations/5682474363912192/">website.
    </a></p>
</div>

<!-- *********************************************************************** -->
<div class="www_subsection">
  <a>LLVM</a>
</div>
<!-- *********************************************************************** -->

<!-- *********************************************************************** -->
<div class="www_subsubsection">
  <a name="debuginfo_codegen_mismatch">Debug Info should have no
          effect on codegen</a>
</div>
<!-- *********************************************************************** -->

<div class="www_text">
  <p><b>Description of the project:</b>
      Adding Debug Info (compiling with `clang -g`) shouldn't change the
      generated code at all. Unfortunately we have bugs. These are usually not
      too hard to fix and a good way to discover new part of the codebase!
      We suggest building object files both ways and disassembling the
          text sections, which will give cleaner diffs than comparing .s files.
  </p>

  <p><b>Expected results:</b> Reduced test cases, bug reports with analysis
          (e.g., which pass is responsible), possibly patches.</p>

  <p><b>Confirmed Mentor:</b> Paul Robinson</p>
  <p><b>Desirable skills:</b> Intermediate knowledge of C++, some familiarity
        with x86 or ARM instruction set.</p>
</div>


<!-- *********************************************************************** -->
<div class="www_subsection">
  <a>Clang</a>
</div>
<!-- *********************************************************************** -->

<!-- *********************************************************************** -->
<div class="www_subsubsection">
  <a name="clang-astimporter-fuzzer">Implement an ASTImporter fuzzer</a>
</div>
<!-- *********************************************************************** -->

<div class="www_text">
  <p><b>Description of the project: </b>
    Clang contains an ASTImporter which allows moving declarations and
    statements from one Clang AST to another. This is for example used for
    static analysis across translation units and in LLDB's expression
    evaluator.
  </p>
  <p>
    The current ASTImporter works as intended when moving simple C code from
    one AST to another. However, more complicated declarations such as C++'s
    OOP features and templates are not fully implemented and can cause crashes
    or invalid AST nodes. The bug reports related to these crashes are often
    filed against LLDB's expression evaluator and are rarely submited with a
    minimal reproducer. This makes improving ASTImporter a time-consuming and
    tedious task.
  </p>
  <p>
    This project is about writing a fuzzer to proactively discover these
    ASTImporter bugs and provide minimal reproducers which make understanding
    and fixing the underlying bug easier.
  </p>
  <p>
    A possible implementation of such a fuzzer and driver could look like this:

  <ul>
    <li>Generate some source code that can be imported (either fully randomly
        or based on existing source code from a user-given code corpus).</li>
    <li>Import randomly a few declarations from this AST. The AST in which
        they are imported to can already be populated with declarations.</li>
    <li>Run Clang's code generator over our imported AST.</li>
    <li>If we hit an assert during the import or CodeGen steps we probably
        found an ASTImporter bug.</li>
    <li>The fuzzer driver should now reduce the size of the source code
        until it is as small as possible and still reproduces the crash (e.g.
        by running Creduce with an automatically generated test script).</li>
    <li>The reproducer should now be stored in a format so that it can just be
        copied into Clang's regression test suite for the ASTImporter (see
        the <a href="https://github.com/llvm/llvm-project/tree/master/clang/test/Import">clang/test/Import/</a> directory).
        The reproducer must still reproduce the found bug when run as part
        of the test suite.
        </li>
  </ul>
  This is just one possible approach and students are welcome to submit their
  own ideas on how the fuzzer should operate. Approaches that allow to
  automatically verify more aspects of the imported AST (e.g. the source
  locations of AST nodes, size of RecordDecls) are encouraged. The fuzzer and
  driver should be implemented in C++ and/or Python.
  </p>
  <p><b>Confirmed Mentor:</b> Raphael Isemann, Shafik Yaghmour</p>
  <p><b>Desirable skills:</b> Intermediate knowledge of C++.</p>
</div>

<!-- *********************************************************************** -->
<div class="www_subsubsection">
  <a name="improve-autocompletion">Improve shell autocompletion for Clang</a>
</div>
<!-- *********************************************************************** -->

<div class="www_text">
  <p><b>Description of the project: </b> Clang has a newly implemented autocompletion feature which details can be found at <a href="http://blog.llvm.org/2017/09/clang-bash-better-auto-completion-is.html">LLVM blog</a>. We would like to improve this by adding more flags to autocompletion, supporting more shells (currently it supports only bash) and exporting this feature to other projects such as llvm-opt. Accepted student will be working on Clang Driver, LLVM Options and shell scripts.
  </p>

  <p><b>Expected Results:</b> Autocompletion working on bash and zsh, support llvm-opt options.</p>

  <p><b>Confirmed Mentor:</b> Yuka Takahashi and Vassil Vassilev</p>

  <p><b>Desirable skills:</b>
  Intermediate knowledge of C++ and shell scripting
  </p>
</div>

<!-- *********************************************************************** -->
<div class="www_sectiontitle">
  <a name="gsoc18">Google Summer of Code 2018</a>
</div>
<!-- *********************************************************************** -->

<div class="www_text">
  <p>Google Summer of Code 2018 contributed a lot to the LLVM project. For the list of
  accepted and completed projects, please take a look into Google Summer of Code
  <a href="https://summerofcode.withgoogle.com/archive/2018/organizations/5263452624912384/">website.
  </a></p>
</div>

<!-- *********************************************************************** -->
<div class="www_sectiontitle">
  <a name="gsoc17">Google Summer of Code 2017</a>
</div>
<!-- *********************************************************************** -->

<div class="www_text">
  <p>Google Summer of Code 2017 contributed a lot to the LLVM project. For the list of
  accepted and completed projects, please take a look into Google Summer of Code
  <a href="https://summerofcode.withgoogle.com/archive/2017/organizations/6215410651234304/">website.
  </a></p>
</div>

<!-- *********************************************************************** -->
<div class="www_sectiontitle">
  <a name="what">What is this?</a>
</div>
<!-- *********************************************************************** -->

<div class="www_text">

<p>This document is meant to be a sort of "big TODO list" for LLVM.  Each
project in this document is something that would be useful for LLVM to have, and
would also be a great way to get familiar with the system.  Some of these
projects are small and self-contained, which may be implemented in a couple of
days, others are larger.  Several of these projects may lead to interesting
research projects in their own right.  In any case, we welcome all
contributions.</p>

<p>If you are thinking about tackling one of these projects, please send a mail
to the <a href="http://lists.llvm.org/mailman/listinfo/llvm-dev">LLVM
Developer's</a> mailing list, so that we know the project is being worked on.
Additionally this is a good way to get more information about a specific project
or to suggest other projects to add to this page.
</p>

<p>The projects in this page are open-ended. More specific projects are
filed as unassigned enhancements in the <a href="https://github.com/llvm/llvm-project/issues/">
LLVM bug tracker</a>. See the
<a href="https://github.com/llvm/llvm-project/issues?q=is%3Aopen+is%3Aissue+no%3Aassignee">list of currently outstanding issues</a>
if you wish to help improve LLVM.</p>

</div>

<!-- *********************************************************************** -->
<div class="www_sectiontitle">
  <a name="subprojects">LLVM Subprojects: Clang and More</a>
</div>
<!-- *********************************************************************** -->

<div class="www_text">

<p>In addition to hacking on the main LLVM project, LLVM has several subprojects,
   including Clang and others.  If you are interested in working on these, please
   see their "Open projects" page:</p>

<ul>
<li>The <a href="http://clang.llvm.org/OpenProjects.html">Clang Open
    Projects</a> list.</li>
<li>The <a href="http://polly.llvm.org/projects.html">Polly Open
    Projects</a> list.</li>
<li>The <a href="http://sva.cs.illinois.edu/projects.html">SAFECode Open
    Projects</a> list.</li>
</ul>

</div>

<!-- *********************************************************************** -->
<div class="www_sectiontitle">
  <a name="improving">Improving the current system</a>
</div>
<!-- *********************************************************************** -->

<div class="www_text">

<p>Improvements to the current infrastructure are always very welcome and tend
to be fairly straight-forward to implement.  Here are some of the key areas that
can use improvement...</p>

</div>

<!-- ======================================================================= -->
<div class="www_subsubsection">
  <a name="target-desc">Factor out target descriptions</a>
</div>

<div class="www_text">

<p>Currently, both Clang and LLVM have a separate target description infrastructure,
with some features duplicated, others "shared" (in the sense that Clang has to create
a full LLVM target description to query specific information).</p>

<p>This separation has grown in parallel, since in the beginning they were quite
different and served disparate purposes. But as the compiler evolved, more and
more features had to be shared between the two so that the compiler would behave
properly. An example is when targets have default features on speficic configurations
that don't have flags for. If the back-end has a different "default" behaviour
than the front-end and the latter has no way of enforcing behaviour, it
won't work.</p>

<p>An alternative would be to create flags for all little quirks, but first, Clang
is not the only front-end or tool that uses LLVM's middle/back ends, and second,
that's what "default behaviour" is there for, so we'd be missing the point.</p>

<p>Several ideas have been floating around to fix the Clang driver WRT recognizing
architectures, features and so on (table-gen it, user-specific configuration files,
etc) but none of them touch the critical issue: sharing that information with the
back-end.</p>

<p>Recently, the idea to factor out the target description infrastructure from
both Clang and LLVM into its own library that both use, has been floating around.
This would make sure that all defaults, flags and behaviour are shared, but would
also reduce the complexity (and thus the cost of maintenance) a lot. That would
also allow all tools (lli, llc, lld, lldb, etc) to have the same behaviour
across the board.</p>

<p>The main challenges are:</p>

<ul>
  <li>To make sure the transition doesn't destroy the delicate balance on any
  target, as some defaults are implicit and, some times, unknown.</li>
  <li>To be able to migrate one target at a time, one tool at a time and still
  keep the old infrastructure intact.</li>
  <li>To make it easy for detecting target's features for both front-end and
  back-end features, and to merge both into a coherent set of properties.</li>
  <li>To provide a bridge to the new system for tools that haven't migrated,
  especially the off-the-tree ones, that will need some time (one release,
  at least) to migrate..</li>
</ul>

</div>

<!-- ======================================================================= -->
<div class="www_subsubsection">
  <a name="code-cleanups">Implementing Code Cleanup bugs</a>
</div>

<div class="www_text">

<p>
The <a href="https://github.com/llvm/llvm-project/issues/">LLVM bug tracker</a> occasionally
has <a href="https://github.com/llvm/llvm-project/labels/code-cleanup">"code-cleanup" bugs</a> filed in it.
Taking one of these and fixing it is a good way to get your feet wet in the
LLVM code and discover how some of its components work.  Some of these include
some major IR redesign work, which is high-impact because it can simplify a lot
of things in the optimizer.
</p>

<p>
Some specific ones that would be great to have:

<ul>
<li><a href="/PR10367">Fix the design of GlobalAlias to not require dest type to match source type</a></li>
<li><a href="/PR10368">Redesign ConstantExpr's</a></li>
<li><a href="/PR11944">Static constructors should be purged from LLVM</a></li>
</ul>
</p>

<p>Additionally, there are performance improvements in LLVM that need to get
fixed. These are marked with the <tt>slow-compile</tt> keyword. Use
<a href="https://github.com/llvm/llvm-project/issues?q=is%3Aopen+is%3Aissue+label%3Aslow-compile">
this LLVM bug tracker query</a>
to find them.</p>

</div>

<!-- ======================================================================= -->
<div class="www_subsubsection">
  <a name="llvmtest">Add programs to the llvm-test testsuite</a>
</div>

<div class="www_text">

<p>
The <a href="docs/TestingGuide.html#wholeprograms">llvm-test</a> testsuite is
a large collection of programs we use for nightly testing of generated code
performance, compile times, correctness, etc.  Having a large testsuite gives
us a lot of coverage of programs and enables us to spot and improve any
problem areas in the compiler.</p>

<p>
One extremely useful task, which does not require in-depth knowledge of
compilers, would be to extend our testsuite to include <a href=
"http://nondot.org/sabre/LLVMNotes/#benchmarks">new programs and benchmarks</a>.
In particular, we are interested in cpu-intensive programs that have few
library dependencies, produce some output that can be used for correctness
testing, and that are redistributable in source form.  Many different programs
are suitable, for example, see <a
href="http://nondot.org/sabre/LLVMNotes/#benchmarks">this list</a> for some
potential candidates.
</p>

</div>

<!-- ======================================================================= -->
<div class="www_subsubsection">
  <a name="programs">Compile programs with the LLVM Compiler</a>
</div>

<div class="www_text">

<p>We are always looking for new testcases and benchmarks for use with LLVM.  In
particular, it is useful to try compiling your favorite C source code with LLVM.
If it doesn't compile, try to figure out why or report it to the <a
href="http://lists.llvm.org/pipermail/llvm-bugs/">llvm-bugs</a> list.  If you
get the program to compile, it would be extremely useful to convert the build
system to be compatible with the LLVM Programs testsuite so that we can check it
into SVN and the automated tester can use it to track progress of the
compiler.</p>

<p>When testing a code, try running it with a variety of optimizations, and with
all the back-ends: CBE, llc, and lli.</p>

</div>


<!-- ======================================================================= -->
<div class="www_subsubsection">
  <a name="benchmark">Benchmark the LLVM compiler</a>
</div>

<div class="www_text">

<p>Find benchmarks either using our <a
href="/nightlytest/">test results</a> or on your own,
where LLVM code generators do not produce optimal code or where another
compiler produces better code.  Try to minimize the test case that demonstrates
the issue.  Then, either <a href="https://github.com/llvm/llvm-project/issues/">submit a
bug</a> with your testcase and the code that LLVM produces vs. the code that it
<em>should</em> produce, or even better, see if you can improve the code
generator and submit a patch.  The basic idea is that it's generally quite easy
for us to fix performance problems if we know about them, but we generally don't
have the resources to go finding out why performance is bad.</p>

</div>


<!-- ======================================================================= -->
<div class="www_subsubsection">
  <a name="statistics">Benchmark Statistics and Warning System</a>
</div>

<div class="www_text">

<p>The <a href='http://llvm.org/perf/db_default/v4/nts/recent_activity'>
LNT perf database</a> has some nice features like detect moving average,
standard deviations, variations, etc. But the report page give too much emphasis
on the individual variation (where noise can be higher than signal), eg.
<a href='http://llvm.org/perf/db_default/v4/nts/graph?plot.0=10.341.3&highlight_run=8943'>
this case</a>.</p>

<p>The first part of the project would be to create an analysis tool that would
track moving averages and report:
<ul>
 <li>If the current result is higher/lower than the previous moving average by
     more than (configurable) S standard deviations</li>
 <li>If the current moving average is more than S standard deviations of the
     Base run</li>
 <li>If the last A moving averages are in constant increase/decrease of more
     than P percent</li>
</ul>

<p>The second part would be to create a web page which would show all related
benchmarks (possibly configurable, like a dashboard) and show the basic statistics
with red/yellow/green colour codes to show status and links to more detailed
analysis of each benchmark.</p>

<p>A possible third part would be to be able to automatically cross reference
different builds, so that if you group them by architecture/compiler/number
of CPUs, this automated tool would understand that the changes are more common
to one particular group.</p>

</div>

<!-- ======================================================================= -->
<div class="www_subsubsection">
  <a name="coverage">Improving Coverage Reports</a>
</div>

<div class="www_text">

<p>The <a href='http://llvm.org/reports/coverage/'>
LLVM Coverage Report</a> has a nice interface to show what source lines are
covered by the tests, but it doesn't mentions which tests, which revision and
what architecture is covered.</p>

<p>A project to renovate LCOV would involve:
<ul>
 <li>Making it run on a buildbot, so that we know what commits / architectures
     are covered</li>
 <li>Update the web page to show that information</li>
 <li>Develop a system that would report every buildbot build into the web page
     in a searchable database, like LNT</li>
</ul>

<p>Another idea is to enable the test suite to run all built backends, not only
   the host architecture, so that coverage report can be built in a fast machine
   and have one report per commit without needing to update the buildbots.</p>

</div>

<!-- ======================================================================= -->
<div class="www_subsubsection">
  <a name="misc_imp">Miscellaneous Improvements</a>
</div>

<div class="www_text">

<ol>

<li>Completely rewrite bugpoint.  In addition to being a mess, bugpoint suffers
from a number of problems where it will "lose" a bug when reducing.  It should
be rewritten from scratch to solve these and other problems.</li>
<li><a href="https://llvm.org/PR2116">Add support for
transactions to the PassManager</a> for improved bugpoint.</li>
<li><a href="https://llvm.org/PR539">Improve bugpoint to
support running tests in parallel on MP machines</a>.</li>
<li>Add MC assembler/disassembler and JIT support to the SPARC port.</li>
<li>Move more optimizations out of the <tt>-instcombine</tt> pass and into
InstructionSimplify.  The optimizations that should be moved are those that
do not create new instructions, for example turning <tt>sub i32 %x, 0</tt>
into <tt>%x</tt>.  Many passes use InstructionSimplify to clean up code as
they go, so making it smarter can result in improvements all over the place.</li>
</ol>

</div>

<!-- *********************************************************************** -->
<div class="www_sectiontitle">
  <a name="new">Adding new capabilities to LLVM</a>
</div>
<!-- *********************************************************************** -->

<div class="www_text">

<p>Sometimes creating new things is more fun than improving existing things.
These projects tend to be more involved and perhaps require more work, but can
also be very rewarding.</p>

</div>

<!-- ======================================================================= -->
<div class="www_subsubsection">
  <a name="llvm_ir">Extend the LLVM intermediate representation</a>
</div>

<div class="www_text">

<p>Many proposed <a href="http://nondot.org/sabre/LLVMNotes/">extensions and
improvements to LLVM core</a> are awaiting design and implementation.</p>

<ol>
<li><a href="http://nondot.org/sabre/LLVMNotes/DebugInfoImprovements.txt">Improvements
for Debug Information Generation</a></li>
<li><a href="/PR1269">EH support for non-call exceptions</a></li>
<li>Many ideas for feature requests are stored in LLVM bugzilla.  Search
<a href="https://github.com/llvm/llvm-project/issues?q=is%3Aissue+is%3Aopen+label%3Anew-feature">for bugs with a "new-feature" keyword</a>.</li>
</ol>

</div>

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<div class="www_subsubsection">
  <a name="pointeranalysis">Pointer and Alias Analysis</a>
</div>

<div class="www_text">

<p>We have a <a href="docs/AliasAnalysis.html">strong base for development</a> of
both pointer analysis based optimizations as well as pointer analyses
themselves.  We want to take advantage of this:</p>

<ol>
<li>The globals mod/ref pass does an inexpensive bottom-up context sensitive
  alias analysis.  There are some inexpensive things that we could do to better
  capture the effects of functions that access pointer arguments.  This can be
  really important for C++ methods, which spend lots of time accessing pointers
  off 'this'.</li>

<li>The alias analysis API supports the getModRefBehavior method, which allows
  the implementation to give details analysis of the functions. For example, we
  could implement <a href="/PR1604">full knowledge of
    printf/scanf</a> side effects, which would be useful.  This feature is in
  place but not being used for anything right now.</li>

<li>We need some way to reason about errno.  Consider a loop like this:

<pre>
    for ()
      x += sqrt(loopinvariant);
</pre>

<p>We'd like to transform this into:</p>

<pre>
    t = sqrt(loopinvariant);
    for ()
      x += t;
</pre>

<p>This transformation is safe, because the value of errno isn't
otherwise changed in the loop and the exit value of errno from the
loop is the same.  We currently can't do this, because sqrt clobbers
errno, so it isn't "readonly" or "readnone" and we don't have a good
way to model this.</p>

<p>The important part of this project is figuring out how to describe
errno in the optimizer: each libc #defines errno to something different
it seems.  Maybe the solution is to have a __builtin_errno_addr() or
something and change sys headers to use it.</p>

<li>There are lots of ways to optimize out and <a
href="/PR452">improve handling of
memcpy/memset</a>.</li>

</ol>

</div>

<!-- ======================================================================= -->
<div class="www_subsubsection">
  <a name="profileguided">Profile-Guided Optimization</a>
</div>

<div class="www_text">

<p>We now have a unified infrastructure for writing profile-guided
transformations, which will work either at offline-compile-time or in the JIT,
but we don't have many transformations.  We would welcome new profile-guided
transformations as well as improvements to the current profiling system.
</p>

<p>Ideas for profile-guided transformations:</p>

<ol>
<li>Superblock formation (with many optimizations)</li>
<li>Loop unrolling/peeling</li>
<li>Profile directed inlining</li>
<li>Code layout</li>
<li>...</li>
</ol>

<p>Improvements to the existing support:</p>

<ol>
<li>The current block and edge profiling code that gets inserted is very simple
and inefficient.  Through the use of control-dependence information, many fewer
counters could be inserted into the code.  Also, if the execution count of a
loop is known to be a compile-time or runtime constant, all of the counters in
the loop could be avoided.</li>

<li>You could implement one of the "static profiling" algorithms which analyze a
piece of code an make educated guesses about the relative execution frequencies
of various parts of the code.</li>

<li>You could add path profiling support, or adapt the existing LLVM path
profiling code to work with the generic profiling interfaces.</li>
</ol>

</div>

<!-- ======================================================================= -->
<div class="www_subsubsection">
  <a name="compaction">Code Compaction</a>
</div>

<div class="www_text">
<p>LLVM aggressively optimizes for performance, but does not yet optimize for code size.
With a new ARM backend, there is increasing interest in using LLVM for embedded systems
where code size is more of an issue.
</p>

<p>Someone interested in working on implementing code compaction in LLVM might want to read
<a href="http://citeseer.ist.psu.edu/425696.html">this</a> article, describing using
link-time optimizations for code size optimization.
</p>

</div>

<!-- ======================================================================= -->
<div class="www_subsubsection">
  <a name="xforms">New Transformations and Analyses</a>
</div>

<div class="www_text">

<ol>
  <li>Implement a Loop Dependence Analysis Infrastructure<br>
    - Design some way to represent and query dep analysis</li>
  <li>Value range propagation pass</li>
  <li>More fun with loops:
    <a href="http://www.cs.ualberta.ca/~amaral/cascon/CDP04/tal.html">
      Predictive Commoning
    </a>
  </li>
  <li>Type inference (aka. devirtualization)</li>
  <li><a href="http://nondot.org/sabre/LLVMNotes/BuiltinUnreachable.txt">Value
      assertions</a> (also <a href="/PR810">PR810</a>).</li>
</ol>

</div>

<!-- ======================================================================= -->
<div class="www_subsubsection">
  <a name="codegen">Code Generator Improvements</a>
</div>

<div class="www_text">

<ol>
<li>Generalize target-specific backend passes that could be target-independent,
    by adding necessary target hooks and making sure all IR/MI features (such as
    register masks and predicated instructions) are properly handled. Enable these
    for other targets where doing so is demonstrably beneficial.
    For example:
      <ol><li>lib/Target/Hexagon/RDF*</li>
          <li>lib/Target/AArch64/AArch64AddressTypePromotion.cpp</li>
     </ol>
    </li>
<li>Merge the delay slot filling logic that is duplicated into (at least)
    the Sparc and Mips backends into a single target independent pass.
     Likewise, the branch shortening logic in several targets should be merged
     together into one pass.</li>
<li>Implement 'stack slot coloring' to allocate two frame indexes to the same
    stack offset if their live ranges don't overlap.  This can reuse a bunch of
    analysis machinery from LiveIntervals.  Making the stack smaller is good
    for cache use and very important on targets where loads have limited
    displacement like ppc, thumb, mips, sparc, etc.  This should be done as
    a pass before prolog epilog insertion.  This is now done for register
    allocator temporaries, but not for allocas.</li>
<li>Implement 'shrink wrapping', which is the intelligent placement of callee
    saved register save/restores.  Right now PrologEpilogInsertion always saves
    every (modified) callee save reg in the prolog and restores it in the
    epilog, however, some paths through a function (e.g. an early exit) may
    not use all regs.  Sinking the save down the CFG avoids useless work on
    these paths. Work has started on this, please inquire on llvm-dev.</li>
<li>Implement interprocedural register allocation. The CallGraphSCCPass can be
    used to implement a bottom-up analysis that will determine the *actual*
    registers clobbered by a function. Use the pass to fine tune register usage
    in callers based on *actual* registers used by the callee.</li>
<li>Add support for 16-bit x86 assembly and real mode to the assembler and
    disassembler, for use by BIOS code. This includes both 16-bit instruction
    encodings as well as privileged instructions (lgdt, lldt, ltr, lmsw, clts,
    invd, invlpg, wbinvd, hlt, rdmsr, wrmsr, rdpmc, rdtsc) and the control and
    debug registers.
</ol>

</div>

<!-- ======================================================================= -->
<div class="www_subsubsection">
  <a name="misc_new">Miscellaneous Additions</a>
</div>

<div class="www_text">

<ol>
<li>Port the <a href="http://www-sop.inria.fr/mimosa/fp/Bigloo/">Bigloo</A>
Scheme compiler, from Manuel Serrano at INRIA Sophia-Antipolis, to
output LLVM bytecode. It seems that it can already output .NET
bytecode, JVM bytecode, and C, so LLVM would ostensibly be another good
candidate.</li>
<li>Write a new frontend for some other language (Java? OCaml? Forth?)</li>
<li>Random test vector generator: Use a C grammar to generate random C code,
e.g., <a href="http://code.google.com/p/quest-tester/">quest</a>;
run it through llvm-gcc, then run a random set of passes on it using opt.
Try to crash <tt><a href="/docs/CommandGuide/html/opt.html">opt</a></tt>. When
<tt>opt</tt> crashes, use <tt><a
href="/docs/CommandGuide/html/bugpoint.html">bugpoint</a></tt> to reduce the
test case and post it to a website or mailing list.  Repeat ad infinitum.</li>
<li>Add sandbox features to the Interpreter: catch invalid memory accesses,
  potentially unsafe operations (access via arbitrary memory pointer) etc.
</li>
<li>Port <a href="http://valgrind.org">Valgrind</a> to use LLVM code generation
  and optimization passes instead of its own.</li>
<li>Write LLVM IR level debugger (extend Interpreter?)</li>
<li>Write an LLVM Superoptimizer.  It would be interesting to take ideas from
    this superoptimizer for x86:
<a href="http://theory.stanford.edu/~aiken/publications/papers/asplos06.pdf">paper #1</a> and <a href="http://theory.stanford.edu/~sbansal/superoptimizer.html">paper #2</a> and adapt them to run on LLVM code.<p>

It would seem that operating on LLVM code would save a lot of time
because its semantics are much simpler than x86.  The cost of operating
on LLVM is that target-specific tricks would be missed.<p>

The outcome would be a new LLVM pass that subsumes at least the
instruction combiner, and probably a few other passes as well.  Benefits
would include not missing cases missed by the current combiner and also
more easily adapting to changes in the LLVM IR.<p>

All previous superoptimizers have worked on linear sequences of code.
It would seem much better to operate on small subgraphs of the program
dependency graph.</li>
</ol>

</div>

<!-- *********************************************************************** -->
<div class="www_sectiontitle">
  <a name="using">Projects using LLVM</a>
</div>
<!-- *********************************************************************** -->

<div class="www_text">

  <p>
  In addition to projects that enhance the existing LLVM infrastructure, there
  are projects that improve software that uses, but is not included with, the
  LLVM compiler infrastructure.  These projects include open-source software
  projects and research projects that use LLVM.  Like projects that enhance the
  core LLVM infrastructure, these projects are often challenging and rewarding.
  </p>

</div>

<!-- ======================================================================= -->
<div class="www_subsubsection">
  <a name="encodeanalysis">Encode Analysis Results in MachineInstr IR</a>
</div>

<div class="www_text">
  <p>
  At least one project (and probably more) needs to use analysis information
  (such as call graph analysis) from within a MachineFunctionPass, however,
  most analysis passes operate at the LLVM IR level.  In some cases, a value
  (e.g., a function pointer) cannot be mapped from the MachineInstr level back
  to the LLVM IR level reliably, making the use of existing LLVM analysis
  passes from within a MachineFunctionPass impossible (or at least brittle).
  </p>

  <p>
  This project is to encode analysis information from the LLVM IR level into
  the MachineInstr IR when it is generated so that it is available to a
  MachineFunctionPass.  The exemplar is call graph analysis (useful for
  control-flow integrity instrumentation, analysis of code reuse defenses, and
  gadget compilers); however, other LLVM analyses may be useful.
  </p>
</div>

<!-- ======================================================================= -->
<div class="www_subsubsection">
  <a name="codelayoutjit">Code Layout in the LLVM JIT</a>
</div>

<div class="www_text">
  <p>
  Implement an on-demand function relocator in the LLVM JIT. This can help
  improve code locality using runtime profiling information. The idea is to use
  a relocation table for every function.  The relocation entries need to be
  updated upon every function relocation (take a look at
  <a href="https://people.cs.umass.edu/~emery/pubs/stabilizer-asplos13.pdf">
  this article</a>).
  A (per-function) basic block reordering would be a useful extension.
  </p>
</div>

<!-- ======================================================================= -->
<div class="www_subsubsection">
  <a name="fieldlayout">Improved Structure Splitting and Field Reordering</a>
</div>

<div class="www_text">
  <p>
  The goal of this project is to implement better data layout optimizations
  using the model of reference affinity.  This
  <a href="http://www.cs.rochester.edu/~cding/Documents/Publications/pldi04.pdf">
  paper</a>
  provides some background information.
  </p>
</div>

<!-- ======================================================================= -->
<div class="www_subsubsection">
  <a name="slimmer">Finish the Slimmer Project</a>
</div>

<div class="www_text">
  <p>
  Slimmer is a prototype tool, built using LLVM, that uses dynamic analysis to
  find potential performance bugs in programs.  Development on Slimmer started
  during Google Summer of Code in 2015 and resulted in an initial prototype,
  but evaluation of the prototype and improvements to make it portable and
  robust are still needed.  This project would have a student pick up and
  finish the Slimmer work.  The source code of Slimmer and
  its current documentation can be found at its
  <a href="https://github.com/james0zan/Slimmer">Github</a> web page.
  </p>
</div>

<!-- *********************************************************************** -->

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