| commit | 596d534ac3524052df210be8d3c01a33b2260a42 | [log] [tgz] |
|---|---|---|
| author | Dan Liew <dliew@apple.com> | Wed Jan 06 17:41:46 2021 -0800 |
| committer | Dan Liew <dliew@apple.com> | Fri Jan 22 23:34:43 2021 -0800 |
| tree | 7d8e9e08d9b8dc79ffc6e458e2d5f9c8451ffba9 | |
| parent | 49231c1f80803ae0f15963cce708cedf6e44088f [diff] |
[ASan] Stop blocking child thread progress from parent thread in `pthread_create` interceptor. Previously in ASan's `pthread_create` interceptor we would block in the `pthread_create` interceptor waiting for the child thread to start. Unfortunately this has bad performance characteristics because the OS scheduler doesn't know the relationship between the parent and child thread (i.e. the parent thread cannot make progress until the child thread makes progress) and may make the wrong scheduling decision which stalls progress. It turns out that ASan didn't use to block in this interceptor but was changed to do so to try to address http://llvm.org/bugs/show_bug.cgi?id=21621/. In that bug the problem being addressed was a LeakSanitizer false positive. That bug concerns a heap object being passed as `arg` to `pthread_create`. If: * The calling thread loses a live reference to the object (e.g. `pthread_create` finishes and the thread no longer has a live reference to the object). * Leak checking is triggered. * The child thread has not yet started (once it starts it will have a live reference). then the heap object will incorrectly appear to be leaked. This bug is covered by the `lsan/TestCases/leak_check_before_thread_started.cpp` test case. In b029c5101fb49b3577a1c322f42ef9fc616f25bf ASan was changed to block in `pthread_create()` until the child thread starts so that `arg` is kept alive for the purposes of leaking check. While this change "works" its problematic due to the performance problems it causes. The change is also completely unnecessary if leak checking is disabled (via detect_leaks runtime option or CAN_SANITIZE_LEAKS compile time config). This patch does two things: 1. Takes a different approach to solving the leak false positive by making LSan's leak checking mechanism treat the `arg` pointer of created but not started threads as reachable. This is done by implementing the `ForEachRegisteredThreadContextCb` callback for ASan. 2. Removes the blocking behaviour in the ASan `pthread_create` interceptor. rdar://problem/63537240 Differential Revision: https://reviews.llvm.org/D95184
This directory and its sub-directories contain source code for LLVM, a toolkit for the construction of highly optimized compilers, optimizers, and run-time environments.
The README briefly describes how to get started with building LLVM. For more information on how to contribute to the LLVM project, please take a look at the Contributing to LLVM guide.
Taken from https://llvm.org/docs/GettingStarted.html.
Welcome to the LLVM project!
The LLVM project has multiple components. The core of the project is itself called “LLVM”. This contains all of the tools, libraries, and header files needed to process intermediate representations and converts it into object files. Tools include an assembler, disassembler, bitcode analyzer, and bitcode optimizer. It also contains basic regression tests.
C-like languages use the Clang front end. This component compiles C, C++, Objective-C, and Objective-C++ code into LLVM bitcode -- and from there into object files, using LLVM.
Other components include: the libc++ C++ standard library, the LLD linker, and more.
The LLVM Getting Started documentation may be out of date. The Clang Getting Started page might have more accurate information.
This is an example work-flow and configuration to get and build the LLVM source:
Checkout LLVM (including related sub-projects like Clang):
git clone https://github.com/llvm/llvm-project.git
Or, on windows, git clone --config core.autocrlf=false https://github.com/llvm/llvm-project.git
Configure and build LLVM and Clang:
cd llvm-project
mkdir build
cd build
cmake -G <generator> [options] ../llvm
Some common build system generators are:
Ninja --- for generating Ninja build files. Most llvm developers use Ninja.Unix Makefiles --- for generating make-compatible parallel makefiles.Visual Studio --- for generating Visual Studio projects and solutions.Xcode --- for generating Xcode projects.Some Common options:
-DLLVM_ENABLE_PROJECTS='...' --- semicolon-separated list of the LLVM sub-projects you'd like to additionally build. Can include any of: clang, clang-tools-extra, libcxx, libcxxabi, libunwind, lldb, compiler-rt, lld, polly, or debuginfo-tests.
For example, to build LLVM, Clang, libcxx, and libcxxabi, use -DLLVM_ENABLE_PROJECTS="clang;libcxx;libcxxabi".
-DCMAKE_INSTALL_PREFIX=directory --- Specify for directory the full path name of where you want the LLVM tools and libraries to be installed (default /usr/local).
-DCMAKE_BUILD_TYPE=type --- Valid options for type are Debug, Release, RelWithDebInfo, and MinSizeRel. Default is Debug.
-DLLVM_ENABLE_ASSERTIONS=On --- Compile with assertion checks enabled (default is Yes for Debug builds, No for all other build types).
cmake --build . [-- [options] <target>] or your build system specified above directly.
The default target (i.e. ninja or make) will build all of LLVM.
The check-all target (i.e. ninja check-all) will run the regression tests to ensure everything is in working order.
CMake will generate targets for each tool and library, and most LLVM sub-projects generate their own check-<project> target.
Running a serial build will be slow. To improve speed, try running a parallel build. That's done by default in Ninja; for make, use the option -j NNN, where NNN is the number of parallel jobs, e.g. the number of CPUs you have.
For more information see CMake
Consult the Getting Started with LLVM page for detailed information on configuring and compiling LLVM. You can visit Directory Layout to learn about the layout of the source code tree.