======================
Using Polly with Clang
======================

This documentation discusses how Polly can be used in Clang to automatically
optimize C/C++ code during compilation.


.. warning::

  Warning: clang/LLVM/Polly need to be in sync (compiled from the same SVN
  revision).

Make Polly available from Clang
===============================

Polly is available through clang, opt, and bugpoint, if Polly was checked out
into tools/polly before compilation. No further configuration is needed.

Optimizing with Polly
=====================

Optimizing with Polly is as easy as adding -O3 -mllvm -polly to your compiler
flags (Polly is only available at -O3).

.. code-block:: console

  clang -O3 -mllvm -polly file.c

Automatic OpenMP code generation
================================

To automatically detect parallel loops and generate OpenMP code for them you
also need to add -mllvm -polly-parallel -lgomp to your CFLAGS.

.. code-block:: console

  clang -O3 -mllvm -polly -mllvm -polly-parallel -lgomp file.c

Automatic Vector code generation
================================

Automatic vector code generation can be enabled by adding -mllvm
-polly-vectorizer=stripmine to your CFLAGS.

.. code-block:: console

  clang -O3 -mllvm -polly -mllvm -polly-vectorizer=stripmine file.c

Extract a preoptimized LLVM-IR file
===================================

Often it is useful to derive from a C-file the LLVM-IR code that is actually
optimized by Polly. Normally the LLVM-IR is automatically generated from the C
code by first lowering C to LLVM-IR (clang) and by subsequently applying a set
of preparing transformations on the LLVM-IR. To get the LLVM-IR after the
preparing transformations have been applied run Polly with '-O0'.

.. code-block:: console

  clang -O0 -mllvm -polly -S -emit-llvm file.c

Further options
===============
Polly supports further options that are mainly useful for the development or the
analysis of Polly. The relevant options can be added to clang by appending
-mllvm -option-name to the CFLAGS or the clang command line.

Limit Polly to a single function
--------------------------------

To limit the execution of Polly to a single function, use the option
-polly-only-func=functionname.

Disable LLVM-IR generation
--------------------------

Polly normally regenerates LLVM-IR from the Polyhedral representation. To only
see the effects of the preparing transformation, but to disable Polly code
generation add the option polly-no-codegen.

Graphical view of the SCoPs
---------------------------
Polly can use graphviz to show the SCoPs it detects in a program. The relevant
options are -polly-show, -polly-show-only, -polly-dot and -polly-dot-only. The
'show' options automatically run dotty or another graphviz viewer to show the
scops graphically. The 'dot' options store for each function a dot file that
highlights the detected SCoPs. If 'only' is appended at the end of the option,
the basic blocks are shown without the statements the contain.

Change/Disable the Optimizer
----------------------------

Polly uses by default the isl scheduling optimizer. The isl optimizer optimizes
for data-locality and parallelism using the Pluto algorithm.
To disable the optimizer entirely use the option -polly-optimizer=none.

Disable tiling in the optimizer
-------------------------------

By default both optimizers perform tiling, if possible. In case this is not
wanted the option -polly-tiling=false can be used to disable it. (This option
disables tiling for both optimizers).

Import / Export
---------------

The flags -polly-import and -polly-export allow the export and reimport of the
polyhedral representation. By exporting, modifying and reimporting the
polyhedral representation externally calculated transformations can be
applied. This enables external optimizers or the manual optimization of
specific SCoPs.

Viewing Polly Diagnostics with opt-viewer
-----------------------------------------

The flag -fsave-optimization-record will generate .opt.yaml files when compiling
your program. These yaml files contain information about each emitted remark.
Ensure that you have Python 2.7 with PyYaml and Pygments Python Packages.
To run opt-viewer:

.. code-block:: console

   llvm/tools/opt-viewer/opt-viewer.py -source-dir /path/to/program/src/ \
      /path/to/program/src/foo.opt.yaml \
      /path/to/program/src/bar.opt.yaml \
      -o ./output

Include all yaml files (use \*.opt.yaml when specifying which yaml files to view)
to view all diagnostics from your program in opt-viewer. Compile with `PGO
<https://clang.llvm.org/docs/UsersManual.html#profiling-with-instrumentation>`_ to view
Hotness information in opt-viewer. Resulting html files can be viewed in an internet browser.