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).

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.

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.

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’.

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.