Using Polly with Clang

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

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

Switching the OpenMP backend

The following CL switch allows to choose Polly's OpenMP-backend:

-polly-omp-backend[=BACKEND]

choose the OpenMP backend; BACKEND can be 'GNU' (the default) or 'LLVM';

The OpenMP backends can be further influenced using the following CL switches:

-polly-num-threads[=NUM]

set the number of threads to use; NUM may be any positive integer (default: 0, which equals automatic/OMP runtime);

-polly-scheduling[=SCHED]

set the OpenMP scheduling type; SCHED can be 'static', 'dynamic', 'guided' or 'runtime' (the default);

-polly-scheduling-chunksize[=CHUNK]

set the chunksize (for the selected scheduling type); CHUNK may be any strictly positive integer (otherwise it will default to 1);

Note that at the time of writing, the GNU backend may only use the polly-num-threads and polly-scheduling switches, where the latter also has to be set to "runtime".

Example: Use alternative backend with dynamic scheduling, four threads and chunksize of one (additional switches).

-mllvm -polly-omp-backend=LLVM -mllvm -polly-num-threads=4
-mllvm -polly-scheduling=dynamic -mllvm -polly-scheduling-chunksize=1

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

Isolate the Polly passes

Polly's analysis and transformation passes are run with many other passes of the pass manager's pipeline. Some of passes that run before Polly are essential for its working, for instance the canonicalization of loop. Therefore Polly is unable to optimize code straight out of clang's -O0 output.

To get the LLVM-IR that Polly sees in the optimization pipeline, use the command:

clang file.c -c -O3 -mllvm -polly -mllvm -polly-dump-before-file=before-polly.ll

This writes a file 'before-polly.ll' containing the LLVM-IR as passed to polly, after SSA transformation, loop canonicalization, inlining and other passes.

Thereafter, any Polly pass can be run over 'before-polly.ll' using the 'opt' tool. To found out which Polly passes are active in the standard pipeline, see the output of

clang file.c -c -O3 -mllvm -polly -mllvm -debug-pass=Arguments

The Polly's passes are those between '-polly-detect' and '-polly-codegen'. Analysis passes can be omitted. At the time of this writing, the default Polly pass pipeline is:

opt before-polly.ll -polly-simplify -polly-optree -polly-delicm -polly-simplify -polly-prune-unprofitable -polly-opt-isl -polly-codegen

Note that this uses LLVM's old/legacy pass manager.

For completeness, here are some other methods that generates IR suitable for processing with Polly from C/C++/Objective C source code. The previous method is the recommended one.

The following generates unoptimized LLVM-IR ('-O0', which is the default) and runs the canonicalizing passes on it ('-polly-canonicalize'). This does /not/ include all the passes that run before Polly in the default pass pipeline. The '-disable-O0-optnone' option is required because otherwise clang adds an 'optnone' attribute to all functions such that it is skipped by most optimization passes. This is meant to stop LTO builds to optimize these functions in the linking phase anyway.

clang file.c -c -O0 -Xclang -disable-O0-optnone -emit-llvm -S -o - | opt -polly-canonicalize -S

The option '-disable-llvm-passes' disables all LLVM passes, even those that run at -O0. Passing -O1 (or any optimization level other than -O0) avoids that the 'optnone' attribute is added.

clang file.c -c -O1 -Xclang -disable-llvm-passes -emit-llvm -S -o - | opt -polly-canonicalize -S

As another alternative, Polly can be pushed in front of the pass pipeline, and then its output dumped. This implicitly runs the '-polly-canonicalize' passes.

clang file.c -c -O3 -mllvm -polly -mllvm -polly-position=early -mllvm -polly-dump-before-file=before-polly.ll

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:

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 to view Hotness information in opt-viewer. Resulting html files can be viewed in an internet browser.