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

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.

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