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[doc] Overhaul doc on preparing IR for processing by Polly.

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The previously documented method did not work (anymore).

Suggested-by: Philip Pfaffe <philip.pfaffe@gmail.com>
llvm-svn: 329446
This commit is contained in:
Michael Kruse 2018-04-06 19:24:18 +00:00
parent dbddadd52f
commit 9b67e569c6
1 changed files with 66 additions and 8 deletions
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74
polly/docs/UsingPollyWithClang.rst
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@ -47,18 +47,76 @@ Automatic vector code generation can be enabled by adding -mllvm
clang -O3 -mllvm -polly -mllvm -polly-vectorizer=stripmine file.c
Extract a preoptimized LLVM-IR file
===================================
Isolate the Polly passes
========================
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'.
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:
.. code-block:: console
clang -O0 -mllvm -polly -S -emit-llvm file.c
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
.. code-block:: console
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:
.. code-block:: console
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.
.. code-block:: console
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.
.. code-block:: console
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.
.. code-block:: console
clang file.c -c -O3 -mllvm -polly -mllvm -polly-position=early -mllvm -polly-dump-before-file=before-polly.ll
Further options
===============