[llvm-dev] Is it possible to reproduce the result of opt -O3 manually?

Rahim Mammadli via llvm-dev llvm-dev at lists.llvm.org
Tue May 14 06:42:53 PDT 2019

Dear Bhatu and Mehdi,

Thank you for your helpful suggestions.

Indeed -print-after-all option is quite useful to find out the differences
between the two approaches. As Bhatu pointed out, when providing -O3 as an
option some of the passes are initialized differently when compared to
simply passing a list of passes to opt. Some of the passes even depend on
OptLevel as a parameter which influences the behavior of a pass, e.g. the
following passes directly depend on the OptLevel parameter:
* createSimpleLoopUnrollPass()
* createLoopUnrollAndJamPass()
* createLoopUnrollPass()

However, the only way of setting OptLevel and SizeLevel parameters through
CLI is by providing -O<> optimization flag, which in turn results in all of
the associated passes being added to the list of passes. Is there a way of
setting OptLevel = 3 and SizeLevel = 0 without running all the passes
associated with -O3? That would probably get me closest to replicating O3,
but I cannot figure out how to do it.

Kind Regards,

Rahim Mammadli

On Mon, 13 May 2019 at 09:49, Bhatu <CS12B1010 at iith.ac.in> wrote:

> I think this has to do with how the pass manager is populated when we give
> -O3 vs when we give particular pass names.
> Some passes have multiple createXYZPass() methods that accept arguments
> too. These methods call non-default pass constructors, which in turn cause
> the passes to behave in a different manner.
> eg:
> Pass *llvm::createLICMPass() { return new LegacyLICMPass(); }
> Pass *llvm::createLICMPass(unsigned LicmMssaOptCap,
>                            unsigned LicmMssaNoAccForPromotionCap) {
>   return new LegacyLICMPass(LicmMssaOptCap, LicmMssaNoAccForPromotionCap);
> }
> or
> Pass *createLoopVectorizePass() { return new LoopVectorize(); }
> Pass *createLoopVectorizePass(bool InterleaveOnlyWhenForced,
>                               bool VectorizeOnlyWhenForced) {
>   return new LoopVectorize(InterleaveOnlyWhenForced,
> VectorizeOnlyWhenForced);
> }
> When we give pass names, opt calls the default constructor (eg:
> LoopVectorize()) whereas when we give O3, it can call a different version.
> You can check in PassManagerBuilder.cpp (populateModulePassManager,
> populateFunctionPassManager) to see where different versions are being
> populated. Those must be the points in the pipeline where the IR starts
> differing.
> On Sat, May 11, 2019 at 10:09 PM Mehdi AMINI via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>> Hi,
>> On Thu, May 9, 2019 at 5:20 PM Rahim Mammadli via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>> Dear developers,
>>> I am trying to reproduce the results of applying opt -O3 to a source
>>> file in the form of LLVM IR. I want to get the same IR by manually ordering
>>> the passes used by O3 and passing them to opt.
>>> To illustrate what I am doing on an example, as an input I use linpack
>>> benchmark from the LLVM test suite[1]:
>>> 1. First I produce the intermediate representation using clang:
>>> clang -O3 -Xclang -disable-llvm-optzns -emit-llvm -S linpack-pc.c -o
>>> linpack-pc.ll
>>> 2. Then I use opt to optimize the IR:
>>> opt -S -O3 -o linpack-pc-3.ll linpack-pc.ll
>>> Now my goal is to produce the IR identical to linpack-pc-3.ll by passing
>>> a sequence of optimizations to opt. To get the list of optimizations used
>>> by opt for O3, I run this:
>>> opt -O3 -disable-output -debug-pass=Arguments linpack-pc.ll
>>> Which produces (shortened to avoid wasting space):
>>> Pass Arguments:  -tti -targetlibinfo -tbaa ...
>>> Pass Arguments:  -targetlibinfo -tti -tbaa ...
>>> Pass Arguments:  -domtree
>>> So apparently there are three sequences of passes applied to IR as part
>>> of O3. I wasn't able to reproduce the same IR as linpack-pc-3.ll using
>>> these passes, I tried applying passes sequentially or concatenating them
>>> and passing as a single sequence to opt. Neither produced the needed
>>> output. Moreover the performance of the final executable downgraded by
>>> about 35%. I'm using LLVM 3.8 and my OS is Ubuntu 16.04.
>>> [1]
>>> https://github.com/llvm/llvm-test-suite/blob/master/SingleSource/Benchmarks/Linpack/linpack-pc.c
>> I'd very much appreciate if you could help me with this. Thank you.
>> Your approach seems sensible to me. I usually have been debugging this
>> kind of problem by piping the output of the two runs with
>> `-print-after-all` to files and diff them to find out where the difference
>> pops in.
>> --
>> Mehdi
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> --
> Regards
> Bhatu
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