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

[Mehdi AMINI via llvm-dev]

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
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