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

Rahim Mammadli via llvm-dev llvm-dev at lists.llvm.org
Thu May 9 05:04:46 PDT 2019

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

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.


I'd very much appreciate if you could help me with this. Thank you.

Kind Regards,

Rahim Mammadli
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