[llvm-dev] replicating -O3 optimizations manually

[Emanuele Del Sozzo via llvm-dev]

Hello llvm-dev,

my name is Emanuele and I am an intern in ARM. As part of the project I am doing here, I would like to manually replicate the optimizations that LLVM applies when I type -O3. In other words, I would like to know what are the compilation flags/passes that -O3 triggers.

I noticed that GCC reports, on its website, all the flags that are enforced by -O3 (https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html), but I wasn't able to find something similar within LLVM documentation. On the other hand, I found that this command displays all the optimization passes applied by opt when -O3 flag is on:

llvm-as < /dev/null | opt -O3 -disable-output -debug-pass=Arguments

I tried to apply the same optimization passes through opt, but, even though the performance are similar, the resulting binary is slower than the one generated using -O3 (also the binaries differ, of course).

Again, I found this other command that does something similar (it lists the sequence of optimization passes applied):

clang -O3 -mllvm -debug-pass=Arguments file.c

In this case, the performance are still different and some of the optimization passes listed in the last block of passes (e.g. -machinemoduleinfo, -stack-protector, etc.) are unknown to opt.


Said that, my question is: how can I find out what optimization passes/flags -O3 enforces in order to manually apply the same optimizations and have, hopefully, the same binary and performance?


I am currently using LLVM version 5.0.2.


Thank you for both your help and your time!


Best regards

Emanuele

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