[llvm-dev] Fine Grained Optimization Control (original) (raw)

Navid Rahimi via llvm-dev llvm-dev at lists.llvm.org
Sat Mar 27 19:24:33 PDT 2021

Hi everyone,

tl;dr: I want to control which optimization and transformation can and will run on my code. Does Clang/LLVM permit such an approach?

I am doing this with GCC. But at first, it seems for some reason GCC does not allow optimizations to run unless I am passing -Ox flag (x>=1). The approach I thought would work is using -O3 and disabling all the optimizations one by one with -fno-XXX, then passing each optimization I want with -fXXX. Even after doing that it seems GCC does take the flags seriously. Sometimes it might consider the -fXXX flags, but sometimes it totally ignores.

I was investigating this issue more recently due to a project I am involved in. I realized that there are two sets of optimizations and transformation can happen in Clang/LLVM. Clang can do a few optimizations itself on AST and then LLVM will run its own optimizations. Please correct me if I am wrong.

Here is a list of few questions I am trying to find an answer for:

  1. I am looking for a list of optimizations that Clang might do. Where can I find them?
  2. I am looking for a list of optimizations that LLVM might do. Where can I find them?
  3. Is there any way to disable/enable specific Clang optimization?
  4. Is there any way to disable/enable specific LLVM optimization?
  5. Would LLVM/Clang respect specific optimization flags?

I appreciate immensely any help regarding these questions.

Best wishes, Navid. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20210327/be468a4e/attachment.html>

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