Fine Grained Optimization Control

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#1

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.

#2

Hi Navid,

comments inlined.

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?

There is no unified approach to this as far as I know. The closest
I'm aware of was some research prototype:
https://compilers.cs.uni-saarland.de/projects/noise/

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.

I'm not aware of optimizations/transformation we do on the AST,
except the things that "have to" happen on that level.

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?

I doubt there are "optimzations" to speak of, constant propagation
can happen though.

2) I am looking for a list of optimizations that LLVM might do. Where can I
find them?

Most passes that exist in LLVM are listed in
llvm/lib/Passes/PassRegistry.def

There are (outdated) lists online as well.

3) Is there any way to disable/enable specific Clang optimization?

Most, if not all, are mandatory.

4) Is there any way to disable/enable specific LLVM optimization?

Some, not all, have command line flags to disable them, I would do:
opt -help-hidden | grep disable
opt -help-hidden | grep enable

if I needed a list.

5) Would LLVM/Clang respect specific optimization flags?

I don't think you can build your own optimization pipelines via clang
but you can emit IR and do it with opt.

I appreciate immensely any help regarding these questions.

Hope this helps, others might have more information.

~ Johannes

#3

Thanks Johannes. That makes this makes it more understandable to me. What can I do for optimization that doesn’t have flag? How should I approach disabling them.

#4

I recommend adding such a flag to the pass you want to disable.
Whenever `runOnXXX` is called, check the flag and exit if set.

~ Johannes

#5

Makes sense thank you. I have to look at the dependencies between different optimization passes too I guess.