llvm dev - Mar 2021 - Fine Grained Optimization Control

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.
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Hi Navid,

comments inlined.

On 3/27/21 9:24 PM, Navid Rahimi via llvm-dev wrote:
> 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

>
> Best wishes,
> Navid.
>
>
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