[cfe-users] [llvm-dev] Fine Grained Optimization Control

Johannes Doerfert via cfe-users cfe-users at lists.llvm.org
Sat Mar 27 22:10:40 PDT 2021

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:

> 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

There are (outdated) lists online as well.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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