[llvm-dev] Fine Grained Optimization Control

[Navid Rahimi via llvm-dev]

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

On Sun, Mar 28, 2021 at 17:33 Johannes Doerfert <johannesdoerfert at gmail.com>
wrote:

> 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
>
>
> On 3/28/21 5:27 PM, Navid Rahimi wrote:
> > 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.
> >
> > On Sat, Mar 27, 2021 at 22:10 Johannes Doerfert <
> johannesdoerfert at gmail.com>
> > wrote:
> >
> >> 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.
> >>>
> >>>
> >>> _______________________________________________
> >>> LLVM Developers mailing list
> >>> llvm-dev at lists.llvm.org
> >>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
-- 
Best wishes,
Navid.
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