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

Navid Rahimi via llvm-dev llvm-dev at lists.llvm.org
Sun Mar 28 15:27:17 PDT 2021

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 There are (outdated) lists online as well.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

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