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 <johannesdoerf...@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 There are (outdated) lists online as well.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-...@lists.llvm.org > > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev > -- Best wishes, Navid.
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