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

Johannes Doerfert via llvm-dev llvm-dev at lists.llvm.org
Sun Mar 28 17:33:22 PDT 2021

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 Most passes that exist in LLVM are listed in llvm/lib/Passes/PassRegistry.def There are (outdated) lists online as well. 3) Is there any way to disable/enable specific Clang optimization?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.

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