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There are (outdated) lists online as well.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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