[llvm-dev] Fine Grained Optimization Control

[Johannes Doerfert via llvm-dev]

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 There are (outdated) lists online as well.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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