Re: [cfe-users] [llvm-dev] Fine Grained Optimization Control

2021-03-28 Thread Johannes Doerfert via cfe-users

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There are (outdated) lists online as well.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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Re: [cfe-users] [llvm-dev] Fine Grained Optimization Control

2021-03-28 Thread Johannes Doerfert via cfe-users

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 
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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