[llvm-dev] [RFC] Adding function attributes to represent codegen optimization level

Wed Apr 4 23:40:06 PDT 2018

Yes, this is very useful RFC for being able to control optimization level at function level. We may also want to provide fp-model=<fast|precised …> control at function level via attributes as well. However, the “merge” actions of all these attributes need to be carefully defined and designed for LTO and IPO optimizations.

Thanks,
Xinmin

From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Chad Rosier via llvm-dev
Sent: Wednesday, April 4, 2018 8:54 AM
To: Martin J. O'Riordan <MartinO at theheart.ie>; 'LLVM Developers' <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] [RFC] Adding function attributes to represent codegen optimization level

Hi Martin,
I think this is another example of why we might consider having such function level attributes.. yes.

 Chad

On 4/4/2018 11:42 AM, Martin J. O'Riordan via llvm-dev wrote:
Sorry, my reply “to all” left out LLVM-Dev

From: Martin J. O'Riordan [mailto:MartinO at theheart.ie]
Sent: 04 April 2018 16:41
To: 'David Blaikie' <dblaikie at gmail.com><mailto:dblaikie at gmail.com>; 'mcrosier at codeaurora.org<mailto:mcrosier at codeaurora.org>' <mcrosier at codeaurora.org><mailto:mcrosier at codeaurora.org>; 'Chandler Carruth' <chandlerc at gmail.com><mailto:chandlerc at gmail.com>; 'Eric Christopher' <echristo at gmail.com><mailto:echristo at gmail.com>
Subject: RE: [llvm-dev] [RFC] Adding function attributes to represent codegen optimization level

Would implementing GCC’s ‘__attribute__((optimize(...)))’ help?

I can’t find any good documentation for this attribute, but it seems that GCC supports this in two forms:

__attribute__((optimize(2)))
meaning optimise equivalent to ‘-O2’, and:

__attribute_((optimize("fblah", "O1")))
meaning optimise with the provided discrete options.

            MartinO

From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of David Blaikie via llvm-dev
Sent: 04 April 2018 15:41
To: mcrosier at codeaurora.org<mailto:mcrosier at codeaurora.org>; Chandler Carruth <chandlerc at gmail.com<mailto:chandlerc at gmail.com>>; Eric Christopher <echristo at gmail.com<mailto:echristo at gmail.com>>
Cc: llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] [RFC] Adding function attributes to represent codegen optimization level

On Tue, Apr 3, 2018 at 12:47 PM via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
All,
A recent commit, D43040/r324557, changed the behavior of the gold plugin
when compiling with LTO.  The change now causes the codegen optimization
level to default to CodeGenOpt::Default (i.e., -O2) rather than use the
LTO optimization level.  The argument was made that the LTO optimization
level should control the amount of cross-module optimizations done by
LTO, but it should not control the codegen optimization level; that
should be based off of the optimization level used during the initial
compilation phase (i.e., bitcode generation).

At least as I recall the discussion around optnone/optsize in the pass was that these were in some way special semantically distinct properties (optnone being "good for debugging" (or good for debugging compilers - what's the baseline behavior before optimizations are applied), optsize being "make this fit into something it wouldn't otherwise fit into") but that the gradiations of -ON didn't fit into this kind of model & wouldn't ever be implemented as function attributes.

CC'd Chandler & Eric who I think had opinions/were involved in those previous discussions.

Assuming the argument is reasonable (it make sense to me), I was hoping
to solicit feedback on how to proceed.  The suggestion in D43040/r324557
was to add function attributes to represent the compile-time
optimization level (which also seems reasonable to me).

As a first step, I've put together two patches: 1) an llvm patch that
adds the function attributes to the LLVM IR and 2) a clang patch that
attaches these attributes to each function based on the codegen
optimization level.  I then use the function level attributes to
"reconstruct" to codegen optimization level used with LTO.

Please understand this is very much a WIP and just a very small step
towards a final solution.

Here are the patches for reference:
Clang: D45226
LLVM: D45225

Regards,
  Chad

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