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

[via llvm-dev]

On 2018-04-04 22:00, Mehdi AMINI wrote:
> Le mar. 3 avr. 2018 à 12:47, via llvm-dev <llvm-dev at lists.llvm.org> a
> écrit :
> 
>> 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).
> 
> I actually don't understand this clearly.
> 
> Unless we're saying that we would change the IR optimization level
> either using the -OX flag during LTO (which is clumsy, because what is
> a "cross-module optimization" alone?), why would the `-OX` flag change
> the Codegen optimization level when passed to clang without LTO, but
> it wouldn't during LTO?

I'm simply stating the argument made by Peter in r324557; this is not my 
opinion.  Personally, I think it seems reasonable to allow the 
optimization flag used during the link step to control the codegen 
optimization level.  However, this is no longer the case after r324557.

FWIW, I would be very much on-board with reverting r324557 and then 
changing lld to mirror the behavior of the gold plugin, but I don't know 
if that's the consensus in the community.

> Are we encoding O1/O2/O3 optimization level into function attributes
> and trying to honor these during the LTO IR optimization pipeline as
> well?

No.  The intent of these attributes are to control the codegen pipeline 
only.  Of course this is all based on the assumption that using the 
optimization level used during bitcode generation should also be used 
with LTO in the codegen pipeline.

I don't have a strong opinion either way.  I just want codgen to respect 
the fact that I specified -O3 during both the bitcode generation and 
link steps, but that's not the case anymore.  :)

  Chad

> 
> Thanks,
> 
> --
> Mehdi
> 
>> 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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