[llvm] r324557 - gold-plugin: Do not set codegen opt level based on LTO opt level.

Fri Apr 6 10:37:59 PDT 2018

On Fri, Apr 6, 2018 at 10:15 AM, Chad Rosier <mcrosier at codeaurora.org>
wrote:

> Hi Peter,
> On 4/6/2018 12:25 PM, Peter Collingbourne wrote:
>
> Unfortunately reverting would regress the user that requested this patch.
> They use -plugin-opt=O0 and expect performance consistent with setting cg
> opt level to default.
>
>
> Using -O0 with LTO seems like a very odd/uncommon use case.
>

It can be used to get program transformations that rely on whole-program
information (such as control flow integrity
<https://clang.llvm.org/docs/ControlFlowIntegrity.html>) without requesting
cross-module optimizations. This is important for users who want CFI but do
not want to pay the link time or code size cost of cross-module
optimizations.

Can the user use '-mllvm -cg-opt-level=2' as an alternative solution to
> override the codegen optimization level used during LTO?
>

This flag is not present in the gold plugin, only in llvm-lto2.

>
>
>
> Instead of reverting can we make a change that just sets the cg opt level
> to aggressive if -plugin-opt=O3 is passed?
>
>
> I'd prefer not.  IMO, this would just further muddy the water.  The
> '-mllvm -cg-opt-level=2' suggestion seems like a much less intrusive
> fix/workaround.
>

Adding the flag would be going in the wrong direction IMHO. What we should
be working towards is removing the LTO configuration option entirely. We
can do that by moving the default/aggressive selection into the LTO API
implementation so that the behaviour is consistent between linkers. Then
once the attribute is ready we can remove that code.

Peter

>
>
>
> Peter
>
>
> On Fri, Apr 6, 2018, 09:18 Chad Rosier <mcrosier at codeaurora.org> wrote:
>
>> Thanks, Rafael.  Sound reasonable to you, Peter?
>>
>>
>> On 4/6/2018 12:12 PM, Rafael Avila de Espindola wrote:
>> > I am OK with reverting it until we have a better framework in place.
>> >
>> > Cheers,
>> > Rafael
>> >
>> > Chad Rosier via llvm-commits <llvm-commits at lists.llvm.org> writes:
>> >
>> >> Peter/Rafael,
>> >>
>> >> Over the past few days I've been investigating several regressions with
>> >> our weekly LTO configuration.  As a result, I identified that the
>> >> following regressions were due to this commit:
>> >>
>> >> SPEC2006/h264ref: -2.68%
>> >> SPEC2006/libquantum: -4.36%
>> >> SPEC2006/perlbench: -2.67%
>> >> SPEC2017/mcf: -4.47%
>> >> SPEC2017/perlbench: -3.82%
>> >>
>> >> I did not test SPEC2000, SPEC2006fp, or SPEC2017fp, but I'm seeing
>> >> similar small regressions in our weekly runs there as well (i.e., the
>> >> above is just what I was able to test last night, but isn't likely the
>> >> only regressions).  I'd like to kindly request that this change be
>> >> reverted until something is in place to properly propagate the codegen
>> >> optimization level.
>> >>
>> >>    Chad
>> >>
>> >>
>> >> On 2/7/2018 9:41 PM, Peter Collingbourne via llvm-commits wrote:
>> >>> Author: pcc
>> >>> Date: Wed Feb  7 18:41:22 2018
>> >>> New Revision: 324557
>> >>>
>> >>> URL: http://llvm.org/viewvc/llvm-project?rev=324557&view=rev
>> >>> Log:
>> >>> gold-plugin: Do not set codegen opt level based on LTO opt level.
>> >>>
>> >>> The LTO opt level should not affect the codegen opt level, and indeed
>> >>> it does not affect it in lld. Ideally the codegen opt level should
>> >>> be controlled by an IR-level attribute based on the compile-time opt
>> >>> level, but that hasn't been implemented yet.
>> >>>
>> >>> Differential Revision: https://reviews.llvm.org/D43040
>> >>>
>> >>> Modified:
>> >>>       llvm/trunk/tools/gold/gold-plugin.cpp
>> >>>
>> >>> Modified: llvm/trunk/tools/gold/gold-plugin.cpp
>> >>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/tools/gold/
>> gold-plugin.cpp?rev=324557&r1=324556&r2=324557&view=diff
>> >>> ============================================================
>> ==================
>> >>> --- llvm/trunk/tools/gold/gold-plugin.cpp (original)
>> >>> +++ llvm/trunk/tools/gold/gold-plugin.cpp Wed Feb  7 18:41:22 2018
>> >>> @@ -727,20 +727,6 @@ static int getOutputFileName(StringRef I
>> >>>      return FD;
>> >>>    }
>> >>>
>> >>> -static CodeGenOpt::Level getCGOptLevel() {
>> >>> -  switch (options::OptLevel) {
>> >>> -  case 0:
>> >>> -    return CodeGenOpt::None;
>> >>> -  case 1:
>> >>> -    return CodeGenOpt::Less;
>> >>> -  case 2:
>> >>> -    return CodeGenOpt::Default;
>> >>> -  case 3:
>> >>> -    return CodeGenOpt::Aggressive;
>> >>> -  }
>> >>> -  llvm_unreachable("Invalid optimization level");
>> >>> -}
>> >>> -
>> >>>    /// Parse the thinlto_prefix_replace option into the \p OldPrefix
>> and
>> >>>    /// \p NewPrefix strings, if it was specified.
>> >>>    static void getThinLTOOldAndNewPrefix(std::string &OldPrefix,
>> >>> @@ -767,7 +753,6 @@ static std::unique_ptr<LTO> createLTO(In
>> >>>
>> >>>      Conf.MAttrs = MAttrs;
>> >>>      Conf.RelocModel = RelocationModel;
>> >>> -  Conf.CGOptLevel = getCGOptLevel();
>> >>>      Conf.DisableVerify = options::DisableVerify;
>> >>>      Conf.OptLevel = options::OptLevel;
>> >>>      if (options::Parallelism)
>> >>>
>> >>>
>> >>> _______________________________________________
>> >>> llvm-commits mailing list
>> >>> llvm-commits at lists.llvm.org
>> >>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
>> >> _______________________________________________
>> >> llvm-commits mailing list
>> >> llvm-commits at lists.llvm.org
>> >> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
>>
>>
>

-- 
-- 
Peter
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