[llvm-dev] defaults for FP contraction [e.g. fused multiply-add]: suggestion and patch to be slightly more aggressive and to make Clang`s optimization settings closer to having the same meaning as when they are given to GCC [at least for "-O3"]

[Stephen Canon via llvm-dev]

Gating this on -Owhatever is dangerous, .  We should simply default to the pragma “on” state universally.

– Steve

> On Sep 9, 2016, at 5:17 PM, Abe Skolnik via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Dear all,
> 
> In the process of investigating a performance difference between Clang & GCC when both compile the same non-toolchain program while using the "same"* compiler flags, I have found something that may be worth changing in Clang, developed a patch, and confirmed that the patch has its intended effect.
> 
> *: "same" in quotes b/c the essence of the problem is that the _meaning_ of "-O3" on Clang differs from that of "-O3" on GCC in at least one way.
> 
> The specific problem here relates to the default settings for FP contraction, e.g. fused multiply-add.  At -O2 and higher, GCC defaults FP contraction to "fast", i.e. always on.  I`m not suggesting that Clang/LLVM/both need to do the same, since Clang+LLVM has good support for "#pragma STDC FP_CONTRACT".
> 
> If we keep Clang`s default for FP contraction at "on" [which really means "according to the pragma"] but change the default value of the _pragma_ [currently off] to on at -O3, then Clang will be more competitive with GCC at high optimization settings without resorting to the more-brutish "fast by default" at plain -O3 [as opposed to "-Ofast", "-O3 -ffast-math", etc.].
> 
> Since I don`t know what Objective-C [and Objective-C++] have to say about FP operations, I have made my patch very selective based on language.  Also, I noticed that the CUDA front-end seems to already have its own defaults for FP contraction, so there`s no need to change this for every language.
> 
> I needed to change one test case because it made an assumption that FP contraction is off by default when compiling with "-O3" but without any additional optimization-related flags.
> 
> Patch relative to upstream code with Git ID b0768e805d1d33d730e5bd44ba578df043dfbc66
> ------------------------------------------------------------------------------------
> 
> diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
> index 619ea9c..d02d873 100644
> --- a/clang/lib/Frontend/CompilerInvocation.cpp
> +++ b/clang/lib/Frontend/CompilerInvocation.cpp
> @@ -2437,6 +2437,13 @@ bool CompilerInvocation::CreateFromArgs(CompilerInvocation &Res,
>   if (Arch == llvm::Triple::spir || Arch == llvm::Triple::spir64) {
>     Res.getDiagnosticOpts().Warnings.push_back("spir-compat");
>   }
> +
> +  // If there will ever be e.g. "LangOpts.C", replace "LangOpts.C11 || LangOpts.C99" with "LangOpts.C" on the next line.
> +  if (    (LangOpts.C11 || LangOpts.C99 || LangOpts.CPlusPlus)                      // ...
> +  /*...*/ && ( CodeGenOptions::FPC_On == Res.getCodeGenOpts().getFPContractMode() ) // ... // just being careful
> +  /*...*/ && (Res.getCodeGenOpts().OptimizationLevel >= 3) )
> +    LangOpts.DefaultFPContract = 1;
> +
>   return Success;
> }
> 
> diff --git a/clang/test/CodeGen/fp-contract-pragma.cpp b/clang/test/CodeGen/fp-contract-pragma.cpp
> index 1c5921a..0949272 100644
> --- a/clang/test/CodeGen/fp-contract-pragma.cpp
> +++ b/clang/test/CodeGen/fp-contract-pragma.cpp
> @@ -13,6 +13,7 @@ float fp_contract_2(float a, float b, float c) {
> // CHECK: _Z13fp_contract_2fff
> // CHECK: %[[M:.+]] = fmul float %a, %b
> // CHECK-NEXT: fadd float %[[M]], %c
> +  #pragma STDC FP_CONTRACT OFF
>   {
>     #pragma STDC FP_CONTRACT ON
>   }
> 
> Please give me any and all feedback you may have on this suggested change and this proposed patch.
> 
> Regards,
> 
> Abe
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