[llvm] r343663 - Improve static analysis of cold basic blocks

[Vedant Kumar via llvm-commits]

Hi Aditya,

Thanks for working on this :).

> On Oct 2, 2018, at 11:21 PM, Aditya Kumar via llvm-commits <llvm-commits at lists.llvm.org> wrote:
> 
> Author: hiraditya
> Date: Tue Oct  2 23:21:05 2018
> New Revision: 343663
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=343663&view=rev
> Log:
> Improve static analysis of cold basic blocks
> 
> Differential Revision: https://reviews.llvm.org/D52704
> 
> Reviewers: sebpop, tejohnson, brzycki, SirishP
> Reviewed By: sebpop
> 
> Modified:
>    llvm/trunk/lib/Transforms/IPO/HotColdSplitting.cpp
> 
> Modified: llvm/trunk/lib/Transforms/IPO/HotColdSplitting.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/IPO/HotColdSplitting.cpp?rev=343663&r1=343662&r2=343663&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Transforms/IPO/HotColdSplitting.cpp (original)
> +++ llvm/trunk/lib/Transforms/IPO/HotColdSplitting.cpp Tue Oct  2 23:21:05 2018
> @@ -111,6 +111,18 @@ bool blockEndsInUnreachable(const BasicB
>   return succ_empty(&BB);
> }
> 
> +static bool exceptionHandlingFunctions(const CallInst *CI) {

Could you use verb phrases for function names (see https://www.llvm.org/docs/CodingStandards.html#name-types-functions-variables-and-enumerators-properly <https://www.llvm.org/docs/CodingStandards.html#name-types-functions-variables-and-enumerators-properly>)?


> +  auto F = CI->getCalledFunction();
> +  if (!F)
> +    return false;
> +  auto FName = F->getName();
> +  return FName == "__cxa_begin_catch" ||
> +         FName == "__cxa_free_exception" ||
> +         FName == "__cxa_allocate_exception" ||
> +         FName == "__cxa_begin_catch" ||
> +         FName == "__cxa_end_catch”;

Given how hot this might be, would it make sense to return early if, say, the name doesn’t begin with “__”? Ideally we’d use some sort of off-the-shelf multi-string matching function, but absent that..

thanks,
vedant

> +}
> +
> static
> bool unlikelyExecuted(const BasicBlock &BB) {
>   if (blockEndsInUnreachable(BB))
> @@ -122,7 +134,8 @@ bool unlikelyExecuted(const BasicBlock &
>     if (const CallInst *CI = dyn_cast<CallInst>(&I)) {
>       // The block is cold if it calls functions tagged as cold or noreturn.
>       if (CI->hasFnAttr(Attribute::Cold) ||
> -          CI->hasFnAttr(Attribute::NoReturn))
> +          CI->hasFnAttr(Attribute::NoReturn) ||
> +          exceptionHandlingFunctions(CI))
>         return true;
> 
>       // Assume that inline assembly is hot code.
> 
> 
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