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

[Vedant Kumar via llvm-commits]

Hm, it should be reasonable to add a test for this as well.

vedant

> On Oct 3, 2018, at 6:36 PM, Vedant Kumar via llvm-commits <llvm-commits at lists.llvm.org> wrote:
> 
> 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 <mailto: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 <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 <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 <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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