[lld] r317026 - Revert r316305: Remove a fast lookup table from MergeInputSection.

[Rafael Avila de Espindola via llvm-commits]

One last thing I remembered. Most of the regression is because of massive branch mis prediction, which is not surprising in a hot binary search.

In http://cglab.ca/~morin/misc/arraylayout-v2/ there more efficient algorithms that don't need mis predicted branches.

Last time I tried clang was creating more branches than expected, but that might be fixed now.

Cheers,
Rafael


On November 3, 2017 1:21:42 PM PDT, Rafael Avila de Espindola <rafael.espindola at gmail.com> wrote:
>I found a fascinating fact about what regressed and what improved with
>the cache:
>
>Testcases that are compiled by gcc and have debug info benefit
>tremendously: scylla got 20% faster and mozilla 15%.
>
>Tests without debug information benefited a bit.
>
>The one test we have with clang debug info (clang-gdb-index) regressed
>a
>bit (1.3%).
>
>So there is some difference that causes gcc produced debug info to hit
>a
>lot more relocations to a symbol in a merge section. Not sure what that
>is.
>
>Cheers,
>Rafael
>
>Rui Ueyama via llvm-commits <llvm-commits at lists.llvm.org> writes:
>
>> Author: ruiu
>> Date: Tue Oct 31 12:14:06 2017
>> New Revision: 317026
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=317026&view=rev
>> Log:
>> Revert r316305: Remove a fast lookup table from MergeInputSection.
>>
>> This reverts commit r316305 because performance regression was
>observed.
>>
>> Modified:
>>     lld/trunk/ELF/InputSection.cpp
>>     lld/trunk/ELF/InputSection.h
>>
>> Modified: lld/trunk/ELF/InputSection.cpp
>> URL:
>http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/InputSection.cpp?rev=317026&r1=317025&r2=317026&view=diff
>>
>==============================================================================
>> --- lld/trunk/ELF/InputSection.cpp (original)
>> +++ lld/trunk/ELF/InputSection.cpp Tue Oct 31 12:14:06 2017
>> @@ -968,9 +968,24 @@ uint64_t MergeInputSection::getOffset(ui
>>    if (!Live)
>>      return 0;
>>  
>> -  const SectionPiece &Piece = *getSectionPiece(Offset);
>> +  // Initialize OffsetMap lazily.
>> +  llvm::call_once(InitOffsetMap, [&] {
>> +    OffsetMap.reserve(Pieces.size());
>> +    for (size_t I = 0; I < Pieces.size(); ++I)
>> +      OffsetMap[Pieces[I].InputOff] = I;
>> +  });
>> +
>> +  // Find a string starting at a given offset.
>> +  auto It = OffsetMap.find(Offset);
>> +  if (It != OffsetMap.end())
>> +    return Pieces[It->second].OutputOff;
>> +
>> +  // If Offset is not at beginning of a section piece, it is not in
>the map.
>> +  // In that case we need to search from the original section piece
>vector.
>> +  const SectionPiece &Piece = *this->getSectionPiece(Offset);
>>    if (!Piece.Live)
>>      return 0;
>> +
>>    uint64_t Addend = Offset - Piece.InputOff;
>>    return Piece.OutputOff + Addend;
>>  }
>>
>> Modified: lld/trunk/ELF/InputSection.h
>> URL:
>http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/InputSection.h?rev=317026&r1=317025&r2=317026&view=diff
>>
>==============================================================================
>> --- lld/trunk/ELF/InputSection.h (original)
>> +++ lld/trunk/ELF/InputSection.h Tue Oct 31 12:14:06 2017
>> @@ -263,6 +263,9 @@ private:
>>    void splitStrings(ArrayRef<uint8_t> A, size_t Size);
>>    void splitNonStrings(ArrayRef<uint8_t> A, size_t Size);
>>  
>> +  mutable llvm::DenseMap<uint32_t, uint32_t> OffsetMap;
>> +  mutable llvm::once_flag InitOffsetMap;
>> +
>>    llvm::DenseSet<uint64_t> LiveOffsets;
>>  };
>>  
>>
>>
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-- 
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