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

[Rafael Avila de Espindola via llvm-commits]

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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