[lld] r284594 - Add a faster binary search.

[Rui Ueyama via llvm-commits]

This seems a binary search, so how it is different from std::upper_bound?

On Wed, Oct 19, 2016 at 7:17 AM, Rafael Espindola via llvm-commits <
llvm-commits at lists.llvm.org> wrote:

> Author: rafael
> Date: Wed Oct 19 09:17:36 2016
> New Revision: 284594
>
> URL: http://llvm.org/viewvc/llvm-project?rev=284594&view=rev
> Log:
> Add a faster binary search.
>
> Even with the hash table cache, binary search was still pretty
> hot. This can be made even faster with prefetching.
>
> Idea from http://cglab.ca/~morin/misc/arraylayout-v2/
>
> I will suggest moving this to llvm.
>
> Modified:
>     lld/trunk/ELF/InputSection.cpp
>
> Modified: lld/trunk/ELF/InputSection.cpp
> URL: http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/
> InputSection.cpp?rev=284594&r1=284593&r2=284594&view=diff
> ============================================================
> ==================
> --- lld/trunk/ELF/InputSection.cpp (original)
> +++ lld/trunk/ELF/InputSection.cpp Wed Oct 19 09:17:36 2016
> @@ -651,6 +651,19 @@ SectionPiece *MergeInputSection<ELFT>::g
>    return const_cast<SectionPiece *>(This->getSectionPiece(Offset));
>  }
>
> +template <class It, class T, class Compare>
> +static It fastUpperBound(It First, It Last, const T &Value, Compare Comp)
> {
> +  size_t Size = std::distance(First, Last);
> +  assert(Size != 0);
> +  while (Size != 1) {
> +    size_t H = Size / 2;
> +    const It MI = First + H;
> +    Size -= H;
> +    First = Comp(Value, *MI) ? First : First + H;
> +  }
> +  return Comp(Value, *First) ? First : First + 1;
> +}
> +
>  template <class ELFT>
>  const SectionPiece *
>  MergeInputSection<ELFT>::getSectionPiece(uintX_t Offset) const {
> @@ -659,7 +672,7 @@ MergeInputSection<ELFT>::getSectionPiece
>      fatal(getName(this) + ": entry is past the end of the section");
>
>    // Find the element this offset points to.
> -  auto I = std::upper_bound(
> +  auto I = fastUpperBound(
>        Pieces.begin(), Pieces.end(), Offset,
>        [](const uintX_t &A, const SectionPiece &B) { return A <
> B.InputOff; });
>    --I;
>
>
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