[lld] r284594 - Add a faster binary search.

[Rafael Avila de Espindola via llvm-commits]

Same semantics, but a faster implementation.

On October 19, 2016 1:58:14 PM EDT, Rui Ueyama <ruiu at google.com> wrote:
>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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-- 
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