[lld] r284594 - Add a faster binary search.

[Rui Ueyama via llvm-commits]

This seems pretty straightforward binary search, so I'm wondering why
std::upper_bound is so slow. Just curious.

On Wed, Oct 19, 2016 at 11:23 AM, Rafael Avila de Espindola <
rafael.espindola at gmail.com> wrote:

> 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/InputSecti
>>> on.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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>>>
>>
>>
> --
> Sent from my Android device with K-9 Mail. Please excuse my brevity.
>
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