[PATCH] D45227: Instead of using std::copy, clear the vector first and add new elements. NFC.

[Rui Ueyama via llvm-commits]

I think that the new code is easier to read than before, and the
performance is the same as we reserve space before adding an element?

On Mon, Apr 9, 2018 at 11:41 AM David Blaikie <dblaikie at gmail.com> wrote:

> Out of curiosity - what was the motivation for this change? I'd expect
> that copy would be faster than repeated push_back (though maybe LLVM can
> optimize it to match)
>
> On Tue, Apr 3, 2018 at 1:00 PM Rui Ueyama via Phabricator via llvm-commits
> <llvm-commits at lists.llvm.org> wrote:
>
>> ruiu created this revision.
>> ruiu added a reviewer: pcc.
>> Herald added subscribers: arichardson, emaste.
>> Herald added a reviewer: espindola.
>>
>> Instead of using std::copy, clear the vector first and add new elements.
>> NFC.
>>
>>
>> https://reviews.llvm.org/D45227
>>
>> Files:
>>   lld/ELF/Writer.cpp
>>
>>
>> Index: lld/ELF/Writer.cpp
>> ===================================================================
>> --- lld/ELF/Writer.cpp
>> +++ lld/ELF/Writer.cpp
>> @@ -1137,24 +1137,23 @@
>>    // of the second block of cold code can call the hot code without a
>> thunk. So
>>    // we effectively double the amount of code that could potentially
>> call into
>>    // the hot code without a thunk.
>> -  size_t UnorderedInsPt = 0;
>> +  size_t InsPt = 0;
>>    if (Target->ThunkSectionSpacing && !OrderedSections.empty()) {
>>      uint64_t UnorderedPos = 0;
>> -    for (; UnorderedInsPt != UnorderedSections.size(); ++UnorderedInsPt)
>> {
>> -      UnorderedPos += UnorderedSections[UnorderedInsPt]->getSize();
>> +    for (; InsPt != UnorderedSections.size(); ++InsPt) {
>> +      UnorderedPos += UnorderedSections[InsPt]->getSize();
>>        if (UnorderedPos > UnorderedSize / 2)
>>          break;
>>      }
>>    }
>>
>> -  std::copy(UnorderedSections.begin(),
>> -            UnorderedSections.begin() + UnorderedInsPt,
>> ISD->Sections.begin());
>> -  std::vector<InputSection *>::iterator SectionsPos =
>> -      ISD->Sections.begin() + UnorderedInsPt;
>> +  ISD->Sections.clear();
>> +  for (InputSection *IS : makeArrayRef(UnorderedSections).slice(0,
>> InsPt))
>> +    ISD->Sections.push_back(IS);
>>    for (std::pair<InputSection *, int> P : OrderedSections)
>> -    *SectionsPos++ = P.first;
>> -  std::copy(UnorderedSections.begin() + UnorderedInsPt,
>> UnorderedSections.end(),
>> -            SectionsPos);
>> +    ISD->Sections.push_back(P.first);
>> +  for (InputSection *IS : makeArrayRef(UnorderedSections).slice(InsPt))
>> +    ISD->Sections.push_back(IS);
>>  }
>>
>>  static void sortSection(OutputSection *Sec,
>>
>>
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>
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