[PATCH] D115993: [ELF] Optimize RelocationSection<ELFT>::writeTo

[Fangrui Song via Phabricator via llvm-commits]

MaskRay added a comment.

I have tried a serial radix sort whose performance is similar to the parallel qsort.
It has more code and needs to sacrifice the D62141 <https://reviews.llvm.org/D62141> ordering property (the property doesn't matter and can be sacrificed if justified):

  // Sort by (!IsRelative,SymIndex). DT_REL[A]COUNT requires us to
  // place R_*_RELATIVE first. SymIndex is to improve locality.
  if (sort) {
    unsigned num = symTab->getNumSymbols();
    SmallVector<unsigned, 0> cnt(num);
    for (const DynamicReloc &rel : relocs)
      ++cnt[rel.r_sym];
    for (unsigned j = 0, i = 0; i != num; ++i) {
      unsigned t = j + cnt[i];
      cnt[i] = j;
      j = t;
    }
    auto tmp = relocs;
    unsigned j = 0, k = relocs.size();
    for (const DynamicReloc &rel : relocs)
      tmp[cnt[rel.r_sym]++] = rel;
  
    j = 0;
    for (const DynamicReloc &rel : tmp)
      if (rel.type == target->relativeRel)
        relocs[j++] = rel;
      else
        relocs[--k] = rel;
    std::reverse(relocs.begin() + k, relocs.end());
  }


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https://reviews.llvm.org/D115993