[PATCH] D45571: [ELF] - Speedup MergeInputSection::splitStrings

[George Rimar via Phabricator via llvm-commits]

grimar added a comment.

In https://reviews.llvm.org/D45571#1066162, @espindola wrote:

> One interesting thing about the current setup is that we first read the bytes in the string looking for the 0 that terminates the string. We then read them again to hash them. At least with a simple hash like djb (what is implemented in hashGnu) it should not be too hard to read each byte once. Would you mind giving that a try?


No much difference for me:

  Function Name                                   Total CPU(%) Total CPU (ms)
  
  * After the change:
  - lld.exe (PID: 15032)                          100.00        4166
   + lld::elf::MergeInputSection::splitIntoPieces  22.40         933
  
  * Default (xxHash64):
  - lld.exe                                         100.00%        4254
   + lld::elf::MergeInputSection::splitStrings       21.86%         930

My test code was:

  static size_t findNull(StringRef S, size_t EntSize, uint32_t& H) {
    uint32_t Hash = 5381;
  
    // Optimize the common case.
    if (EntSize == 1) {
      const char *Data = S.data();
      while (uint8_t C = *Data++)
        Hash = (Hash << 5) + Hash + C;
      H = Hash;
      return Data - S.data();
    }
  
    llvm_unreachable("scylla?");
  
    for (unsigned I = 0, N = S.size(); I != N; I += EntSize) {
      const char *B = S.begin() + I;
      if (std::all_of(B, B + EntSize, [](char C) { return C == 0; }))
        return I;
    }
  }
  
  
  // Split SHF_STRINGS section. Such section is a sequence of
  // null-terminated strings.
  void MergeInputSection::splitStrings(ArrayRef<uint8_t> Data, size_t EntSize) {
    size_t Off = 0;
    bool IsAlloc = Flags & SHF_ALLOC;
    StringRef S = toStringRef(Data);
  
    while (!S.empty()) {
      uint32_t Hash;
      size_t Size = findNull(S, EntSize, Hash);
  
      Pieces.emplace_back(Off, Hash, !IsAlloc);
      S = S.substr(Size);
      Off += Size;
    }
  }

  


https://reviews.llvm.org/D45571