[lld] r303801 - Close the gap between ELF and COFF ICF implementations. NFC.

[Aleksey Shlyapnikov via llvm-commits]

http://lab.llvm.org:8011/builders/sanitizer-windows/builds/11813 reports
lld related failures, please check.

On Wed, May 24, 2017 at 12:56 PM, Rui Ueyama via llvm-commits <
llvm-commits at lists.llvm.org> wrote:

> Author: ruiu
> Date: Wed May 24 14:56:29 2017
> New Revision: 303801
>
> URL: http://llvm.org/viewvc/llvm-project?rev=303801&view=rev
> Log:
> Close the gap between ELF and COFF ICF implementations. NFC.
>
> We originally wrote the ICF code for COFF and ported it to ELF.
> They started diverging since then. This patch closes the gap.
>
> Modified:
>     lld/trunk/COFF/ICF.cpp
>
> Modified: lld/trunk/COFF/ICF.cpp
> URL: http://llvm.org/viewvc/llvm-project/lld/trunk/COFF/ICF.
> cpp?rev=303801&r1=303800&r2=303801&view=diff
> ============================================================
> ==================
> --- lld/trunk/COFF/ICF.cpp (original)
> +++ lld/trunk/COFF/ICF.cpp Wed May 24 14:56:29 2017
> @@ -56,7 +56,6 @@ private:
>
>    std::vector<SectionChunk *> Chunks;
>    int Cnt = 0;
> -  std::atomic<uint32_t> NextId = {1};
>    std::atomic<bool> Repeat = {false};
>  };
>
> @@ -98,10 +97,10 @@ void ICF::segregate(size_t Begin, size_t
>          });
>      size_t Mid = Bound - Chunks.begin();
>
> -    // Split [Begin, End) into [Begin, Mid) and [Mid, End).
> -    uint32_t Id = NextId++;
> +    // Split [Begin, End) into [Begin, Mid) and [Mid, End). We use Mid as
> an
> +    // equivalence class ID because every group ends with a unique index.
>      for (size_t I = Begin; I < Mid; ++I)
> -      Chunks[I]->Class[(Cnt + 1) % 2] = Id;
> +      Chunks[I]->Class[(Cnt + 1) % 2] = Mid;
>
>      // If we created a group, we need to iterate the main loop again.
>      if (Mid != End)
> @@ -186,6 +185,7 @@ void ICF::forEachClass(std::function<voi
>    // call Fn sequentially.
>    if (Chunks.size() < 1024) {
>      forEachClassRange(0, Chunks.size(), Fn);
> +    ++Cnt;
>      return;
>    }
>
> @@ -196,6 +196,7 @@ void ICF::forEachClass(std::function<voi
>      size_t End = (I == NumShards - 1) ? Chunks.size() : (I + 1) * Step;
>      forEachClassRange(I * Step, End, Fn);
>    });
> +  ++Cnt;
>  }
>
>  // Merge identical COMDAT sections.
> @@ -203,22 +204,15 @@ void ICF::forEachClass(std::function<voi
>  // contents and relocations are all the same.
>  void ICF::run(const std::vector<Chunk *> &Vec) {
>    // Collect only mergeable sections and group by hash value.
> -  for (Chunk *C : Vec) {
> -    auto *SC = dyn_cast<SectionChunk>(C);
> -    if (!SC)
> -      continue;
> -
> -    if (isEligible(SC)) {
> -      // Set MSB to 1 to avoid collisions with non-hash classs.
> -      SC->Class[0] = getHash(SC) | (1 << 31);
> -      Chunks.push_back(SC);
> -    } else {
> -      SC->Class[0] = NextId++;
> -    }
> -  }
> -
> -  if (Chunks.empty())
> -    return;
> +  for (Chunk *C : Vec)
> +    if (auto *SC = dyn_cast<SectionChunk>(C))
> +      if (isEligible(SC))
> +        Chunks.push_back(SC);
> +
> +  // Initially, we use hash values to partition sections.
> +  for (SectionChunk *SC : Chunks)
> +    // Set MSB to 1 to avoid collisions with non-hash classs.
> +    SC->Class[0] = getHash(SC) | (1 << 31);
>
>    // From now on, sections in Chunks are ordered so that sections in
>    // the same group are consecutive in the vector.
> @@ -229,14 +223,12 @@ void ICF::run(const std::vector<Chunk *>
>
>    // Compare static contents and assign unique IDs for each static
> content.
>    forEachClass([&](size_t Begin, size_t End) { segregate(Begin, End,
> true); });
> -  ++Cnt;
>
>    // Split groups by comparing relocations until convergence is obtained.
>    do {
>      Repeat = false;
>      forEachClass(
>          [&](size_t Begin, size_t End) { segregate(Begin, End, false); });
> -    ++Cnt;
>    } while (Repeat);
>
>    log("ICF needed " + Twine(Cnt) + " iterations");
>
>
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