Commit 40cbc004 authored by Hans Wennborg's avatar Hans Wennborg
Browse files

Merging r351898: 

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r351898 | pcc | 2019-01-23 00:51:35 +0100 (Wed, 23 Jan 2019) | 13 lines

COFF, ELF: Adjust ICF hash computation to account for self relocations.

It turns out that sections in PGO instrumented object files on Windows
contain a large number of relocations pointing to themselves. With
r347429 this can cause many sections to receive the same hash (usually
zero) as a result of a section's hash being xor'ed with itself.

This patch causes the COFF and ELF linkers to avoid this problem
by adding the hash of the relocated section instead of xor'ing it.
On my machine this causes the regressing test case
provided by Mozilla to terminate in 2m41s.

Differential Revision: https://reviews.llvm.org/D56955
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llvm-svn: 351963
parent f669262c

lld/COFF/ICF.cpp

+1 −1
Original line number Diff line number Diff line
@@ -272,7 +272,7 @@ void ICF::run(ArrayRef<Chunk *> Vec) { 
    uint32_t Hash = SC->Class[1];

    for (Symbol *B : SC->symbols())

      if (auto *Sym = dyn_cast_or_null<DefinedRegular>(B))

        Hash ^= Sym->getChunk()->Class[1];

        Hash += Sym->getChunk()->Class[1];

    // Set MSB to 1 to avoid collisions with non-hash classs.

    SC->Class[0] = Hash | (1U << 31);

  });

lld/ELF/ICF.cpp

+1 −1
Original line number Diff line number Diff line
@@ -432,7 +432,7 @@ static void combineRelocHashes(InputSection *IS, ArrayRef<RelTy> Rels) { 
    Symbol &S = IS->template getFile<ELFT>()->getRelocTargetSym(Rel);

    if (auto *D = dyn_cast<Defined>(&S))

      if (auto *RelSec = dyn_cast_or_null<InputSection>(D->Section))

        Hash ^= RelSec->Class[1];

        Hash += RelSec->Class[1];

  }

  // Set MSB to 1 to avoid collisions with non-hash IDs.

  IS->Class[0] = Hash | (1U << 31);