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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

llvm-svn: 351898
This commit is contained in:
Peter Collingbourne 2019-01-22 23:51:35 +00:00
parent 881cae7a45
commit 3426111145
2 changed files with 2 additions and 2 deletions
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2
lld/COFF/ICF.cpp
View File

@ -271,7 +271,7 @@ 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);
});

2
lld/ELF/ICF.cpp
View File

@ -431,7 +431,7 @@ 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);