commit e67de3fa5ebbf23fd0685e1c04e3e424f1d50986
author Peter Collingbourne <peter@pcc.me.uk> Tue Jan 22 23:54:49 2019 +0000
committer Peter Collingbourne <peter@pcc.me.uk> Tue Jan 22 23:54:49 2019 +0000
tree beaea1f91692810f587d3efc98914b318c739131
parent f2a1028469505507e44e9a8b67aa64b378a81572

COFF, ELF: ICF: Perform 2 rounds of relocation hash propagation.

LLD's performance on PGO instrumented Windows binaries was still not
great even with the fix in D56955; out of the 2m41s linker runtime,
around 2 minutes were still being spent in ICF. I looked into this more
closely and discovered that the vast majority of the runtime was being
spent segregating .pdata sections with the following relocation chain:

.pdata -> identical .text -> unique PGO counter (not eligible for ICF)

This patch causes us to perform 2 rounds of relocation hash
propagation, which allows the hash for the .pdata sections to
incorporate the identifier from the PGO counter. With that, the amount
of time spent in ICF was reduced to about 2 seconds. I also found that
the same change led to a significant ICF performance improvement in a
regular release build of Chromium's chrome_child.dll, where ICF time
was reduced from around 1s to around 700ms.

With the same change applied to the ELF linker, median of 100 runs
for lld-speed-test/chrome reduced from 4.53s to 4.45s on my machine.

I also experimented with increasing the number of propagation rounds
further, but I did not observe any further significant performance
improvements linking Chromium or Firefox.

Differential Revision: https://reviews.llvm.org/D56986

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@351899 91177308-0d34-0410-b5e6-96231b3b80d8