[llvm-dev] [lld] We call SymbolBody::getVA redundantly a lot...

Sean Silva via llvm-dev llvm-dev at lists.llvm.org
Tue Feb 28 04:19:32 PST 2017

tl;dr: it looks like we call SymbolBody::getVA about 5x more times than we
need to

Should we cache it  or something? (careful with threads).

Here is a link to a PDF of my Mathematica notebook which has all the
details of my investigation:

There seem to be two main regimes that we redundantly call

1. most redundant calls on the same symbol (about 80%) happen in quick
succession with few intervening calls for other symbols. Most likely we are
processing a bunch of relocations right next to each other that all refer
to the same symbol (or small set of symbols); e.g. within a TU

2. there is a long-ish tail (about 20% of calls to SymbolBody::getVA) which
happen at a long temporal distance from any previous call to
SymbolBody::getVA on the same symbol. I don't know off the top of my head
where these are coming from, but it doesn't sound like relocations. A quick
grepping shows a bunch of source locations that match getVA, so it's hard
at a glance to see. Any ideas where these other calls are coming from?

The particular link I was looking at was a release without debug info link,
using `-O0 --no-gc-sections --no-threads`. The particular test case is LLD

-- Sean Silva
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