[llvm-dev] RFC: Improving performance of HashString

Scott Smith via llvm-dev llvm-dev at lists.llvm.org
Mon Apr 24 17:37:53 PDT 2017

I've been working on improving the startup performance of lldb, and ran
into an issue with llvm::HashString.  It works a character at a time, which
creates a long dependency chain in the processor.  On the other hand, the
function is very short, which probably works well for short identifiers.

I don't know how the mix of identifier length seen by lldb compares with
that seen by llvm/clang; I imagine they're pretty similar.

I have to different proposals, and wanted to gauge which would be preferred:

1. Use xxhash instead.

2. Use the Intel native crc32q instruction to process 8 bytes at a time,
then fall back to byte at a time.  Non sse 4.2 capable processors (either
early or non Intel/AMD x86) would use the existing algorithm, or possibly
#1 above.

For my test, both result in approximately the same # of cycles (within

#1 uses 3+% more instructions.
#2 requires (realistically) runtime detection of cpu capabilities, because
distributions would want generic x86/x86_64 compatible binaries, not
separate binaries per cpu feature.

I'm leaning toward #1 despite the instruction increase.  My only worry is
the effect on compile times for code with lots of short identifiers.  I
haven't tested that though (and I don't have a suitable benchmark suite for
that) so for all I know I'm worrying about nothing.

FYI the improvement is approximately 11.5% reduction in cycles for my lldb
test (b main, run, quit), so IMO it's pretty significant.
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