[llvm-dev] RFC: Improving performance of HashString

[Scott Smith via llvm-dev]

On Tue, Apr 25, 2017 at 1:11 PM, Davide Italiano <davide at freebsd.org> wrote:

> On Tue, Apr 25, 2017 at 12:55 PM, Vedant Kumar via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> >
> >> On Apr 24, 2017, at 5:37 PM, Scott Smith via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >>
> >> 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.
> >
>
> I'd lean towards this option as we already have a copy of xxHash in
> lib/Support (llvm/Support/xxhash.h).
> We use it for computing the --build-id in lld as fast/non-crytographic
> hash variant and has proven to outperform other candidates (e.g.
> CityHash).
>

That's actually why I suggested it, though AFAIK xxHash has more collisions
than other hash algorithms.

xxHash outperforms CityHash?  https://github.com/rurban/smhasher/  So many
choices!
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