[llvm-dev] RFC: Improving performance of HashString

[Vedant Kumar via llvm-dev]

> On 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).
> 
> Vedant, what do you mean with unsupported?

Apologies. I misread my grep results. I saw an #ifdef containing 'xxhash' and 'support', so I thought xxhash support was conditional in some way. I didn't realize it's a portable hash.

vedant

> 
> -- 
> Davide
> 
> "There are no solved problems; there are only problems that are more
> or less solved" -- Henri Poincare