Re: [ANNOUNCE] Git v2.19.0-rc0

On Thu, 2018-08-23 at 06:26 -0400, Derrick Stolee wrote:

Around the time that my proposed approaches were getting vetoed for 
alignment issues, I figured I was out of my depth here. I reached out to 
Daniel Lemire (of EWAH bitmap fame) on Twitter [1]. His blog is full of 
posts of word-based approaches to different problems, so I thought he 
might know something off the top of his head that would be applicable. 
His conclusion (after looking only a short time) was to take a 'hasheq' 
approach [2] like Peff suggested [3]. Since that requires auditing all 
callers of hashcmp to see if hasheq is appropriate, it is not a good 
solution for 2.19 but (in my opinion) should be evaluated as part of the 
2.20 cycle.

That was an interesting blog post, indeed. It had an interesting
comments section. One comment especially caught my eyes was [a]:

"So the way gcc (and maybe clang) handles this is specifically by
recognizing memcmp and checking whether a only a 2-way result is needed
and then essentially replacing it with a memcmp_eq call.

..."

I find this to be an interesting note. It seems GCC does optimize when
we clearly indicate that we use the result of the memcmp as a boolean.
So would that help in anyway? Maybe it would help in writing a `hasheq`
method easily? I'm not sure.

[1] https://twitter.com/stolee/status/1032312965754748930

[2] 
https://lemire.me/blog/2018/08/22/avoid-lexicographical-comparisons-when-testing-for-string-equality/

[3] 
https://public-inbox.org/git/20180822030344.GA14684@sigill.intra.peff.net/

[4] 
https://public-inbox.org/git/7ea416cf-b043-1274-e161-85a8780b8e1c@gmail.com/

[a]: 
https://lemire.me/blog/2018/08/22/avoid-lexicographical-comparisons-when-testing-for-string-equality/#comment-344073

--
Sivaraam