Re: [ANNOUNCE] Git v2.19.0-rc0

From: Kaartic Sivaraam <kaartic.sivaraam@gmail.com>
Date: Sep 2, 2018, 18:53 UTC
Message-ID: <39ec35843c3164a9da75466b768636cb7871690d.camel@gmail.com>
In-Reply-To: <f854aba0-6d28-7f2b-aad2-858983c4af36@gmail.com>

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

Show 12 quoted lines

> 
> 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.

Show 10 quoted lines

> [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