From: Jeff King <peff@peff.net>
Date: Wed, 09 Aug 2017 15:06:45 GMT
Subject: Re: [PATCH 2/2] hashcmp: use memcmp instead of open-coded loop
Message-ID: <20170809150644.ny565zdhki5zgdzr@sigill.intra.peff.net>
In-Reply-To: <2b111a78-dea4-85ff-3d8f-94d2eee0ca26@web.de>

On Wed, Aug 09, 2017 at 04:55:43PM +0200, René Scharfe wrote:

> > I also wondered if using memcmp() could be a hint to the compiler to use
> > an intrinsic or some other trick, especially because the "len" here is a
> > constant. But in a toy function compiled with "gcc -S", it looks like we
> > do keep the call to memcmp (so the speedup really is glibc, and not some
> > compiler magic).
> 
> GCC 7 inlines memcmp() if we only need a binary result:
> 
> 	https://godbolt.org/g/iZ11Ne

Cute.  It turns it into a series of 8-byte xors. The original open-coded
loop doesn't end up nearly as optimized with gcc-7.

I suspect many calls in practice are of the binary-result type. So some
of the speedup I saw may have been from compiler improvements and not
libc improvements. Still, I think the general argument is the same,
replacing "if your libc memcmp is fast" with "if your libc/compiler
makes memcmp fast".

-Peff