Re: [PATCH 2/2] hashcmp: use memcmp instead of open-coded loop
- Date: Wed, 9 Aug 2017 11:06:45 -0400
- From: Jeff King <peff@xxxxxxxx>
- Subject: Re: [PATCH 2/2] hashcmp: use memcmp instead of open-coded loop
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:
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".