[PATCH v1] x86: memcmp-avx2-movbe.S and memcmp-evex-movbe.S fix overflow bug.

[Siddhesh Poyarekar]

On 6/7/21 2:00 PM, Noah Goldstein via Libc-alpha wrote:
> Fix bugs introducted in commits:
> 
> author	Noah Goldstein <goldstein.w.n@gmail.com>
> Mon, 17 May 2021 17:57:24 +0000 (13:57 -0400)
> commit	4ad473e97acdc5f6d811755b67c09f2128a644ce
> 
> And
> 
> author	Noah Goldstein <goldstein.w.n@gmail.com>
> Mon, 17 May 2021 17:56:52 +0000 (13:56 -0400)
> commit	16d12015c57701b08d7bbed6ec536641bcafb428
> 
> Which added a bug which would cause pointer + length overflow to lead
> to an early return as opposed to a Segmentation Fault.

If we end up making this change, IMO it should come with an explicit 
note that this behaviour is not guaranteed for invalid inputs in other 
implementations of memcmp or for that matter, in future versions of this 
memcmp.

An input that causes pointer + length overflow is undefined behaviour 
and IMO we shouldn't try to define it for glibc.  This change may not 
have a noticeable performance impact but future requests to guarantee 
this behaviour may not necessarily be this straightforward and IMO we 
should not bind ourselves to it.

Siddhesh