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To: Noah Goldstein <goldstein.w.n@gmail.com>,
 Siddhesh Poyarekar <siddhesh@gotplt.org>
Cc: GNU C Library <libc-alpha@sourceware.org>
References: <20210607083011.855616-1-goldstein.w.n@gmail.com>
 <c070432c-ac11-4c81-7c5e-becf026a65ef@gotplt.org>
 <CAFUsyf+K7zMDSm-WFaP=_4+XWEQG3G=Utn0+vh91+DZi6yASEw@mail.gmail.com>
From: Paul Eggert <eggert@cs.ucla.edu>
Organization: UCLA Computer Science Department
Subject: Re: [PATCH v1] x86: memcmp-avx2-movbe.S and memcmp-evex-movbe.S fix
 overflow bug.
Message-ID: <8690911e-3cac-836b-2a8a-8b5b2ca74b02@cs.ucla.edu>
Date: Mon, 7 Jun 2021 10:45:41 -0700
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On 6/7/21 10:28 AM, Noah Goldstein via Libc-alpha wrote:
>=20
> Is it actually UB? The caller is not causing overflow. The implementati=
on
> method is. It is possible to implement without overflow.

The C Standard says that unless otherwise specified, when you pass an=20
array (pointer + size) to a standard function, all the addresses in the=20
array must be valid. It's valid if (say) memcmp is multithreaded and=20
compares the first halves of the two arrays in parallel with comparing=20
the second halves.

If I understand things correctly this patch isn't fixing a conformance=20
bug; it's merely a QoI issue, where by "quality" one means "I want this=20
particular undefined behavior to cause a core dump".