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From: =?UTF-8?Q?Cristian_Rodr=C3=ADguez?= <crrodriguez@opensuse.org>
Date: Mon, 12 Dec 2022 11:57:36 -0300
Message-ID: <CAPBLoAcL=5bZaQ_XvoNJEtHBeChNrkXb2orPMwYXEErb6Wg5Kg@mail.gmail.com>
Subject: Re: Missing optimization: mempcpy(3) vs memcpy(3)
To: Wilco Dijkstra <Wilco.Dijkstra@arm.com>
Cc: "Alejandro Colomar (man-pages)" <alx.manpages@gmail.com>, GNU C Library <libc-alpha@sourceware.org>, 
	"gcc@gcc.gnu.org" <gcc@gcc.gnu.org>
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On Mon, Dec 12, 2022 at 11:35 AM Wilco Dijkstra via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> Hi,
>
> I don't believe there is a missing optimization here: compilers expand mempcpy
> by default into memcpy since that is the standard library call. That means even
> if your source code contains mempcpy, there will never be any calls to mempcpy.
>
> The reason is obvious: most targets support optimized memcpy in the C library
> while very few optimize mempcpy. The same is true for bzero, bcmp and bcopy.
>
> Targets can do it differently, IIRC x86 is the only target that emits calls both to
> memcpy and mempcpy.

yeah, x86_64 at least uses both, but I think open coded mempcpy needs
to be transformed into a library call anyway.

Other optimizations that are actually missing are:

-  the cases where of snprintf %s could become memccpy

- open coded memccpy could also be turned into library calls.