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Date: Wed, 21 Apr 2021 08:32:54 -0400
From: Jesse DeGuire <jesse.a.deguire@...il.com>
To: musl@...ts.openwall.com, √Črico Nogueira <ericonr@...root.org>
Subject: Re: [PATCH] shorten __aeabi_memset by one instruction

On Wed, Apr 21, 2021 at 4:25 AM Szabolcs Nagy <nsz@...t70.net> wrote:
>
> * √Črico Nogueira <ericonr@...root.org> [2021-04-20 16:15:19 -0300]:
> > when building for armhf, this makes libc.so text smaller by 4 bytes:
> > 606619 to 606615
> > ---
> >  src/string/arm/__aeabi_memset.s | 3 +--
> >  1 file changed, 1 insertion(+), 2 deletions(-)
> >
> > diff --git a/src/string/arm/__aeabi_memset.s b/src/string/arm/__aeabi_memset.s
> > index f9f60583..980774e8 100644
> > --- a/src/string/arm/__aeabi_memset.s
> > +++ b/src/string/arm/__aeabi_memset.s
> > @@ -24,8 +24,7 @@ __aeabi_memset:
> >       cmp   r1, #0
> >       beq   2f
> >       adds  r1, r0, r1
> > -1:   strb  r2, [r0]
> > -     adds  r0, r0, #1
> > +1:   strb  r2, [r0], #1
>
> this is not available before armv7 as thumb instruction (and it
> has 32bit thumb encoding, so you replace two 16bit instructions
> with a 32bit one.)
>
> normally this asm is compiled in arm mode even if your toolchain
> defaults to thumb (i'm not sure why), but if you select a cpu or
> arch that only supports thumb then the assembler will try to use
> thumb and fail e.g. on -march=armv6-m (but i'm not sure if musl
> supports that compilation mode throughout)
>

Musl currently does not build on Thumb1-only devices (Armv6-m and
Armv8-m.base). I submitted a patch some time ago to allow it to build,
though life and other obstacles have made me much too slow in actually
getting a functioning toolchain together to test it properly.

> >       cmp   r1, r0
> >       bne   1b
> >  2:   bx    lr
> > --
> > 2.31.1

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