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Date: Wed, 21 Apr 2021 10:24:58 +0200
From: Szabolcs Nagy <nsz@...t70.net>
To: √Črico Nogueira <ericonr@...root.org>
Cc: musl@...ts.openwall.com
Subject: Re: [PATCH] shorten __aeabi_memset by one instruction

* √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)

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

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