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Date: Wed, 21 Apr 2021 13:38:48 -0400
From: Rich Felker <dalias@...c.org>
To: Érico Nogueira <ericonr@...root.org>,
musl@...ts.openwall.com
Subject: Re: [PATCH] shorten __aeabi_memset by one instruction
On Wed, Apr 21, 2021 at 10:24:58AM +0200, Szabolcs Nagy wrote:
> * Érico 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)
Should we hold off on doing anything about this for now then? I'd
rather avoid making more work for future, and this is pure *junk* code
that we do not expect to be called from anywhere (it's extremely slow)
and only there to satisfy broken tooling generating calls to it rather
than to the standard functions.
Rich
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