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Message-ID: <20170618191104.GK27071@port70.net>
Date: Sun, 18 Jun 2017 21:11:04 +0200
From: Szabolcs Nagy <nsz@...t70.net>
To: musl@...ts.openwall.com
Subject: Re: [PATCH] fix arm run-time abi string functions
* Szabolcs Nagy <nsz@...t70.net> [2017-06-18 20:50:07 +0200]:
> * Alexander Monakov <amonakov@...ras.ru> [2017-06-18 15:14:51 +0300]:
> > Sorry for a bit of extreme nit-picking, but
> >
> > On Tue, 21 Feb 2017, Szabolcs Nagy wrote:
> > > +__aeabi_memmove8:
> > > +__aeabi_memmove4:
> > > +__aeabi_memmove:
> > > + cmp r0, r1
> > > + bls 2f
> > > + cmp r2, #0
> > > + bxeq lr
> > > + add r0, r0, r2
> > > + add r2, r1, r2
> > > +1: ldrb r3, [r2, #-1]!
> > > + cmp r1, r2
> > > + strb r3, [r0, #-1]!
> > > + bne 1b
> > > + bx lr
> > > +__aeabi_memcpy8:
> > > +__aeabi_memcpy4:
> > > +__aeabi_memcpy:
> > > +2: cmp r2, #0
> > > + bxeq lr
> > > + sub r0, r0, #1
> > > + add r2, r1, r2
> > > +1: ldrb r3, [r1], #1
> > > + cmp r1, r2
> > > + strb r3, [r0, #1]!
> > > + bne 1b
> > > + bx lr
> >
> > here it's possible to hoist and deduplicate 'cmp r2, #0; bxeq lr' by placing it
> > prior to the direction check.
> >
>
> i dont think there is a difference
> (either way, both functions need to do this cmp once,
> the executed insns are the same you just changed the
> ordering a bit)
>
> > > +__aeabi_memclr8:
> > > +__aeabi_memclr4:
> > > +__aeabi_memclr:
> > > + mov r2, #0
> > > +__aeabi_memset8:
> > > +__aeabi_memset4:
> > > +__aeabi_memset:
> > > + cmp r1, #0
> > > + bxeq lr
> > > + and r2, r2, #255
> >
> > here I don't see why the 'and' is useful, I don't think the following strb
> > needs high bits cleared?
> >
>
> indeed
> (i wrote naive c code and compiled with gcc..
> for some reason it did the and so i kept it, but
> strb seems to ignore top bits according to spec)
>
> shall i resend without and?
>
i cannot reproduce the gcc issue with and now, however..
> > > + add r1, r0, r1
> > > +1: strb r2, [r0], #1
it seems strb with postindex+writeback is armv6t,
so does not work in armv4t, i'll send an updated patch.
> > > + cmp r1, r0
> > > + bne 1b
> > > + bx lr
> >
> > Alexander
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