Follow @Openwall on Twitter for new release announcements and other news
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <20170618185007.GI27071@port70.net>
Date: Sun, 18 Jun 2017 20:50:07 +0200
From: Szabolcs Nagy <nsz@...t70.net>
To: musl@...ts.openwall.com
Subject: Re: [PATCH] fix arm run-time abi string functions

* 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?

> > +	add   r1, r0, r1
> > +1:	strb  r2, [r0], #1
> > +	cmp   r1, r0
> > +	bne   1b
> > +	bx    lr
> 
> Alexander

Powered by blists - more mailing lists

Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.