Re: [PATCH] ARM: convert all "mov.* pc, reg" to "bx reg" for ARMv6+ (part1)

[Will Deacon <will.deacon@arm.com>]

Hi Mans,

On Tue, Jul 01, 2014 at 06:24:43PM +0100, Måns Rullgård wrote:
> Russell King - ARM Linux <linux@arm.linux.org.uk> writes:
> > As you point out, "bx lr" /may/ be treated specially (I've actually been
> 
> Most, if not all, Cortex-A cores do this according the public TRMs.
> They also do the same thing for "mov pc, lr" so there will probably be
> no performance gain from this change.  It's still a good idea though,
> since we don't know what future cores will do.

Funnily enough, that's not actually true (and is more or less what prompted
this patch after discussion with Russell). There are cores out there that
don't predict mov pc, lr at all (let alone do anything with the return
stack).

> > discussing this with Will Deacon over the last couple of days, who has
> > also been talking to the hardware people in ARM, and Will is happy with
> > this patch as in its current form.)  This is why I've changed all
> > "mov pc, reg" instructions which return in some way to use this macro,
> > and left others (those which are used to call some function and return
> > back to the same point) alone.
> 
> In that case the patch should be fine.  Your patch description didn't
> make it clear that only actual returns were being changed.

I'm led to believe that some predictors require lr in order to update the
return stack, whilst others don't. That part is all horribly
micro-architectural, so the current patch is doing the right thing by
sticking to the ARM ARM but enabling us to hook into other registers later
on if we choose.

Cheers,

Will