Will Deacon <will.deacon@xxxxxxx> writes: > Hi Mans, > > On Tue, Jul 01, 2014 at 06:24:43PM +0100, Måns Rullgård wrote: >> Russell King - ARM Linux <linux@xxxxxxxxxxxxxxxx> 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). Even ones where the TRM says they do? I suppose the only way to know for sure is to measure it. >> > 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. Agreed. -- Måns Rullgård mans@xxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
