[PATCH 1/6] ARM: prefetch: remove redundant "cc" clobber

[Nicolas Pitre]

On Wed, 24 Jul 2013, Will Deacon wrote:

> Hi Nicolas,
> 
> On Tue, Jul 23, 2013 at 08:48:11PM +0100, Nicolas Pitre wrote:
> > On Tue, 23 Jul 2013, Will Deacon wrote:
> > 
> > > The pld instruction does not affect the condition flags, so don't bother
> > > clobbering them.
> > > 
> > > Signed-off-by: Will Deacon <will.deacon at arm.com>
> > 
> > Acked-by: Nicolas Pitre <nico at linaro.org>
> 
> Thanks for reviewing the series, much appreciated.
> 
> > You know the legacy reason why the cc clobber was there, right?
> > It certainly doesn't apply anymore.
> 
> I was under the impression that it was due to ancient GCC behaviour and the "cc"
> was there to prevent re-ordering. For interest, what are the specifics?

In the old days, you had to take care of the predicate for conditional 
instructions in your inline asm code.  So for example, if you had:

	if (foo)
		inline asm ("mov %0, #1" : "=r" (bar));

That wouldn't work most of the time because even if the if condition was 
false, gcc would not branch over the inline asm code but expect you to 
take care of the condition code.  That means the above needed to be 
written as:

	if (foo)
		inline asm ("mov%? %0, #1" : "=r" (bar));

Because having %? added all over the place was cumbersome, the 
alternative was to add "cc" to the clobber list so gcc would then branch 
across the inline asm when the condition is false.

But of course many people forgot about those subtle details quite often, 
and then gcc was changed so inline asm was made non predicated.


Nicolas