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

Will Deacon will.deacon at arm.com
Wed Jul 24 12:35:31 EDT 2013

On Wed, Jul 24, 2013 at 05:16:59PM +0100, Nicolas Pitre wrote:
> On Wed, 24 Jul 2013, Will Deacon wrote:
> > On Tue, Jul 23, 2013 at 08:48:11PM +0100, Nicolas Pitre wrote:
> > > 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.

Holy sh*t....

Next time I complain to the tools guys about inline asm, I'll count my
blessings so they don't try and resurrect anything as horrendous as what
you've just described.

Thanks for the insight!


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