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

[Russell King - ARM Linux]

On Wed, Jul 24, 2013 at 05:35:31PM +0100, Will Deacon wrote:
> 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.

And the result of its removal is all the horrid macro crap in asm/tlbflush.h
to work around the lack of this feature - otherwise we end up with seven
instances of:

	tst
	bne	1f
	mcr
1:

for every call to a TLB function.  GCC _could_ have detected the lack of
%? and the lack of "cc" clobber and either warned or implicitly branched
around the block.  There was no need to completely remove the facility.

So now, instead, we have to do lots of preprocessor junk to work out what
instructions are always executed, which are never executed, and which
might be executed.  All that additional complexity thanks to the removal
of this brilliant feature.

That was a truely sad day when that happened.