Re: A plan for eliminating cc0

[James E Wilson <wilson at specifixinc dot com>]

Steven Bosscher wrote:
> As far as I can tell, there is no architecture requirement that p1 and
> p2 must be a register pair (ie. p6,p7 or p2,p3, etc.), but that seems
> to be the only form that GCC can produce.
> Perhaps this was done this way to avoid insn patterns with two sets?

It was done this way because no one has written real predication support 
yet.  And in this context, ifcvt.c doesn't count, because it only has 
the simplest of possible predication support.

In the absence of real predication support, the two predicates are 
always inverses of each other, and hence there is no advantage to 
treating them as separate registers.  We need both predicates so we can 
reverse branches, but there is no reason why we would ever need to treat 
them as anything other than a register pair.  Treating them as a 
register pair makes everything simpler, the md patterns, the support for 
reversing branches, the PR register allocation, etc.  They look just 
like the condition codes available on other targets when we do things 
this way.

When we have real predication support, the predicate register support in 
ia64.md will have to be generalized.  Until then, it seems to be an 
unnecessary luxury.
--
Jim Wilson, GNU Tools Support, http://www.SpecifixInc.com