RFC: using clz for comparing to zero on the tree level.

[Richard Earnshaw]

> On Thu, May 08, 2003 at 12:36:35PM -0400, David Edelsohn wrote:
> > 	Not Alpha and not ARM V5, which have single instruction clz?
> 
> PPC:
> 	cntlz	tmp, in
> 	srwi	out, tmp, 5
> 
> Alpha:
> 	cmpeq	in, 0, out
> 
> ARM:
> 	cmp	in, #0
> 	movne	out, #0
> 	moveq	out, #1
> 
> Hmm, so it might be a win for ARM.  You'll have to work out
> with Richard Earnshaw how you want to arrange these costs.
> At the moment arm_rtx_costs doesn't tell you that the SEQ
> operation takes 3 instructions.

for an seq expander the shortest (though not necessarily fastest) sequence 
on ARMv5 would be

	clz	t1, in
	mov	out, t1, lsr #5

But that's probably best left as a back-end expansion of the canonical RTL:

	(set (out) (eq (in) (const_int 0)))

which can be expanded after reload with a split pattern.

R.