RFC: Simplify rules for ctz/clz patterns and RTL

[Segher Boessenkool]

> * I would like to do the same for __builtin_ctz, but there is a catch.
> The synthetic ctz sequence in terms of popcount (as presently
> implemented by ia64.md, and potentially usable for at least i386 and
> rs6000 as well if moved to optabs.c) produces the canonical behavior at
> zero, but the synthetic sequence in terms of clz (as presently
> implemented by optabs.c) produces the value -1 at zero.  I have not 
> been
> able to think of any refinement to that sequence that would reliably
> produce GET_MODE_BITSIZE(mode) at zero in an efficient manner.
> Furthermore, -1 is the value most convenient for implementing ffs in
> terms of ctz.  Opinions and/or clever bit manipulation hacks would be
> much appreciated.

I suppose you're using (assuming 32-bit)

	ctz(x) := 31 - clz(x & -x)

now, which gives -1 for 0; and the version you're looking for is

	ctz(x) := 32 - clz(~x & (x-1))

which gives 32 for 0.

(Straight from the venerable PowerPC Compiler Writer's Guide, btw).

What does the popcount version look like?  Never seen that before,
but I think it will be really expensive on PowerPC.


Segher