Re: [PATCH] Optimize "a || b" into "(a | b) != 0"

[Richard Henderson <rth at redhat dot com>]

On Fri, May 17, 2002 at 09:48:03PM -0600, Roger Sayle wrote:
> 	* fold-const.c (fold_truthop): Transform "a || b" into "(a|b) != 0"
> 	and "!p && !q" into "(p|q) == 0" under suitable conditions.

I guess this is ok for now.

I recall that Jan mentioned implementing some of this in ifcvt which
might be better at throttling itself when the chain gets too long.
Also consider that a long chain like

int foo(int a0, int a1, int a2, int a3, int a4, int a5, int a6, int a7)
{
  return a0 || a1 || a2 || a3 || a4 || a5 || a6 || a7;
}

is _not_ best implemented with a string of IORs on ia64.  There the
absolute best we could do with IORs is

	or  t1 = a0, a1
	or  t2 = a2, a3
	or  t3 = a4, a5
	or  t4 = a6 a7
	;;
	or  t1 = t1, t2
	or  t3 = t3, t4
	;;
	or  t1 = t1, t3
	;;
	cmp.ne p6, p7 = t1, r0
	;;
	(do something with p6)		; 4 cycles

(At present, I don't think we'll even re-organize the compares, so
we'll wind up with an 9-cycle linear chain of IORs.)

However, it also has a feature called a parallel comparison 
in which we can perform lots of comparisons on an open collector:

	cmp.ne p6, p7 = r0, r0
	;;
	cmp.ne.or.andcm  p6, p7 = a0, r0
	cmp.ne.or.andcm  p6, p7 = a1, r0
	cmp.ne.or.andcm  p6, p7 = a2, r0
	cmp.ne.or.andcm  p6, p7 = a3, r0
	cmp.ne.or.andcm  p6, p7 = a4, r0
	cmp.ne.or.andcm  p6, p7 = a5, r0
	cmp.ne.or.andcm  p6, p7 = a6, r0
	cmp.ne.or.andcm  p6, p7 = a7, r0
	cmp.ne.or.andcm  p6, p7 = a8, r0
	;;
	(do something with p6)		; 2 cycles

(I'm sort-of fudging here with "2", assuming that the initialization
can be folded into some free slot beforehand.  Which really isn't 
going out on a limb too far.)


r~