Re: i386 flags register clober in inline assembly

[Richard Henderson <rth@twiddle.net>]

On Sat, Nov 17, 2001 at 06:48:22PM -0800, Linus Torvalds wrote:
> That sounds pretty ideal - have some way of telling gcc to add a "seta
> %reg", while at the same time telling gcc that if it can elide the "seta"
> and use a direct jump instead, do so..

Hmm.  It appears to be easy to do with machine-dependent builtins.  E.g.

	int x;
	__asm__ __volatile__(LOCK "subl %1,%0"
			     : "=m"(v->counter) : "ir"(i) : "memory");
	x = __builtin_ia32_sete();
	if (x) {
		...
	}

Now, you'd have to be careful in where that __builtin_ia32_sete
gets placed, but I'd guess that immediately after an asm would
be relatively safe.  No 100% guarantees on that, unfortunately.

And the sete _ought_ to get merged with the if test by combine
or cse with no extra code.

It wouldn't take too much effort to try this out either...


r~