Optimisations and undefined behaviour

[Segher Boessenkool]

On Sun, Nov 08, 2015 at 08:11:08PM +0100, Florian Weimer wrote:
> On 11/06/2015 01:32 PM, David Brown wrote:
> > How about this case:
> > 
> > int foo(int x) {
> > 	if (x > 1290) {
> > 		printf("X is wrong here %d, but we don't care\n", x);
> > 	}
> > 	return x*x*x;
> > }
> > 
> > The compiler can eliminate the check and the printf.
> 
> I don't think the compiler can do that because printf has an externally
> visible effect, which is sequenced before the undefined behavior, so
> this program transformation would not be permitted under the as-if rule.

The compiler is free to transform it to

int foo(int x) {
	int t = x*x*x;
	if (x > 1290) {
		printf("X is wrong here %d, but we don't care\n", x);
	}
	return t;
}

because x*x*x does not have any observable behaviour, and then it is
obvious it _can_ remove the printf and conditional.  Undefined behaviour
is not observable behaviour.

No?


Segher