On 11/09/2015 12:09 PM, Florian Weimer wrote:
> On 11/09/2015 11:11 AM, Andrew Haley wrote:
>> On 08/11/15 19:34, Segher Boessenkool wrote:
>>> 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.
>
> I'm not sure if this is a valid transformation for printf, even if
> targets stdout and does not use any custom format specifiers. Isn't it
> a cancellation point? But let's assume it's not.
>
>> Yes, that is correct. And, indeed, the hardware is free to do taht
>> too. With speculative execution, the "as if" rule is not limited to
>> the compiler.
>
> Can we disallow that optimization as a quality-of-implementation matter?
> What would be the benefit of such optimizations, other than
> discouraging programmers from using C or C++?
There isn't really any way to distinguish between wanted optimizations
and unwanted ones. If GCC determines that a statement is unreachable
it can be deleted, and this depends on its knowledge of UB. Like this:
void foo(int b) {
if (b > 0) {
int m = b * 3 / 6;
if (m < 0)
die();
}
}
}
Deleting such unreachable code happens all the time. IMO we should
not disable this optimization.
> I'm worried that this particular line of argument would also allow the
> movement of undefined behavior which occurs after an infinite loop in
> front of it, even if this loop performs I/O.
Sure. But it can already do that even if the compiler does not move
anything. The I/O writes to a stream, the UB causes a segfault which
kills a process, the stream never gets written.
Andrew.
