Parameters declared restrict and compiler warnings

Question

Neither gcc 5 nor clang 3.6 give warnings where the constraints of the restrict qualifier are violated, even when called with -Wall. Consider the following code fragment:

extern void f(char *restrict p, char *restrict q);

void g(char *p)
{
    f(p, p);
}

Naively, I'd expect that the violation can be determined statically, and I was expecting that -Wall would give a warning. Have I missed a flag somewhere, or is there some problem with giving warnings that I'm not seeing?

Answer 1

Starting with version 8, gcc gives a helpful warning for the above code:

a.c: In function ‘g’:
a.c:5:5: warning: passing argument 1 to restrict-qualified parameter aliases with argument 2 [-Wrestrict]
     f(p, p);
     ^

Answer 2

The restrict keyword is an explicit assurance from the programmer that the pointers in question don't alias. In essence, it allows the compiler to omit alias analysis for those pointers, because the programmer has already provided the presumed answer. In addition to enabling better optimization, this can also save compilation time. In large programs, analysis can be quite expensive, so that's potentially a big deal in its own right.

So, the answer to your question is, I believe, "the compilers aren't looking, because the code tells them not to bother"