Optimization attributes const vs. pure

[Johannes Bauer <dfnsonfsduifb@xxxxxx>]

Hi list,

I have a question about the optimizations conducted by gcc when
functions are declared pure or const. I'm using gcc 4.6.2 on Gentoo
Linux x86-64. Here's a minimal example:

----8<----8<---- int.c ----8<----8<----8<
#include <string.h>
#include "int.h"
int intcmp(const struct mi *a, const struct mi *b) {
	return memcmp(a, b, sizeof(struct mi));
}



----8<----8<---- int.h ----8<----8<----8<
struct mi {
	int foo;
	unsigned char x[16];
};

int intcmp(const struct mi *a, const struct mi *b) __attribute__ ((pure));


----8<----8<---- main.c ----8<----8<----8<
#include <stdio.h>
#include <string.h>
#include "int.h"
int main() {
	struct mi a, b;
	memset(&a, 0, sizeof(struct mi));
	memset(&b, 0, sizeof(struct mi));
	for (unsigned int i = 0; i < 100; i++) {
		fprintf(stderr, "%d\n", intcmp(&a, &b));
	}
	return 0;
}


I'm compiling using

$ gcc -O3 -march=nocona -std=c99 main.c int.c -o out

This is the observation I'm making: The generated assembly does not
differ from a function declared as "pure" from a function not declared
with any special attribute.

As soon as I declare the function (wrongly!) as const, the optimization
that I'd expect with "pure" occurs (i.e. intcmp() is only called once).

Now from my understanding I cannot use the attribute "const" for
intcmp() since intcmp() dereferences the pointers that it's passed (and
thus accesses global memory in a reading fashion).

So "pure" would be the perfect fit: Global memory is read but not
modified (which is also asserted by passing the arguments as "const").
Why is gcc then not doing the optimization that I'd want it to perform?

Best regards,
Joe