G++ missing warning

uninit.c

#include <stdio.h>
#include <stdlib.h>

int main() 
{
	int a, b;
	float f, g = 1;
	if(rand() < 0.5) {
		b = 0;
	} else {
		b = 5;
	}
	for(a = 0; a < b; ++a) {
		/* you would expect the following line to trigger a warning "f is used uninitialized"... 
		   but that doesn't happen without optimization (-O)! */
		f = f + a;
	}
	g += f;
	printf("%f\n", g);
	return 0;
}