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Re: Optimization attributes const vs. pure
- From: Ángel González <keisial at gmail dot com>
- To: Johannes Bauer <dfnsonfsduifb at gmx dot de>
- Cc: gcc-help at gcc dot gnu dot org
- Date: Mon, 28 May 2012 16:40:14 +0200
- Subject: Re: Optimization attributes const vs. pure
- References: <4FC37103.9000300@gmx.de>
On 28/05/12 14:35, Johannes Bauer wrote:
> 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?
I have no idea.
I wondered if it feared fprintf changing global variables on which
intcmp() depended,
so I modified it to create the following program, where there is no
side-effect.
But it still exhibits the same behavior (intcmp called 100 times if
pure, 1 if const).
#include <stdio.h>
#include <string.h>
#include "int.h"
int main() {
struct mi a, b;
int vals[100];
memset(&a, 0, sizeof(struct mi));
memset(&b, 0, sizeof(struct mi));
for (unsigned int i = 0; i < 100; i++) {
vals[i] = intcmp(&a, &b);
}
for (unsigned int i = 0; i < 100; i++) {
fprintf(stderr, "%d\n", vals[i]);
}
return 0;
}