John Fine wrote:
I don't actually know, but maybe the optimizer figures out that c is
never used and eliminates it.
I actually had an "c" stomped on. Looks like a correctness issue to me.
Here's what happens if you initialize "c":
> gcc align.c; ./a.out
sizeof(Bar) = 129
sizeof(Foo) = 640
&bar[0]=0x8049180, addr%128=0x0
&bar[1]=0x8049201, addr%128=0x1
&bar[2]=0x8049282, addr%128=0x2
&bar[3]=0x8049303, addr%128=0x3
foo[0].bar[0].c[128] = a
foo[0].bar[1].c[128] = b
foo[0].bar[2].c[128] = c
foo[0].bar[3].c[128] = d
> gcc -O1 align.c; ./a.out
sizeof(Bar) = 129
sizeof(Foo) = 640
&bar[0]=0x8049180, addr%128=0x0
&bar[1]=0x8049200, addr%128=0x0
&bar[2]=0x8049280, addr%128=0x0
&bar[3]=0x8049300, addr%128=0x0
foo[0].bar[0].c[128] = b
foo[0].bar[1].c[128] = c
foo[0].bar[2].c[128] = d
foo[0].bar[3].c[128] =
-----------
#include <stdio.h>
#define ALIGNED(n) __attribute__((__aligned__(n)))
typedef struct Bar {
char c[129];
} Bar ALIGNED(128);
typedef struct Foo {
Bar bar[4];
} Foo;
Foo foo[4];
main()
{
int i, j;
Foo *foop = &foo[0];
printf("sizeof(Bar) = %d\n", sizeof(Bar));
printf("sizeof(Foo) = %d\n", sizeof(Foo));
for (i=0; i < 4; i++) {
Bar *bar = &foop->bar[i];
printf("&bar[%d]=%p, addr%128=0x%lx\n",
i, bar, ((unsigned long)bar) % 128);
for (j=0; j < 129; j++) {
bar->c[j] = 'a' + i;
}
}
printf("foo[0].bar[0].c[128] = %c\n", foo[0].bar[0].c[128]);
printf("foo[0].bar[1].c[128] = %c\n", foo[0].bar[1].c[128]);
printf("foo[0].bar[2].c[128] = %c\n", foo[0].bar[2].c[128]);
printf("foo[0].bar[3].c[128] = %c\n", foo[0].bar[3].c[128]);
}
