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__attribute__ aligned could be more efficient


Hello everyone,

Consider the following program.

#include <stdio.h>
#include <stddef.h>
struct foo1 { char s[80]; };
struct foo2 { char s[80]; }  __attribute__ ((aligned (64)));
struct bar1 { struct foo1 f; int i; };
struct bar2 { struct foo2 f; int i; };
#define P(arg) printf("sizeof(" #arg ") = %u\n", (unsigned)sizeof(arg))

int main(void)
{
  P(struct foo1);
  P(struct foo2);
  P(struct bar1); printf("offset=%u\n", (unsigned)offsetof(struct bar1, i));
  P(struct bar2); printf("offset=%u\n", (unsigned)offsetof(struct bar2, i));
  return 0;
}

$ gcc -O3 -Wall align.c 
$ ./a.out
sizeof(struct foo1) = 80
sizeof(struct foo2) = 128
sizeof(struct bar1) = 84
offset=80
sizeof(struct bar2) = 192
offset=128

I didn't expect sizeof(struct bar2) to be 192.

NB: I first sent this message to the wrong mailing list, where
Andreas pointed out that the "aligned" attribute effectively
changes the *size* of bar2.

In my real-life scenario, foo2 is typically used to define a
static object of type foo2, and the padding is expected.

But in some cases, we want to define a super-struct like bar2,
where the whole struct is 64-byte aligned, not the inner struct.

So how do we drop / cancel / override the alignment request
for foo2 in the bar2 definition? This doesn't work:

struct bar3 {
  struct foo2 f __attribute__ ((aligned (4)));
  int i;
} __attribute__ ((aligned (64)));

Do I have to duplicate the definition for struct foo2, one with
the align attribute, one without?

Regards.


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