attribute aligned(4) ignored

[Ian Lance Taylor]

Chris Lattner <sabre@nondot.org> writes:

> Hi All, I'm trying to reduce the alignment of the double in this
> structure to 4 bytes (from 8) on Darwin.  The goal of doing this is to
> reduce the structure to 12 bytes in size.  Here is my testcase:
> 
> struct Test {
>    double D __attribute__((aligned(4)));    // only 4 bytes, not 8!
>    int X;
> };
> 
> int X() {
>    return sizeof(struct Test);    // Should return 12, not 16.
> }
> 
> Despite my attempt above, the structure is still 16 bytes in size, due
> to the tail padding required to pad the structure to ensure 8-byte
> alignment.
> 
> Does anyone know how to reduce the alignment of the double,
> eliminating the tail padding?

How about

   double D __attribute__((aligned(4), packed));

Ian