On Mon, 2021-04-19 at 12:05 +0100, Richard Earnshaw via Gcc-help wrote:
>
>
> On 18/04/2021 15:10, Matwey V. Kornilov via Gcc-help wrote:
> >
> > Hi,
> >
> > When a structure is declared as `__attribute__ ((packed))' what
> > should
> > it alignment be? Unfortunately, I have not found clear statement in
> > the
> > docs, but I've found the following very inconsistent behavior
> > instead.
> > It is not clear to me to what extent `__attribute__ ((packed))'
> > should
> > break alignment rules.
> >
> >
> > Imagine the following code:
> >
> > typedef struct {
> > unsigned char a;
> > unsigned char b;
> > unsigned short c;
> > unsigned int d;
> > unsigned long e;
> > } __attribute__ ((packed)) s;
> >
> > s x = {
> > .a = 1,
> > .b = 2,
> > .c = 3,
> > .d = 4,
> > .e = 5
> > };
> >
>
> Creating a static instance of an object and examining its actual
> alignment doesn't tell you anything concrete, as you've found. A
> compiler is free to over-align an instance of an object if it might
> lead
> to better code generation later on.
>
> A better test would be a case such as
>
> typedef struct {
> unsigned char a;
> unsigned char b;
> unsigned short c;
> unsigned int d;
> unsigned long e;
> } __attribute__ ((packed)) s;
>
> struct t
> {
> long long x; // Force 64-bit alignment
> char y; // One byte into next double-word
> needed.
> };
>
> unsigned u = __builtin_offsetof (t, z);
>
> You should find this stable across all optimization levels. On
> aarch64
> you'll get the answer 9, showing that the object has just byte
> alignment
> (no padding needed).
Would C99 _Alignof operator work for this? Like:
printf("%d\n", _Alignof(s));
--
Xi Ruoyao <xry111@xxxxxxxxxxxxxxxx>
School of Aerospace Science and Technology, Xidian University
