Alignment of struct containing SIMD field - GDC

[Iain Buclaw via Digitalmars-d-learn]

On Wednesday, 1 March 2017 at 06:04:32 UTC, Cecil Ward wrote:
> struct vec_struct {
>     alias field this;
>     bool b;
>     int8 field;
>     }
>
> In this code when you look at the generated x64 code output by 
> GDC it seems to be doing a nice job, because it has got the 
> offset right for the 256-bit YMM 'field' correct.
>
> Does D automatically propagate the alignment restrictions on 
> the field to the allocation of static structs or structs on the 
> stack?
>
> In this case:
>
> struct vec_struct {
>     bool b2;
>     struct {
>         alias field this;
>         bool b;
>         int8 field;
>         }
>     }
> it appears that the offset to 'field' is no longer aligned 
> correctly - offset is 40 bytes in GDC. I don't suppose the 
> compiler will use solely unaligned instructions? In any event, 
> I could take the address of field and then pass that to someone 
> expecting to pick up something with guaranteed correct 
> alignment, if I have understood the D docs.
>

Yeah, it looks like the semantic analysis pass is defeating us 
here.  It's adding the anonymous struct offset to the aligned 
field, ignoring completely the original alignment.

Or... the anonymous struct alignment != largest field alignment, 
which probably is the more likely scenario.

Please raise a bug against DMD.

Simple test case would be:

struct vec_struct {
     bool b2;
     struct {
         bool b;
         int8 field;
     }
}

static assert(vec_struct.b.offsetof == 32);
static assert(vec_struct.field.offsetof == 64);


>
> Noob q: I notice that the GDC opcodes look a bit odd, for 
> example the compiler generates a 256-bit unaligned fetch 
> followed by an aligned binary operation (I think), eg a movdqu 
> followed by a vpaddd r, ymm ptr blah - is the latter 
> aligned-only? Apologies if I have got this wrong, need to read 
> up. Would someone sanity-check me?

The x86 allows unaligned loads.  But if this is referencing the 
above data structure, you shouldn't be seeing this if the fields 
were correctly aligned.