primitive vector types

[Daniel Keep]

Jarrett Billingsley wrote:
> On Thu, Feb 19, 2009 at 9:31 PM, Daniel Keep
> <daniel.keep.lists at gmail.com> wrote:
>>> struct float4
>>> {
>>>     __align(16) float[4] data; // right syntax and value?
>>>     alias data this;
>>> }
> 
>> 1. The alignment thing.  Incidentally, I just did a quick check and
>> don't see any notes in the changelog about __align(n) syntax.  As I
>> remember, there was no way to actually ensure the data was properly
>> aligned.  (There's "Data items in static data segment >= 16 bytes in
>> size are now paragraph aligned." but that doesn't help when the vectors
>> are on, say, the stack or in the heap.)
> 
> It's just align(16).
> 
> http://www.digitalmars.com/d/1.0/attribute.html#align

Yeah, except it doesn't do what you want in this case.  For example:


module alignment;

import tango.io.Stdout;

struct float4
{
    align(16) float[4] data;
}

void main()
{
    float4 a;
    byte b;
    float4 c;

    Stdout.format("a @ {0} & 0xF == {1}", &a, cast(size_t)&a & 0xF).newline;
    Stdout.format("c @ {0} & 0xF == {1}", &c, cast(size_t)&c & 0xF).newline;
}



Output is:

a @ 12fe68 & 0xF == 8
c @ 12fe78 & 0xF == 8



The only way I found of guaranteeing alignment was to allocate all
vectors on the heap, using a custom allocator to allocate an extra 15
bytes and then align the result appropriately.  Obviously, for 16-byte
vectors, this is completely unacceptable.

  -- Daniel