Altivec + 16 byte alignment

[Gianni Mariani]

Gianni Tedesco wrote:

>Hi,
>
>I'm currently working on a project which uses altivec (as inline
>assembly) to perform 3d affine transformations, and some other 3d vector
>functions.
>
>I have a problem in that altivec requires all inputs to be 16 byte
>aligned, so I am placing my vectors in a structure like this:
>
>struct vector {
>	float x,y,z,w;
>}__attribute__((aligned(16)));
>
>This seems to work for global variables, but when allocated on the
>stack, my structures end up mis-aligned (on to 8-byte boundaries) which
>totally screws up the results of the altivec computation.
>
>Is this a compiler problem, or can't the compiler be expected to
>guarantee alignments that large?
>

This is a problem with the malloc implementation.

Most malloc's support 8 byte alignment.  You could specifically tune one 
of the
many malloc packages out there to be 16 byte aligned and hence this 
would solve
your problem.

>
>gcc version 2.95.4 20010319 (prerelease/franzo/20011204)
>
>PS. Is there any documentation about the vector support directly in C? I
>hear gcc3 has that feature.
>
>Thanks.
>
>PS. Please CC replies, im not a list subscriber.
>
>  
>