Re: Altivec and ATLAS

[R Clint Whaley <rwhaley@cs.utk.edu>]

Nick,

I cc the atlas-comm list, so that people can keep abreast of what is going
on . . .

>I have a question about the L1 copy matmul.  Altivec code generally 
>requires data to be aligned on 128-bit boundaries.  One can work with 
>unaligned data but it requires extra work.  Is it possible to guarantee 
>that the copied version of the matrices in ATLAS are 128-bit aligned 
>even if the original matrices aren't?  Which portion of the code should 
>I look at?  The Altivec extensions include 128-bit aligned versions of 
>malloc and calloc, so perhaps I can just do a one or two line 
>replacement.

ATLAS already guarantees 128 bit alignment for everything except K-cleanup.
This was put in during the last release for SSE and 3DNow! support.  Here
is the relevant thread (note that 16 byte == 128 bit for discussion):
   http://www.netlib.org/atlas/atlas-comm/msg00144.html
Note that this is the alignment of the input matrices A and B *ONLY*, C has
no guaranteed alignment (C is often passed in by the user and not buffered
by ATLAS).  Is A and B enough, or do you believe you will need C aligned as
well (http://www.netlib.org/atlas/atlas-comm/msg00274.html gives a brief
overview of why copying C can be too costly)?

Cheers,
Clint