Re: float array alignment; was Re: [Caml-list] Bigarray access speed

[Xavier Leroy <xavier.leroy@inria.fr>]

malc writes:
> To get maximal speed of the inner loops data needs to be naturally 
> aligned. OCaml does nothing to enforce it for non-big arrays. Bigarrays on 
> the other hand are mmaped(4k on IA32) and you get perfectly aligned data 
> for free. I was thinking that maybe Array can be extended with
> make[create]_aligned, for speed/space tradeoff.

As William Chester said, allocating 8-aligned arrays isn't really
hard, but keeping them 8-aligned across copying collection,
compaction, and structured I/O is quite a pain.

My experiments indicate that the lack of alignment on float arrays 
(or more precisely the fact that they are 4-aligned instead of
8-aligned) has negligible impact on performance for the IA32 (Pentium)
and PowerPC processors, but non-negligible for SPARC and MIPS.
And of course on a 64-bit architecture the problem goes away because
everything in the Caml heap is then 8-aligned.  Since I expect IA32
and PowerPC to remain dominant until we massively switch to 64-bit
processors, there's no urgent need to do something about float array
alignment.

- Xavier Leroy
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