Re: [Caml-list] Alignment of data

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

> I am working on some ppc architecture, and I realize that I have a 
> (very) big slowdown due to bad alignment of data by ocamlopt. I need to 
> have my data aligned in memory depending of the size of the data : 
> floats are to be aligned on 8 bytes, int on 4 bytes, etc....

First, make sure that misalignment is really the source of your
slowdown.  The PowerPC processors I'm familiar with can access
4-aligned 8-byte floats with minimal overhead, while the penalty is
much bigger for other misalignments.  Indeed, the PowerPC calling
conventions mandate that some 8-byte float arguments are passed on the
stack at 4-aligned addresses, so that's strong incentive for the
hardware people to implement those accesses efficiently.

> BUT, after verification, I remark that ocamlopt doesn't align as I need. 
> I tried to use ARCH_ALIGN_DOUBLE, but it doesn't seem to be what I 
> thought, and doesn't change anything for my needs. Is there ANY way to 
> obtain what I need easily or at least quickly ?

Data allocated in the Caml heap is word-aligned, where a word is 4
bytes on a 32-bit platform and 8 bytes on a 64-bit platform.  This is
deeply ingrained in the Caml GC and allocator, so don't expect to
change this easily.

What you can do, however:

1- Use the 64-bit PowerPC port.  Everything will be 8-aligned then.

2- Use a bigarray instead of a float array.  Bigarray data is
allocated outside the heap, at naturally-aligned addresses.

- Xavier Leroy