Dear Xavier Leroy,
thank you for your answer

>> 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.

I am sorry, but I am sure of that. I ran some tests to ensure that the  
problem
is coming from that particular point.

> 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.

I didn't expect to change myself such a deep feature in ocaml, but I  
hoped
that you or somebody in your team could. Could it be possible to have  
everything
8 aligned on a 32-bit platform with minimum efforts ? Any help is  
welcomed !

> What you can do, however:
>
> 1- Use the 64-bit PowerPC port.  Everything will be 8-aligned then.

Is there a 64-bit PowerPC Linux (ELF) support in ocaml ? I thought it  
was only
a 64-bit PowerPC OSX (Darwin) support...

Thank you to Goswin von Brederlow and Pascal Cuoq for their answers,  
but I should
say that I really prefer to use the GC as usual, without rewriting it :)

Christophe