[MLton] latest MLton segfault in gmp

[Matthew Fluet]

On Sat, 10 Oct 2009, Wesley W. Terpstra wrote:
> On Sat, Oct 10, 2009 at 10:27 PM, Wesley W. Terpstra <wesley at terpstra.ca>wrote:
>
>> I've tried compiling with -align 8 and then it works... I'm not sure this
>> is a solution, though; it may have just masked the problem.
>
> Found the smoking gun! Debian builds gmp with -O3 whereas I used -O2 for
> MinGW32. If you look at the assembler output of mpz/mul_exp.c with the two
> options you will notice a difference... the introduction of a 'movdqa'
> instruction, which is an SSE2 instruction that expects 16-byte alignment.
...
> What's the plan going forward? align(AMD64) == 8?

Nice detective work.  However, from what you have described, it doesn't 
seem as though defaulting to '-align 8' on amd64{-linux?} is sufficient. 
That will guarantee that all objects are aligned on 8-byte boundaries, but 
it won't guarantee that IntInf arrays are aligned on 16-byte boundaries. 
Going to '-align 16' would waste quite a bit more space.

On the other hand, I don't see how the gmp.h header guarantees that a 
mp_limb_t* is 16-byte aligned.  It is simply a pointer to a 64-bit 
integer, so it seems that gcc can only assume that the allocated object 
pointed to by a mp_limb_t* is 8-byte aligned.  Am I misunderstanding the 
assembly?  Is it dynamically checking the alignment and using the SSE2 
instructions only if it happens to be 16-byte aligned?  In which case, 
then '-align 8' would be reasonable.