[MLton] Re: r6701

[Matthew Fluet]

On Tue, 19 Aug 2008, Wesley W. Terpstra wrote:
> I'm currently in paper-deadline-before-vacation-pressure-mode, so I haven't
> gotten to finishing the PIC for amd64 external calls or PIC of any kind for
> i386. The amd64 stuff seems easy and I'll get to it soon. Like I discussed
> with you earlier, i386 is quite a bit harder, particularly because %esp is
> not usable in MLton assembly.

For Cygwin and MinGW, which don't support sigaltstack, we can't use %esp 
other than to point to the top of the C stack.  Look for uses of 
reserveEsp in x86-codegen/*.

> Why is %ebp used for the ML stack anyway? Since %esp gets pushed into
> c_stackP, I'd have thought %esp would be perfectly fine for use as the ML
> stack too.

See
   http://mlton.org/pipermail/mlton/2002-May/021982.html
for some comments about using %esp.

> Would it be safe to push 4 bytes to the ML stack, if %esp were
> pointing at it?

No.  One issue is that the ML stack grows from low addresses to high 
addresses, so the push and pop instructions don't move it in the right 
direction.

> ie, could I do this:
> call tmp_label
> tmp_label: pop %eax
> mov gcState-tmp_label+someOffsetIntoGcState(%eax),%eax
> to load a value out of the gcState array into %eax? The concern is that I
> need to be sure that those 4 bytes at %esp are available. I don't really
> understand how the limit checking of the ML stack works.

I'm not sure exactly what the above code is meant to accomplish.