On 16/11/11 18:27, David Matthews wrote:
On 16/11/2011 18:07, Phil Clayton wrote:
The calling conventions for x86_64 appear to pass via registers not only
the initial floating point arguments but also the initial int/pointer
arguments:
http://en.wikipedia.org/wiki/X86_calling_conventions
So I suspect all arguments, not just floating point, would be an issue
for callbacks on x86_64?
There is a separate issue with floats/doubles on x86_64 when calling
from SML to C but that doesn't relate to callbacks. It looks like an
exception is meant to be raised but isn't currently. (Perhaps the
preprocessor symbol X86_64 should be HOSTARCHITECTURE_X86_64 in
foreign.cpp?)
But now I have confused myself. If x86_64 calling conventions pass
initial int/pointer args via registers as well as floating point args,
what is it about double/float args that is problematic? (Comment
foreign.cpp, line 1037.) Also, a comment (foreign.cpp, line 1041)
suggests that the size of doubles is the issue but on x86_64 but
pointers are also 8 bytes.
Right. Those preprocessor symbols are wrong. I think I changed them
elsewhere in the code and somehow those got missed. I think the issue is
that on X86/32 all arguments are passed on the stack but on the X86/64
integer-like arguments are passed in the general registers but floating
point values are passed in floating-point registers. The code in
apply_rec casts everything to (void*) which means that the foreign
function will always be called with the arguments in the general
registers even if it is expecting them in FP registers. This is frankly
horrible and really this should be changed. Instead of interpreting the
type information when the foreign function is called it should compile a
stub which will convert the arguments into the correct format. I did
find a project that would do something like that but it didn't deal with
call-backs at all.
Thanks - knowing that everything is cast to (void *) makes the issue
much clearer.
Phil
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