[Vm-dev] 64-bit compilation beneath a 32-bit image...

[Eliot Miranda]

Hi David, John, All,

    I'm working on integrating callbacks into the FFI and have the old
version of callbacks working.  In integrating with Cog I've realised some
cleanliness can come from a slightly different approach, the details of
which are irrelevant to my question, which is, when we're running a 32-bit
image above a 32-bit VM compiled in 64-bit mode, is there any way from the
image we can ask the VM if it is 64-bit or 32-bit?  Am I to understand
correctly that you, John, have arranged that when we're compiling the iOS VM
on a 64-bit Mac OS X, sizeof(long) = 8, and everything still works?  If not,
the question is moot.  If so, how from the image can I ask whether the VM is
64-bit, e.g. by asking what sizeof(long) or sizeof(void *) is?

If there isn't a way, I propose we add vmParameterAt: 70 to answer the VM's
underlying word size.  Tis is different to vmParameterAt: 40, which is the
image's underlying word size.

I need the answer to this so that when a callback tries to answer e.g. an
integer result it can tell whether it should use some Alien signedLongAt: N
or Alien signedLongLongAt: N to answer the result, and what machine code
(x86 vs x86-64) to use for the callback thunk.

best
Eliot
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