IA32: printing FP register variables

[Ben Combee]

> > At each instruction, the invariant that a is 0 from the bottom and b is
1
> > from the bottom holds.  Note, the bottom of the stack can be known by
the
> > debugger via scanning the FPU tag word from the TOP element looking for
a FP
> > register that is empty.  All this info is provided by an FSAVE or
FSTENV.
>
> That could work, but what if you happen to have all eight registers in
> use?

Then the scan code sees that there are no empty registers, so the eighth
register is the bottom of the stack.  The IA32 FPU itself uses this logic --
if you push something on the stack such that TOP gets incremented to point
to a non-empty value, you get a stack overflow.

Of course, all this is also based on the assumption, explicit in the Win32
ABI, that the FPU stack is empty on entry to a function.  I think this holds
for the x86 Linux ABI as well -- I can't see any logical other way to do it
since you don't generally know how many levels of nesting there are when
your function is called.