Re: IA32: printing FP register variables

[Richard Henderson <rth@cygnus.com>]

On Mon, Jul 12, 1999 at 06:09:50PM -0500, Jim Blandy wrote:
> If GDB can find the base of the FP stack reliably, using the method
> that Ben Combee suggests, then we don't need LRS at all to describe
> variables' homes.  If we simply number the FP registers relative to
> the base of the FP stack, their names won't change as insns push and
> pop FP values.

You can find the base of the stack, it's true.  But that does you
no good whatsoever. 

Consider the following sequence:

	(set (reg f0) (add (reg f0) (reg f1)))
	(set (reg f2) (add (reg f2) (reg f3)))

Assuming no register deaths, so there's no popping to add to the
confusion, this will ultimately get transformed to

	fadd	%st(1)
	fxch	%st(2)
	fadd	%st(3)

Since there's no popping, the base of the FP stack has not changed.
However, f2 and f0 were swapped, so that f2 would be at the top of
the stack, so that we can perform the second operation.

That's not to say that you can't use the FP base to eliminate the
need to encode the pushes and pops into the debug information, but
you can't get away from bazillions of live ranges.


r~