Re: IA32: printing FP register variables

["Ben Combee" <bcombee at metrowerks dot com>]

> > I would suggest that we may use a "negative" ST value.  The debugger can
> > always know the depth of the stack from reading the status registers, so
> > saying that something was in ST(7) could be interpreted as the top-most
> > stack item, ST(6) as one below that, and so on.  As long as the relative
> > position of items on the stack didn't change (this var is always 2 from
the
> > top), this should be OK.
>
> I think you get more stable position designations by considering the
> position relative to the bottom of the stack.

Yes, that's what I meant -- not what I really said, however.  When I was
writing that, I was thinking of memory stack growing downwards in memory,
hence my top/bottom mixmatch.  The temps are almost always at the top, with
allocated variables there at the bottom.

With the CW compiler, we generally use a similar idea to how the new EGCS
stuff works -- actually, we tend to first figure out the register pressure
from temps needed during expression evaluation, then allocate the some of
the variables to the stack locations available at any particular time.  So,
an item at the bottom of the stack will stay there.

I can see Jim's point -- if you use FXCHG to move items around on the stack,
something you need to do for efficient Pentium codegen, then a variable
won't have a stable location referenced from either the top or the bottom.
On newer chips like the Pentium II and K6, we've seen very little gain from
using FXCHG due to the automatic register renaming going on behind the
scenes.

I like the idea of DWARF2's more complex variable mapping ability that was
mentioned -- but, we need a binding to the complete set of IA32 registers:
integer, special purpose, FP/MMX, and SSE.