Re: IA32: printing FP register variables

[Jim Blandy <jimb@cygnus.com>]

> 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.

If you don't know the base of the FP stack, you have to start a new
range at each push, pop, or exchange.  If you do know the base, you
only need to start a new range at an exchange.

So you're right --- the design of the IA32 FPU requires that variables
change homes pretty frequently.  So LRS is necessary.  But numbering
registers relative to the FP stack base will still reduce the number
of ranges substantially, won't it?