virtual stack regs.

[Kenneth Zadeck]

I would like to get some more information about pr32374.

I do not know what virtual_stack_vars are and there is no documentation
in the doc directory.

1) What are these?

2) Why are they uninitialized?

3) If they really are uninitialized, why is it a problem to assign zero
to them.

4) If they are not uninitialized, where is the initialization code? Why
does df not see it?

5) How can I tell if a reg is a virtual_stack_reg?