Re: virtual stack regs.

[Rask Ingemann Lambertsen <rask at sygehus dot dk>]

On Mon, Jun 18, 2007 at 08:28:25PM -0400, Kenneth Zadeck wrote:
> 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.

less -p 'Virtual registers ' gcc/rtl.h
less -p 'enum global_rtl_index' gcc/rtl.h

> 1) What are these?

   They are placeholders. The vregs pass will replace them with
stack_pointer_rtx, (hard_)frame_pointer_rtx and arg_pointer_rtx using
information from STARTING_FRAME_OFFSET, FIRST_PARM_OFFSET and such.

   Here are two examples, where arg_pointer_rtx is (reg/f:HI 14 argp) and
(hard_)frame_pointer_rtx is (reg/f:HI 10 bp):

(insn ... (set (reg/v/f:HI 39 [ tmp ])
        (mem/f/c/i:HI (plus:HI (reg/f:HI 15 virtual-incoming-args)
                (const_int 4 [0x4])) [0 tmp+0 S2 A16])) -1 (nil)
    (expr_list ...
         (nil)))

becomes

(insn ... (set (reg/v/f:HI 39 [ tmp ])
        (mem/f/c/i:HI (plus:HI (reg/f:HI 14 argp)
                (const_int 6 [0x6])) [0 tmp+0 S2 A16])) 9 {*movhi} (nil)
    (expr_list ...
         (nil)))

(where argp is a virtual hard reg later eliminated by reload) because
FIRST_PARM_OFFSET is 2, and

(insn ... (parallel [
             (set (reg:HI 62)
                (plus:HI (reg/f:HI 16 virtual-stack-vars)
                     (reg:HI 61)))
             (clobber (reg:CC 13 cc))
        ]) -1 (nil)
     (nil))

   becomes

(insn ... (parallel [
             (set (reg:HI 62)
                (plus:HI (reg/f:HI 10 bp)
                     (reg:HI 61)))
             (clobber (reg:CC 13 cc))
        ]) 44 {*addhi3} (nil)
     (nil))

because STARTING_FRAME_OFFSET is 0.
 
> 2) Why are they uninitialized?

   There is no meaningful value to assign to them.
 
> 3) If they really are uninitialized, why is it a problem to assign zero
> to them.

   It is possible to mess up the substitution that the vregs pass performs.
IIRC, it happened to me once because I accidentally put one of these virtual
pseudo inside a (clobber) or (use) or something like that. I don't quite
recall the details, but maybe the substitution fails if they appear outside
an operand?
 
> 4) If they are not uninitialized, where is the initialization code? Why
> does df not see it?

   Hmm, how do you handle arg_pointer_rtx, frame_pointer_rtx and the like?
The are all uninitialized until the prologue is emitted, which is some time
after reload.
 
> 5) How can I tell if a reg is a virtual_stack_reg?

   FIRST_VIRTUAL_REGISTER <= regno <= LAST_VIRTUAL_REGISTER

-- 
Rask Ingemann Lambertsen