virtual-stack-vars reference not resolved in vregs

[Paul Koning]

I'm doing cleanup on the pdp11 back end to get rid of a 
number of ICE in the test suite.  One is in 
gcc.c-torture/compile/20001221.c -- it works in GCC 4 but 
fails in GCC 5 and later.

In the dumps, I see in the output from the expand phase a 
large number of memory reference via the "virtual-stack-vars" 
pseudo-register.  In vregs those all become frame pointer 
references in V4, but in V5 and later, about 10% of them 
remain pseudo-register references all the way through final
assembly output generation.  There things blow up because 
it's an invalid base register.

Here are some snippets:

static void
foo ()
{
  long maplength;
  int type;
  {
    const long nibbles = 8;
    char buf1[nibbles + 1];
...

V4, expand output:

;; nibbles_18 = 8;

(insn 7 6 0 (set (mem/c:SI (plus:HI (reg/f:HI 17 virtual-stack-vars)
                (const_int -8 [0xfffffffffffffff8])) [0 nibbles+0 S4 A16])
        (const_int 8 [0x8])) /Users/pkoning/Documents/svn/gcc/gcc/testsuite/gcc.c-torture/compile/20001221-1.c:8 -1
     (nil))

;; _19 = (sizetype) nibbles_18;

(insn 8 7 0 (set (mem/c:HI (plus:HI (reg/f:HI 17 virtual-stack-vars)
                (const_int -10 [0xfffffffffffffff6])) [0 D.1480+0 S2 A16])
        (subreg:HI (mem/c:SI (plus:HI (reg/f:HI 17 virtual-stack-vars)
                    (const_int -8 [0xfffffffffffffff8])) [0 nibbles+0 S4 A16]) 0)) /Users/pkoning/Documents/svn/gcc/gcc/testsuite/gcc.c-torture/compile/20001221-1.c:9 -1
     (nil))

The second insn is picking up the low order half of "nibbles" 
to use as the index; note that Pmode is HI for this target.  
The vregs pass turns it into this:

(insn 7 6 8 2 (set (mem/c:SI (plus:HI (reg/f:HI 14 fp)
                (const_int -8 [0xfffffffffffffff8])) [0 nibbles+0 S4 A16])
        (const_int 8 [0x8])) /Users/pkoning/Documents/svn/gcc/gcc/testsuite/gcc.c-torture/compile/20001221-1.c:8 12 {movsi}
     (nil))
(insn 8 7 9 2 (set (mem/c:HI (plus:HI (reg/f:HI 14 fp)
                (const_int -10 [0xfffffffffffffff6])) [0 D.1480+0 S2 A16])
        (subreg:HI (mem/c:SI (plus:HI (reg/f:HI 17 virtual-stack-vars)
                    (const_int -8 [0xfffffffffffffff8])) [0 nibbles+0 S4 A16]) 0)) /Users/pkoning/Documents/svn/gcc/gcc/testsuite/gcc.c-torture/compile/20001221-1.c:9 14 {movhi}
     (nil))

which is what I would expect.

In V5, the expand output is the same:

(insn 7 6 0 (set (mem/c:SI (plus:HI (reg/f:HI 17 virtual-stack-vars)
                (const_int -8 [0xfffffffffffffff8])) [0 nibbles+0 S4 A16])
        (const_int 8 [0x8])) /Users/pkoning/Documents/svn/gcc/gcc/testsuite/gcc.c-torture/compile/20001221-1.c:8 -1
     (nil))

;; _19 = (sizetype) nibbles_18;

(insn 8 7 0 (set (mem/c:HI (plus:HI (reg/f:HI 17 virtual-stack-vars)
                (const_int -10 [0xfffffffffffffff6])) [0 D.1480+0 S2 A16])
        (subreg:HI (mem/c:SI (plus:HI (reg/f:HI 17 virtual-stack-vars)
                    (const_int -8 [0xfffffffffffffff8])) [0 nibbles+0 S4 A16]) 0)) /Users/pkoning/Documents/svn/gcc/gcc/testsuite/gcc.c-torture/compile/20001221-1.c:9 -1
     (nil))

but now vregs doesn't convert the virtual-stack-vars reference 
in the subreg operand:

(insn 7 6 8 2 (set (mem/c:SI (plus:HI (reg/f:HI 14 fp)
                (const_int -8 [0xfffffffffffffff8])) [0 nibbles+0 S4 A16])
        (const_int 8 [0x8])) /Users/pkoning/Documents/svn/gcc/gcc/testsuite/gcc.c-torture/compile/20001221-1.c:8 12 {movsi}
     (nil))
(insn 8 7 9 2 (set (mem/c:HI (plus:HI (reg/f:HI 14 fp)
                (const_int -10 [0xfffffffffffffff6])) [0 D.1480+0 S2 A16])
        (subreg:HI (mem/c:SI (plus:HI (reg/f:HI 17 virtual-stack-vars)
                    (const_int -8 [0xfffffffffffffff8])) [0 nibbles+0 S4 A16]) 0)) /Users/pkoning/Documents/svn/gcc/gcc/testsuite/gcc.c-torture/compile/20001221-1.c:9 14 {movhi}
     (nil))

Is this something the back end is responsible for getting right, for example 
via the machine description file?  If so, any hints where to start?

	paul