[Bug debug/100960] var-tracking: parameter location in subregister not tracked

["stefansf at linux dot ibm.com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100960

--- Comment #5 from Stefan Schulze Frielinghaus <stefansf at linux dot ibm.com> ---
However, I found another example (see attachment a-t2.c.325r.vartrack) which
does not profit from the patch:

__attribute__((noinline, noclone)) void
fn1 (int x)
{
  __asm volatile ("" : "+r" (x) : : "memory");
}

__attribute__((noinline, noclone)) int
fn2 (int x, int y)
{
  if (x)
    {
      // x is copied into call-saved r11
      fn1 (x);
      // locs of x point to entry value only
      // ignoring r11
      fn1 (x);
    }
  return y;
}

__attribute__((noinline, noclone)) int
fn3 (int x, int y)
{
  return fn2 (x, y);
}

int
main ()
{
  fn3 (36, 25);
  return 0;
}

For fn2 the value for parameter x is 5:5

cselib hash table:
...
(value/u:SI 5:5 @0x5fb9420/0x5f5e600)
 locs:
  from insn 1 (value/u:SI 6:263 @0x5fb9438/0x5f5e630)
  from insn 1 (entry_value:SI (reg:SI 2 %r2 [ xD.2274 ]))
  from insn 1 (reg:SI 2 %r2 [ xD.2274 ])
 no addrs

which is recorded in bb 2. In bb 4 (the true branch of the if) register r2 is
saved in r11:

bb 4 op 0 insn 36 MO_VAL_USE (concat/v:DI (value/u:DI 26:26
@0x5fb9618/0x5f5e9f0)
        (reg:DI 2 %r2 [64]))
bb 4 op 1 insn 36 MO_VAL_SET (concat/u:DI (value/u:DI 26:26
@0x5fb9618/0x5f5e9f0)
        (set (reg/v:DI 11 %r11 [orig:61 xD.2274+-4 ] [61])
            (reg:DI 2 %r2 [64])))
(insn 36 10 11 4 (set (reg/v:DI 11 %r11 [orig:61 xD.2274+-4 ] [61])
        (reg:DI 2 %r2 [64])) 1472 {*movdi_64}
     (nil))
cselib hash table:
(value/u:DI 26:26 @0x5fb9618/0x5f5e9f0)
 locs:
  from insn 36 (reg/v:DI 11 %r11 [orig:61 xD.2274+-4 ] [61])
  from insn 36 (reg:DI 2 %r2 [64])
 no addrs
cselib preserved hash table:
...
(value/u:SI 5:5 @0x5fb9420/0x5f5e600)
 locs:
  from insn 1 (value/u:SI 6:263 @0x5fb9438/0x5f5e630)
  from insn 1 (entry_value:SI (reg:SI 2 %r2 [ xD.2274 ]))
 no addrs

However at bb 4 the relation between r2 and value 5:5 is lost (except the entry
value relation). Thus I cannot record the subvalue relation between 5:5 and
26:26 at least not during creation of 26:26. Since cselib resets its table
after jumps I'm not sure how to proceed here. Any ideas?

I would be also up for the second idea and pretend that the move is not a
DImode copy but a SImode copy. However, I'm not sure how to look up the mode of
the actual type. Any pointers?