[Bug debug/21913] New: REG_EXPR wrong for parameters passed by invisible reference

["jakub at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

/* { dg-do compile } */
/* { dg-options "-O2 -g -m32" } */
union U { unsigned long u; unsigned long long v; };
struct S { long s; long t; };

extern unsigned int foo (unsigned long long *, unsigned int);

void bar (int x, union U y, struct S *z)
{
  if ((x & 3) == 2)
    {
      unsigned long long r = y.v;
      unsigned int b = 1000000000L;
      unsigned int m;
      if ((r >> 32) == 0)
        {
          m = (unsigned int)r % b;
          r = (unsigned int)r / b;
        }
      else
        m = foo (&r, b);
      z->t = m;
      z->s = r;
    }
}

creates following bogus location description for y (which is passed by
invisible reference on this arch).
        .byte   0x54     # DW_OP_reg4
        .byte   0x93     # DW_OP_piece
        .byte   0x4      # uleb128 0x4
        .byte   0x74     # DW_OP_breg4
        .byte   0x4      # sleb128 4
        .byte   0x93     # DW_OP_piece
        .byte   0x4      # uleb128 0x4
The 64-bit union is certainly not located partly in reg4 and partly in what
reg4+4 points to, at least not in this case.

The problem seems to be that GCC uses REG_EXPR y for the register that
contains the address of the union:
(reg:SI 4 4 [ y ])
and also (MEM_EXPR) for the MEM that actually contains it, e.g.:
(mem/s:SI (reg:SI 4 4 [ y ]) [6 y+0 S4 A64])
(mem/s:SI (plus:SI (reg:SI 4 4 [ y ]) (const_int 4 [0x4])) [6 y+4 S4 A32])
The REG_EXPR on the register sounds wrong, if we allow REG_EXPR to not contain
only decls, then it could be ADDR_EXPR <y>, otherwise perhaps nothing would
be better than lying about where the argument is.

-- 
           Summary: REG_EXPR wrong for parameters passed by invisible
                    reference
           Product: gcc
           Version: 4.0.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: debug
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: jakub at gcc dot gnu dot org
                CC: gcc-bugs at gcc dot gnu dot org,roland at redhat dot com
GCC target triplet: ppc-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21913