Question about generated type for common block in fortran

[Bin.Cheng]

Hi,
I am looking into DSE transformation of some fortran codes.  Given
below fortran declarations:

       real*8  a(len) , b(len) ,  c(len) , d(len)
       common /area/  a, b, c, d
       real*8         src1(len), temp1(len), temp2(len), src2(len)
       equivalence    (src1, a), (src2, b), (temp1, c), (temp2, d)

with my limited understanding of fortran, the layout should be like:
struct area
{
  union {double src1[len]; double a[len]};
  union {double temp1[len]; double b[len]};
  union {double temp2[len]; double c[len]};
  union {double src2[len]; double d[len]};
};

When debugging in tree-ssa-dse.c, if I have memory reference like
area.src1[idx], the type for area dumped is like:
(gdb) call debug_generic_expr(ref1->exp.operands[0])
area
(gdb) call debug_generic_expr(ref1->exp.operands[0]->typed.type)
union
{
  real(kind=8) src1[100];
  real(kind=8) a[100];
  real(kind=8) src2[100];
  real(kind=8) b[100];
  real(kind=8) temp1[100];
  real(kind=8) c[100];
  real(kind=8) temp2[100];
  real(kind=8) d[100];
}
I do noticed src1/src2 fields do have different offset, although they
are put into a union type here.

I suppose such type are generated by fortran front-end?  Is it just
because it's easy to do so?  And any background comment/document about
this?
Unfortunately middle end misses lots of dead store, redundant load
because of difficulty in alias check of such type information.  I
guess fully support alias check of this issue would be non-trivial, so
any suggestions will be highly appreciated too.

Thanks,
bin