% cat predcom-fail.c
#define INFTY       987654321
#define Ra /*__restrict*/
#define Rv __restrict
void
P7Viterbi(int * Ra _dc, int * Ra _mc, int * Ra _tpdd, int * Ra _tpmd, int M)
{
  int   i,k;
  int   sc;
  int * Rv dc = _dc;
  int * Rv mc = _mc;
  int * Rv tpdd = _tpdd;
  int * Rv tpmd = _tpmd;

  dc[0] = -INFTY;

  for (k = 1; k < M; k++) {
    dc[k] = dc[k-1] + tpdd[k-1];
    if ((sc = mc[k-1] + tpmd[k-1]) > dc[k]) dc[k] = sc;
    if (dc[k] < -INFTY) dc[k] = -INFTY;
  }
}

./cc1 -Ofast predcom-fail.c should be able to predcom the loop.  It doesn't
because it doesn't see that the (first) dc[] write doesn't conflict with the
mc/tpmd[] reads.  It should be able to see that because all the int pointers
are created with new restrict sets.  If you defined Ra as also __restrict
(making the arguments already restrict qualified) you get the transformation.

The problem is an interaction between creating the datarefs and the
disambiguator.  The data-ref base objects contain the casted inputs:

#(Data Ref:
#  bb: 4
#  stmt: D.2741_19 = *D.2740_18;
#  ref: *D.2740_18;
#  base_object: *(int * restrict) _dc_2(D);
#  Access function 0: {0B, +, 4}_1
#)
vs
#(Data Ref:
#  bb: 4
#  stmt: D.2743_24 = *D.2742_23;
#  ref: *D.2742_23;
#  base_object: *(int * restrict) _tpdd_6(D);
#  Access function 0: {0B, +, 4}_1
#)

The disambiguator used is ptr_derefs_may_alias_p on those two (casted)
pointers.  But the first thing it does is to remove all conversions.
Remembering the original type wouldn't help as we need to look into the
points-to info of the restrict qualified object (i.e. the LHS of that
conversion).

Hence when creating the data-ref we shouldn't look through such casts, that
introduce useful information.  I have a patch.