[Bug tree-optimization/23286] missed fully redundant expression

[steven at gcc dot gnu dot org]

------- Comment #18 from steven at gcc dot gnu dot org  2008-11-23 13:22 -------
The test case of PR38204 shows one of the problems with proof-of-concept patch,
namely the "don't move up too much" problem.  The .pre dump looks like this:

test (int a, int b, int c, int g)
{
  int pretmp.11;
  int e;
  int d;
  int D.1257;
  int D.1256;

<bb 2>:
  pretmp.11_11 = b_3(D) * c_4(D);
  pretmp.11_12 = g_8(D) + pretmp.11_11;  // No need to move this up to here.
  if (a_2(D) != 0)
    goto <bb 3>;
  else
    goto <bb 4>;

<bb 3>:
  d_5 = pretmp.11_11;
  goto <bb 5>;

<bb 4>:
  d_6 = b_3(D) - c_4(D);

<bb 5>:
  # d_1 = PHI <d_5(3), d_6(4)>
  D.1256_7 = pretmp.11_11;
  e_9 = pretmp.11_12;
  D.1257_10 = e_9 + d_1;
  return D.1257_10;

}


Eventually this gives (in the .final_cleanup dump):

;; Function test (test)

test (int a, int b, int c, int g)
{
  int d.21;
  int d;

<bb 2>:
  d.21 = c * b;
  if (a != 0)
    goto <bb 3>;
  else
    goto <bb 4>;

<bb 3>:
  d = d.21;
  goto <bb 5>;

<bb 4>:
  d = b - c;

<bb 5>:
  return (d.21 + g) + d;

}


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23286