[Bug tree-optimization/71815] New: SLSR misses several PHI candidate cases

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71815

            Bug ID: 71815
           Summary: SLSR misses several PHI candidate cases
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rguenth at gcc dot gnu.org
  Target Milestone: ---

Derived from gcc.dg/tree-ssa/slsr-3[56].c which fail with code hoisting
enabled.

int                                                                             
f (int s, int c, int i)                                                         
{                                                                               
  int a1, a3, x1, x3, x;                                                        

  a1 = i * s;                                                                   
  x1 = c + a1;                                                                  
  i = i + 2;                                                                    

  if (x1 > 6)                                                                   
    i = i + 2;                                                                  

  a3 = i * s;                                                                   
  x3 = c + a3;                                                                  

  x = x1 + x3;                                                                  
  return x;                                                                     
}                 

fails to SLSR properly.  With forwprop enabled the conditional i increment
gets rewritten to i = i + 4 and i = i + 2 is sunk into the else arm
which is handled but in a suboptimal way:

  <bb 2>:
  a1_4 = i_2(D) * s_3(D);
  x1_6 = a1_4 + c_5(D);
  if (x1_6 > 6)
    goto <bb 4>;
  else
    goto <bb 3>;

  <bb 3>:
  i_7 = i_2(D) + 2;
  slsr_16 = s_3(D) * 2;
  slsr_14 = a1_4 + slsr_16;
  goto <bb 5>;

  <bb 4>:
  i_8 = i_2(D) + 4;
  slsr_15 = s_3(D) * 4;
  slsr_13 = a1_4 + slsr_15;

  <bb 5>:
  # i_1 = PHI <i_7(3), i_8(4)>
  # slsr_17 = PHI <slsr_14(3), slsr_13(4)>
  a3_9 = slsr_17;
  x3_10 = c_5(D) + a3_9;
  x_11 = x1_6 + x3_10;
  return x_11;

when code-hoisting is enabled on gcc.dg/tree-ssa/slsr-3[56].c we get the same
effect as with -fno-tree-forwprop on the above testcase - the increment after
the if gets hoisted before it, and slsr fails:

  <bb 2>:
  a1_4 = i_2(D) * s_3(D);
  x1_6 = c_5(D) + a1_4;
  i_7 = i_2(D) + 2;
  if (x1_6 > 6)
    goto <bb 3>;
  else
    goto <bb 4>;

  <bb 3>:
  i_8 = i_7 + 2;
  slsr_16 = s_3(D) * 4;

  <bb 4>:
  # i_1 = PHI <i_7(2), i_8(3)>
  a3_9 = i_1 * s_3(D);
  x3_10 = c_5(D) + a3_9;
  x_11 = x1_6 + x3_10;
  return x_11;