[Bug middle-end/63184] [7/8/9 Regression] Fails to simplify comparison

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63184

--- Comment #18 from Richard Biener <rguenth at gcc dot gnu.org> ---
That works but it doesn't perform the folding in the end.  Still combining
&a + b + c + d + e + 2 into (&a + 2) + b + c + d + e is better since most
targets can compute symbol+offset in a single instruction.

We then end up with

  <bb 2> [local count: 1073741824]:
  i.0_1 = i;
  _2 = i.0_1 * 4;
  _3 = (sizetype) _2;
  _5 = &MEM[(void *)&a + 8B] + _3;
  _13 = _3 + 8;
  _7 = &a + _13;
  if (_5 != _7)
    goto <bb 3>; [53.47%]
  else
    goto <bb 4>; [46.53%]

where the issue is that the 2nd opportunity for &a + _3 + 8 only appears
after SLSR which transforms

  _6 = i.0_1 + 2;
  _12 = (sizetype) _6;
  _13 = _12 * 4;
  _7 = &a + _13;

to

  _6 = i.0_1 + 2;
  _12 = (sizetype) _6;
  _13 = _3 + 8;
  _7 = &a + _13;

and we have

      NEXT_PASS (pass_reassoc, false /* insert_powi_p */);
      NEXT_PASS (pass_strength_reduction);

and I'm not sure it's a good idea to swap those two...

If I do we fold things (albeit quite late).

diff --git a/gcc/tree-ssa-reassoc.c b/gcc/tree-ssa-reassoc.c
index a9f45bfd891..fb1f8014633 100644
--- a/gcc/tree-ssa-reassoc.c
+++ b/gcc/tree-ssa-reassoc.c
@@ -5988,6 +5988,31 @@ reassociate_bb (basic_block bb)
                    }
                }

+             /* If the association chain is used in a single
+                POINTER_PLUS_EXPR with an invariant first operand
+                then combine a constant element with the invariant
+                address.  */
+             use_operand_p use_p;
+             gimple *use_stmt;
+             if (ops.length () > 1
+                 && rhs_code == PLUS_EXPR
+                 && TREE_CODE (ops.last ()->op) == INTEGER_CST
+                 && single_imm_use (lhs, &use_p, &use_stmt)
+                 && is_gimple_assign (use_stmt)
+                 && gimple_assign_rhs_code (use_stmt) == POINTER_PLUS_EXPR
+                 && TREE_CODE (gimple_assign_rhs1 (use_stmt)) == ADDR_EXPR)
+               {
+                 last = ops.pop ();
+                 tree addr = gimple_assign_rhs1 (use_stmt);
+                 addr = build1 (ADDR_EXPR, TREE_TYPE (addr),
+                                fold_build2 (MEM_REF,
+                                             TREE_TYPE (TREE_TYPE (addr)),
+                                             addr,
+                                             fold_convert (ptr_type_node,
+                                                           last->op)));
+                 gimple_assign_set_rhs1 (use_stmt, addr);
+               }
+
              tree new_lhs = lhs;
              /* If the operand vector is now empty, all operands were 
                 consumed by the __builtin_powi optimization.  */