[PR81611] accept copies in simple_iv_increment_p

If there are copies between the GIMPLE_PHI at the loop body and the
increment that reaches it (presumably through a back edge), still
regard it as a simple_iv_increment, so that we won't consider the
value in the back edge eligible for forwprop.  Doing so would risk
making the phi node and the incremented conflicting value live
within the loop, and the phi node to be preserved for propagated
uses after the loop.

for  gcc/ChangeLog

	PR tree-optimization/81611
	* tree-ssa-dom.c (simple_iv_increment_p): Skip intervening
	copies.

From-SVN: r257194

gcc/ChangeLog

+2018-01-30  Alexandre Oliva <aoliva@redhat.com>
+
+	PR tree-optimization/81611
+	* tree-ssa-dom.c (simple_iv_increment_p): Skip intervening
+	copies.
+
 2018-01-30  Aaron Sawdey  <acsawdey@linux.vnet.ibm.com>
 
 	* config/rs6000/rs6000.c (rs6000_internal_arg_pointer): Only return

gcc/tree-ssa-dom.c

@@ -1276,8 +1276,11 @@ record_equality (tree x, tree y, class const_and_copies *const_and_copies)
 /* Returns true when STMT is a simple iv increment.  It detects the
    following situation:
 
-   i_1 = phi (..., i_2)
-   i_2 = i_1 +/- ...  */
+   i_1 = phi (..., i_k)
+   [...]
+   i_j = i_{j-1}  for each j : 2 <= j <= k-1
+   [...]
+   i_k = i_{k-1} +/- ...  */
 
 bool
 simple_iv_increment_p (gimple *stmt)
@@ -1305,8 +1308,15 @@ simple_iv_increment_p (gimple *stmt)
     return false;
 
   phi = SSA_NAME_DEF_STMT (preinc);
-  if (gimple_code (phi) != GIMPLE_PHI)
-    return false;
+  while (gimple_code (phi) != GIMPLE_PHI)
+    {
+      /* Follow trivial copies, but not the DEF used in a back edge,
+	 so that we don't prevent coalescing.  */
+      if (!gimple_assign_ssa_name_copy_p (phi))
+	return false;
+      preinc = gimple_assign_rhs1 (phi);
+      phi = SSA_NAME_DEF_STMT (preinc);
+    }
 
   for (i = 0; i < gimple_phi_num_args (phi); i++)
     if (gimple_phi_arg_def (phi, i) == lhs)