Fix PR91821

Index: gcc/tree-vect-loop.c
===================================================================
--- gcc/tree-vect-loop.c	(revision 275988)
+++ gcc/tree-vect-loop.c	(working copy)
@@ -2658,7 +2658,13 @@  pop:
       gimple *use_stmt = USE_STMT (path[i].second);
       tree op = USE_FROM_PTR (path[i].second);
       if (! has_single_use (op)
-	  || ! is_gimple_assign (use_stmt))
+	  || ! is_gimple_assign (use_stmt)
+	  /* The following make sure we can compute the operand index
+	     easily plus it mostly disallows chaining via COND_EXPR condition
+	     operands.  */
+	  || (gimple_assign_rhs1 (use_stmt) != op
+	      && gimple_assign_rhs2 (use_stmt) != op
+	      && gimple_assign_rhs3 (use_stmt) != op))
 	{
 	  fail = true;
 	  break;
@@ -3058,6 +3064,7 @@  vect_is_simple_reduction (loop_vec_info
 	  || !flow_bb_inside_loop_p (loop, gimple_bb (def1_info->stmt))
 	  || vect_valid_reduction_input_p (def1_info)))
     {
+      STMT_VINFO_REDUC_IDX (def_stmt_info) = 1;
       if (dump_enabled_p ())
 	report_vect_op (MSG_NOTE, def_stmt, "detected reduction: ");
       return def_stmt_info;
@@ -3070,6 +3077,7 @@  vect_is_simple_reduction (loop_vec_info
 	  || !flow_bb_inside_loop_p (loop, gimple_bb (def2_info->stmt))
 	  || vect_valid_reduction_input_p (def2_info)))
     {
+      STMT_VINFO_REDUC_IDX (def_stmt_info) = 0;
       if (dump_enabled_p ())
 	report_vect_op (MSG_NOTE, def_stmt, "detected reduction: ");
       return def_stmt_info;
@@ -3084,16 +3092,18 @@  vect_is_simple_reduction (loop_vec_info
          restriction is that all operations in the chain are the same.  */
       auto_vec<stmt_vec_info, 8> reduc_chain;
       unsigned i;
+      bool is_slp_reduc = !nested_in_vect_loop && code != COND_EXPR;
       for (i = path.length () - 1; i >= 1; --i)
 	{
 	  gimple *stmt = USE_STMT (path[i].second);
 	  if (gimple_assign_rhs_code (stmt) != code)
-	    break;
-	  reduc_chain.safe_push (loop_info->lookup_stmt (stmt));
+	    is_slp_reduc = false;
+	  stmt_vec_info stmt_info = loop_info->lookup_stmt (stmt);
+	  STMT_VINFO_REDUC_IDX (stmt_info)
+	    = path[i].second->use - gimple_assign_rhs1_ptr (stmt);
+	  reduc_chain.safe_push (stmt_info);
 	}
-      if (i == 0
-	  && ! nested_in_vect_loop
-	  && code != COND_EXPR)
+      if (is_slp_reduc)
 	{
 	  for (unsigned i = 0; i < reduc_chain.length () - 1; ++i)
 	    {
Index: gcc/tree-vect-patterns.c
===================================================================
--- gcc/tree-vect-patterns.c	(revision 275988)
+++ gcc/tree-vect-patterns.c	(working copy)
@@ -868,6 +868,8 @@  vect_reassociating_reduction_p (stmt_vec
 
   *op0_out = gimple_assign_rhs1 (assign);
   *op1_out = gimple_assign_rhs2 (assign);
+  if (commutative_tree_code (code) && STMT_VINFO_REDUC_IDX (stmt_info) == 0)
+    std::swap (*op0_out, *op1_out);
   return true;
 }
 
Index: gcc/tree-vectorizer.c
===================================================================
--- gcc/tree-vectorizer.c	(revision 275988)
+++ gcc/tree-vectorizer.c	(working copy)
@@ -639,6 +639,7 @@  vec_info::new_stmt_vec_info (gimple *stm
   STMT_VINFO_VECTORIZABLE (res) = true;
   STMT_VINFO_VEC_REDUCTION_TYPE (res) = TREE_CODE_REDUCTION;
   STMT_VINFO_VEC_CONST_COND_REDUC_CODE (res) = ERROR_MARK;
+  STMT_VINFO_REDUC_IDX (res) = -1;
   STMT_VINFO_SLP_VECT_ONLY (res) = false;
 
   if (gimple_code (stmt) == GIMPLE_PHI
Index: gcc/tree-vectorizer.h
===================================================================
--- gcc/tree-vectorizer.h	(revision 275988)
+++ gcc/tree-vectorizer.h	(working copy)
@@ -941,6 +941,10 @@  public:
      vect_force_simple_reduction.  */
   enum vect_reduction_type reduc_type;
 
+  /* On a stmt participating in the reduction the index of the operand
+     on the reduction SSA cycle.  */
+  int reduc_idx;
+
   /* On a reduction PHI the def returned by vect_force_simple_reduction.
      On the def returned by vect_force_simple_reduction the
      corresponding PHI.  */
@@ -1030,6 +1034,7 @@  STMT_VINFO_BB_VINFO (stmt_vec_info stmt_
 #define STMT_VINFO_SIMD_LANE_ACCESS_P(S)   (S)->simd_lane_access_p
 #define STMT_VINFO_VEC_REDUCTION_TYPE(S)   (S)->v_reduc_type
 #define STMT_VINFO_VEC_CONST_COND_REDUC_CODE(S) (S)->const_cond_reduc_code
+#define STMT_VINFO_REDUC_IDX(S)		   (S)->reduc_idx
 
 #define STMT_VINFO_DR_WRT_VEC_LOOP(S)      (S)->dr_wrt_vec_loop
 #define STMT_VINFO_DR_BASE_ADDRESS(S)      (S)->dr_wrt_vec_loop.base_address