Index: gcc/machmode.h
===================================================================
--- gcc/machmode.h	2019-10-25 13:26:59.053879364 +0100
+++ gcc/machmode.h	2019-10-25 13:27:26.201687539 +0100
@@ -258,6 +258,9 @@ #define CLASS_HAS_WIDER_MODES_P(CLASS)
   bool exists () const;
   template<typename U> bool exists (U *) const;
 
+  bool operator== (const T &m) const { return m_mode == m; }
+  bool operator!= (const T &m) const { return m_mode != m; }
+
 private:
   machine_mode m_mode;
 };
Index: gcc/tree-vectorizer.h
===================================================================
--- gcc/tree-vectorizer.h	2019-10-25 13:27:19.317736181 +0100
+++ gcc/tree-vectorizer.h	2019-10-25 13:27:26.209687483 +0100
@@ -329,8 +329,9 @@ typedef std::pair<tree, tree> vec_object
   /* Cost data used by the target cost model.  */
   void *target_cost_data;
 
-  /* If we've chosen a vector size for this vectorization region,
-     this is one mode that has such a size, otherwise it is VOIDmode.  */
+  /* The argument we should pass to related_vector_mode when looking up
+     the vector mode for a scalar mode, or VOIDmode if we haven't yet
+     made any decisions about which vector modes to use.  */
   machine_mode vector_mode;
 
 private:
@@ -1595,8 +1596,9 @@ extern dump_user_location_t find_loop_lo
 extern bool vect_can_advance_ivs_p (loop_vec_info);
 
 /* In tree-vect-stmts.c.  */
+extern tree get_related_vectype_for_scalar_type (machine_mode, tree,
+						 poly_uint64 = 0);
 extern tree get_vectype_for_scalar_type (vec_info *, tree);
-extern tree get_vectype_for_scalar_type_and_size (tree, poly_uint64);
 extern tree get_mask_type_for_scalar_type (vec_info *, tree);
 extern tree get_same_sized_vectype (tree, tree);
 extern bool vect_get_loop_mask_type (loop_vec_info);
Index: gcc/tree-vect-slp.c
===================================================================
--- gcc/tree-vect-slp.c	2019-10-25 13:27:19.313736209 +0100
+++ gcc/tree-vect-slp.c	2019-10-25 13:27:26.205687511 +0100
@@ -3118,7 +3118,12 @@ vect_slp_bb_region (gimple_stmt_iterator
 	  && dbg_cnt (vect_slp))
 	{
 	  if (dump_enabled_p ())
-	    dump_printf_loc (MSG_NOTE, vect_location, "SLPing BB part\n");
+	    {
+	      dump_printf_loc (MSG_NOTE, vect_location,
+			       "***** Analysis succeeded with vector mode"
+			       " %s\n", GET_MODE_NAME (bb_vinfo->vector_mode));
+	      dump_printf_loc (MSG_NOTE, vect_location, "SLPing BB part\n");
+	    }
 
 	  bb_vinfo->shared->check_datarefs ();
 	  vect_schedule_slp (bb_vinfo);
@@ -3138,6 +3143,13 @@ vect_slp_bb_region (gimple_stmt_iterator
 
 	  vectorized = true;
 	}
+      else
+	{
+	  if (dump_enabled_p ())
+	    dump_printf_loc (MSG_NOTE, vect_location,
+			     "***** Analysis failed with vector mode %s\n",
+			     GET_MODE_NAME (bb_vinfo->vector_mode));
+	}
 
       if (mode_i == 0)
 	autodetected_vector_mode = bb_vinfo->vector_mode;
@@ -3145,9 +3157,22 @@ vect_slp_bb_region (gimple_stmt_iterator
       delete bb_vinfo;
 
       if (mode_i < vector_modes.length ()
-	  && known_eq (GET_MODE_SIZE (vector_modes[mode_i]),
-		       GET_MODE_SIZE (autodetected_vector_mode)))
-	mode_i += 1;
+	  && VECTOR_MODE_P (autodetected_vector_mode)
+	  && (related_vector_mode (vector_modes[mode_i],
+				   GET_MODE_INNER (autodetected_vector_mode))
+	      == autodetected_vector_mode)
+	  && (related_vector_mode (autodetected_vector_mode,
+				   GET_MODE_INNER (vector_modes[mode_i]))
+	      == vector_modes[mode_i]))
+	{
+	  if (dump_enabled_p ())
+	    dump_printf_loc (MSG_NOTE, vect_location,
+			     "***** Skipping vector mode %s, which would"
+			     " repeat the analysis for %s\n",
+			     GET_MODE_NAME (vector_modes[mode_i]),
+			     GET_MODE_NAME (autodetected_vector_mode));
+	  mode_i += 1;
+	}
 
       if (vectorized
 	  || mode_i == vector_modes.length ()
Index: gcc/tree-vect-loop.c
===================================================================
--- gcc/tree-vect-loop.c	2019-10-25 13:27:19.309736237 +0100
+++ gcc/tree-vect-loop.c	2019-10-25 13:27:26.201687539 +0100
@@ -2367,6 +2367,17 @@ vect_analyze_loop (class loop *loop, loo
       opt_result res = vect_analyze_loop_2 (loop_vinfo, fatal, &n_stmts);
       if (mode_i == 0)
 	autodetected_vector_mode = loop_vinfo->vector_mode;
+      if (dump_enabled_p ())
+	{
+	  if (res)
+	    dump_printf_loc (MSG_NOTE, vect_location,
+			     "***** Analysis succeeded with vector mode %s\n",
+			     GET_MODE_NAME (loop_vinfo->vector_mode));
+	  else
+	    dump_printf_loc (MSG_NOTE, vect_location,
+			     "***** Analysis failed with vector mode %s\n",
+			     GET_MODE_NAME (loop_vinfo->vector_mode));
+	}
 
       if (res)
 	{
@@ -2400,9 +2411,22 @@ vect_analyze_loop (class loop *loop, loo
 	}
 
       if (mode_i < vector_modes.length ()
-	  && known_eq (GET_MODE_SIZE (vector_modes[mode_i]),
-		       GET_MODE_SIZE (autodetected_vector_mode)))
-	mode_i += 1;
+	  && VECTOR_MODE_P (autodetected_vector_mode)
+	  && (related_vector_mode (vector_modes[mode_i],
+				   GET_MODE_INNER (autodetected_vector_mode))
+	      == autodetected_vector_mode)
+	  && (related_vector_mode (autodetected_vector_mode,
+				   GET_MODE_INNER (vector_modes[mode_i]))
+	      == vector_modes[mode_i]))
+	{
+	  if (dump_enabled_p ())
+	    dump_printf_loc (MSG_NOTE, vect_location,
+			     "***** Skipping vector mode %s, which would"
+			     " repeat the analysis for %s\n",
+			     GET_MODE_NAME (vector_modes[mode_i]),
+			     GET_MODE_NAME (autodetected_vector_mode));
+	  mode_i += 1;
+	}
 
       if (mode_i == vector_modes.length ()
 	  || autodetected_vector_mode == VOIDmode)
@@ -4763,7 +4787,10 @@ vect_create_epilog_for_reduction (stmt_v
 	  && (mode1 = targetm.vectorize.split_reduction (mode)) != mode)
 	sz1 = GET_MODE_SIZE (mode1).to_constant ();
 
-      tree vectype1 = get_vectype_for_scalar_type_and_size (scalar_type, sz1);
+      unsigned int scalar_bytes = tree_to_uhwi (TYPE_SIZE_UNIT (scalar_type));
+      tree vectype1 = get_related_vectype_for_scalar_type (TYPE_MODE (vectype),
+							   scalar_type,
+							   sz1 / scalar_bytes);
       reduce_with_shift = have_whole_vector_shift (mode1);
       if (!VECTOR_MODE_P (mode1))
 	reduce_with_shift = false;
@@ -4781,7 +4808,9 @@ vect_create_epilog_for_reduction (stmt_v
 	{
 	  gcc_assert (!slp_reduc);
 	  sz /= 2;
-	  vectype1 = get_vectype_for_scalar_type_and_size (scalar_type, sz);
+	  vectype1 = get_related_vectype_for_scalar_type (TYPE_MODE (vectype),
+							  scalar_type,
+							  sz / scalar_bytes);
 
 	  /* The target has to make sure we support lowpart/highpart
 	     extraction, either via direct vector extract or through
Index: gcc/tree-vect-stmts.c
===================================================================
--- gcc/tree-vect-stmts.c	2019-10-25 13:27:22.985710263 +0100
+++ gcc/tree-vect-stmts.c	2019-10-25 13:27:26.205687511 +0100
@@ -11111,18 +11111,28 @@ vect_remove_stores (stmt_vec_info first_
     }
 }
 
-/* Function get_vectype_for_scalar_type_and_size.
-
-   Returns the vector type corresponding to SCALAR_TYPE  and SIZE as supported
-   by the target.  */
+/* If NUNITS is nonzero, return a vector type that contains NUNITS
+   elements of type SCALAR_TYPE, or null if the target doesn't support
+   such a type.
+
+   If NUNITS is zero, return a vector type that contains elements of
+   type SCALAR_TYPE, choosing whichever vector size the target prefers.
+
+   If PREVAILING_MODE is VOIDmode, we have not yet chosen a vector mode
+   for this vectorization region and want to "autodetect" the best choice.
+   Otherwise, PREVAILING_MODE is a previously-chosen vector TYPE_MODE
+   and we want the new type to be interoperable with it.   PREVAILING_MODE
+   in this case can be a scalar integer mode or a vector mode; when it
+   is a vector mode, the function acts like a tree-level version of
+   related_vector_mode.  */
 
 tree
-get_vectype_for_scalar_type_and_size (tree scalar_type, poly_uint64 size)
+get_related_vectype_for_scalar_type (machine_mode prevailing_mode,
+				     tree scalar_type, poly_uint64 nunits)
 {
   tree orig_scalar_type = scalar_type;
   scalar_mode inner_mode;
   machine_mode simd_mode;
-  poly_uint64 nunits;
   tree vectype;
 
   if (!is_int_mode (TYPE_MODE (scalar_type), &inner_mode)
@@ -11162,10 +11172,11 @@ get_vectype_for_scalar_type_and_size (tr
   if (scalar_type == NULL_TREE)
     return NULL_TREE;
 
-  /* If no size was supplied use the mode the target prefers.   Otherwise
-     lookup a vector mode of the specified size.  */
-  if (known_eq (size, 0U))
+  /* If no prevailing mode was supplied, use the mode the target prefers.
+     Otherwise lookup a vector mode based on the prevailing mode.  */
+  if (prevailing_mode == VOIDmode)
     {
+      gcc_assert (known_eq (nunits, 0U));
       simd_mode = targetm.vectorize.preferred_simd_mode (inner_mode);
       if (SCALAR_INT_MODE_P (simd_mode))
 	{
@@ -11181,9 +11192,19 @@ get_vectype_for_scalar_type_and_size (tr
 	    return NULL_TREE;
 	}
     }
-  else if (!multiple_p (size, nbytes, &nunits)
-	   || !mode_for_vector (inner_mode, nunits).exists (&simd_mode))
-    return NULL_TREE;
+  else if (SCALAR_INT_MODE_P (prevailing_mode)
+	   || !related_vector_mode (prevailing_mode,
+				    inner_mode, nunits).exists (&simd_mode))
+    {
+      /* Fall back to using mode_for_vector, mostly in the hope of being
+	 able to use an integer mode.  */
+      if (known_eq (nunits, 0U)
+	  && !multiple_p (GET_MODE_SIZE (prevailing_mode), nbytes, &nunits))
+	return NULL_TREE;
+
+      if (!mode_for_vector (inner_mode, nunits).exists (&simd_mode))
+	return NULL_TREE;
+    }
 
   vectype = build_vector_type_for_mode (scalar_type, simd_mode);
 
@@ -11211,9 +11232,8 @@ get_vectype_for_scalar_type_and_size (tr
 tree
 get_vectype_for_scalar_type (vec_info *vinfo, tree scalar_type)
 {
-  tree vectype;
-  poly_uint64 vector_size = GET_MODE_SIZE (vinfo->vector_mode);
-  vectype = get_vectype_for_scalar_type_and_size (scalar_type, vector_size);
+  tree vectype = get_related_vectype_for_scalar_type (vinfo->vector_mode,
+						      scalar_type);
   if (vectype && vinfo->vector_mode == VOIDmode)
     vinfo->vector_mode = TYPE_MODE (vectype);
   return vectype;
@@ -11246,8 +11266,13 @@ get_same_sized_vectype (tree scalar_type
   if (VECT_SCALAR_BOOLEAN_TYPE_P (scalar_type))
     return truth_type_for (vector_type);
 
-  return get_vectype_for_scalar_type_and_size
-	   (scalar_type, GET_MODE_SIZE (TYPE_MODE (vector_type)));
+  poly_uint64 nunits;
+  if (!multiple_p (GET_MODE_SIZE (TYPE_MODE (vector_type)),
+		   GET_MODE_SIZE (TYPE_MODE (scalar_type)), &nunits))
+    return NULL_TREE;
+
+  return get_related_vectype_for_scalar_type (TYPE_MODE (vector_type),
+					      scalar_type, nunits);
 }
 
 /* Function vect_is_simple_use.
Index: gcc/tree-vectorizer.c
===================================================================
--- gcc/tree-vectorizer.c	2019-10-25 13:27:19.317736181 +0100
+++ gcc/tree-vectorizer.c	2019-10-25 13:27:26.209687483 +0100
@@ -1348,7 +1348,7 @@ get_vec_alignment_for_array_type (tree t
   poly_uint64 array_size, vector_size;
 
   tree scalar_type = strip_array_types (type);
-  tree vectype = get_vectype_for_scalar_type_and_size (scalar_type, 0);
+  tree vectype = get_related_vectype_for_scalar_type (VOIDmode, scalar_type);
   if (!vectype
       || !poly_int_tree_p (TYPE_SIZE (type), &array_size)
       || !poly_int_tree_p (TYPE_SIZE (vectype), &vector_size)
