This backport contains 2 patchs from gcc main stream tree. 
The commit id of these patchs list as following in the order of time.

0001-Tweak-gcc.dg-vect-bb-slp-4-01-.c-PR92366.patch
3771033244b3ee1b53a8a00d734580b16384fdd3

0001-tree-vect-slp.c-vect_analyze_slp_instance-Dump-const.patch
140ee00a961fda084c1b4b3f0e7e489a917858f7

diff -Nurp a/gcc/testsuite/gcc.dg/vect/bb-slp-40.c b/gcc/testsuite/gcc.dg/vect/bb-slp-40.c
--- a/gcc/testsuite/gcc.dg/vect/bb-slp-40.c	2020-09-14 21:24:20.899694710 +0800
+++ b/gcc/testsuite/gcc.dg/vect/bb-slp-40.c	2020-09-15 20:54:05.456027442 +0800
@@ -1,5 +1,5 @@
 /* { dg-do compile } */
-/* { dg-options "-O3 -fdump-tree-slp-all" } */
+/* { dg-additional-options "-fvect-cost-model=dynamic" } */
 /* { dg-require-effective-target vect_int } */
 
 char g_d[1024], g_s1[1024], g_s2[1024];
@@ -30,5 +30,5 @@ void foo(void)
 }
 
 /* See that we vectorize an SLP instance.  */
-/* { dg-final { scan-tree-dump-times "Found vectorizable constructor" 1 "slp1" } } */
-/* { dg-final { scan-tree-dump-times "vectorizing stmts using SLP" 1 "slp1" } } */
+/* { dg-final { scan-tree-dump "Analyzing vectorizable constructor" "slp1" } } */
+/* { dg-final { scan-tree-dump "vectorizing stmts using SLP" "slp1" } } */
diff -Nurp a/gcc/testsuite/gcc.dg/vect/bb-slp-41.c b/gcc/testsuite/gcc.dg/vect/bb-slp-41.c
--- a/gcc/testsuite/gcc.dg/vect/bb-slp-41.c	2020-09-14 21:24:20.899694710 +0800
+++ b/gcc/testsuite/gcc.dg/vect/bb-slp-41.c	2020-09-15 20:54:10.424087539 +0800
@@ -1,10 +1,9 @@
-/* { dg-do run } */
-/* { dg-options "-O3 -fdump-tree-slp-all -fno-vect-cost-model" } */
 /* { dg-require-effective-target vect_int } */
 
 #define ARR_SIZE 1000
 
-void foo (int *a, int *b)
+void __attribute__((optimize (0)))
+foo (int *a, int *b)
 {
   int i;
   for (i = 0; i < (ARR_SIZE - 2); ++i)
@@ -56,6 +55,4 @@ int main ()
   return 0;
 
 }
-/* See that we vectorize an SLP instance.  */
-/* { dg-final { scan-tree-dump-times "Found vectorizable constructor" 12 "slp1" } } */
-/* { dg-final { scan-tree-dump-times "vectorizing stmts using SLP" 4 "slp1" } } */
+/* { dg-final { scan-tree-dump-not "vectorizing stmts using SLP" "slp1" } } */
diff -Nurp a/gcc/testsuite/gcc.dg/vect/bb-slp-42.c b/gcc/testsuite/gcc.dg/vect/bb-slp-42.c
--- a/gcc/testsuite/gcc.dg/vect/bb-slp-42.c	1970-01-01 08:00:00.000000000 +0800
+++ b/gcc/testsuite/gcc.dg/vect/bb-slp-42.c	2020-09-15 20:54:14.724139555 +0800
@@ -0,0 +1,48 @@
+/* { dg-require-effective-target vect_int } */
+/* { dg-require-effective-target vect_perm } */
+
+#include "tree-vect.h"
+
+#define ARR_SIZE 1024
+
+void __attribute__((noipa))
+foo (int a[][ARR_SIZE], int *b)
+{
+  int i;
+  for (i = 0; i < ARR_SIZE; ++i)
+    {
+      a[0][i] += b[0];
+      a[1][i] += b[1];
+      a[2][i] += b[2];
+      a[3][i] += b[3];
+    }
+}
+
+int
+main ()
+{
+  int a[4][ARR_SIZE];
+  int b[4];
+
+  check_vect ();
+
+  for (int i = 0; i < 4; ++i)
+    {
+      b[i] = 20 * i;
+      for (int j = 0; j < ARR_SIZE; ++j)
+	a[i][j] = (i + 1) * ARR_SIZE - j;
+    }
+
+  foo (a, b);
+
+  for (int i = 0; i < 4; ++i)
+    for (int j = 0; j < ARR_SIZE; ++j)
+      if (a[i][j] != (i + 1) * ARR_SIZE - j + 20 * i)
+	__builtin_abort ();
+
+  return 0;
+
+}
+
+/* See that we do not try to vectorize the uniform CTORs.  */
+/* { dg-final { scan-tree-dump-not "Analyzing vectorizable constructor" "slp1" } } */
diff -Nurp a/gcc/tree-vect-slp.c b/gcc/tree-vect-slp.c
--- a/gcc/tree-vect-slp.c	2020-09-14 21:24:20.983695752 +0800
+++ b/gcc/tree-vect-slp.c	2020-09-14 16:13:11.077779069 +0800
@@ -2106,6 +2106,10 @@ vect_analyze_slp_instance (vec_info *vin
 	  else
 	    return false;
 	}
+      if (dump_enabled_p ())
+	dump_printf_loc (MSG_NOTE, vect_location,
+			 "Analyzing vectorizable constructor: %G\n",
+			 stmt_info->stmt);
     }
   else
     {
@@ -3049,31 +3053,22 @@ vect_slp_check_for_constructors (bb_vec_
   gimple_stmt_iterator gsi;
 
   for (gsi = bb_vinfo->region_begin;
-      gsi_stmt (gsi) != gsi_stmt (bb_vinfo->region_end); gsi_next (&gsi))
+       gsi_stmt (gsi) != gsi_stmt (bb_vinfo->region_end); gsi_next (&gsi))
     {
-      gimple *stmt = gsi_stmt (gsi);
-
-      if (is_gimple_assign (stmt)
-	  && gimple_assign_rhs_code (stmt) == CONSTRUCTOR
-	  && TREE_CODE (gimple_assign_lhs (stmt)) == SSA_NAME
-	  && TREE_CODE (TREE_TYPE (gimple_assign_lhs (stmt))) == VECTOR_TYPE)
-	{
-	  tree rhs = gimple_assign_rhs1 (stmt);
-
-	  if (CONSTRUCTOR_NELTS (rhs) == 0)
-	    continue;
-
-	  poly_uint64 subparts = TYPE_VECTOR_SUBPARTS (TREE_TYPE (rhs));
+      gassign *stmt = dyn_cast <gassign *> (gsi_stmt (gsi));
+      if (!stmt || gimple_assign_rhs_code (stmt) != CONSTRUCTOR)
+	continue;
 
-	  if (maybe_ne (subparts, CONSTRUCTOR_NELTS (rhs)))
-	    continue;
+      tree rhs = gimple_assign_rhs1 (stmt);
+      if (!VECTOR_TYPE_P (TREE_TYPE (rhs))
+	  || maybe_ne (TYPE_VECTOR_SUBPARTS (TREE_TYPE (rhs)),
+		       CONSTRUCTOR_NELTS (rhs))
+	  || VECTOR_TYPE_P (TREE_TYPE (CONSTRUCTOR_ELT (rhs, 0)->value))
+	  || uniform_vector_p (rhs))
+	continue;
 
-	  if (dump_enabled_p ())
-	    dump_printf_loc (MSG_NOTE, vect_location,
-			     "Found vectorizable constructor: %G\n", stmt);
-	  stmt_vec_info stmt_info = bb_vinfo->lookup_stmt (stmt);
-	  BB_VINFO_GROUPED_STORES (bb_vinfo).safe_push (stmt_info);
-	}
+      stmt_vec_info stmt_info = bb_vinfo->lookup_stmt (stmt);
+      BB_VINFO_GROUPED_STORES (bb_vinfo).safe_push (stmt_info);
     }
 }