This backport contains 1 patch from gcc main stream tree.
The commit id of these patchs list as following in the order of time.

0001-re-PR-bootstrap-92301-Wrong-vectorizer-code-since-r2.patch
b76f4e6c06bd494d2383c4c16d1e1a034da74641

diff -Nurp a/gcc/testsuite/gcc.dg/pr92301.c b/gcc/testsuite/gcc.dg/pr92301.c
--- a/gcc/testsuite/gcc.dg/pr92301.c	1970-01-01 08:00:00.000000000 +0800
+++ b/gcc/testsuite/gcc.dg/pr92301.c	2020-08-24 21:36:23.556000000 +0800
@@ -0,0 +1,35 @@
+/* { dg-do run } */
+/* { dg-options "-O3" } */
+
+unsigned int m;
+
+#define N 128
+unsigned int a[N];
+
+unsigned int
+__attribute__((noipa))
+df_count_refs (_Bool include_defs)
+{
+  int size = 0;
+
+  for (unsigned int regno = 0; regno < m; regno++)
+    if (include_defs)
+      size += a[regno];
+  return size;
+}
+
+int main(int argc, char **argv)
+{
+  for (unsigned i = 0; i < N; i++)
+    a[i] = i;
+
+  if (argc == 1)
+    m = 17;
+
+  unsigned int r = df_count_refs(1);
+  __builtin_printf ("r: %d\n", r);
+  if (r != 136)
+    __builtin_abort ();
+
+  return 0;
+}
diff -Nurp a/gcc/tree-vect-stmts.c b/gcc/tree-vect-stmts.c
--- a/gcc/tree-vect-stmts.c	2020-08-24 21:35:23.664000000 +0800
+++ b/gcc/tree-vect-stmts.c	2020-08-24 21:36:23.556000000 +0800
@@ -474,6 +474,22 @@ process_use (stmt_vec_info stmt_vinfo, t
   basic_block def_bb = gimple_bb (dstmt_vinfo->stmt);
   basic_block bb = gimple_bb (stmt_vinfo->stmt);
 
+  /* case 2: A reduction phi (STMT) defined by a reduction stmt (DSTMT_VINFO).
+     We have to force the stmt live since the epilogue loop needs it to
+     continue computing the reduction.  */
+  if (gimple_code (stmt_vinfo->stmt) == GIMPLE_PHI
+      && STMT_VINFO_DEF_TYPE (stmt_vinfo) == vect_reduction_def
+      && gimple_code (dstmt_vinfo->stmt) != GIMPLE_PHI
+      && STMT_VINFO_DEF_TYPE (dstmt_vinfo) == vect_reduction_def
+      && bb->loop_father == def_bb->loop_father)
+    {
+      if (dump_enabled_p ())
+	dump_printf_loc (MSG_NOTE, vect_location,
+			 "reduc-stmt defining reduc-phi in the same nest.\n");
+      vect_mark_relevant (worklist, dstmt_vinfo, relevant, true);
+      return opt_result::success ();
+    }
+
   /* case 3a: outer-loop stmt defining an inner-loop stmt:
 	outer-loop-header-bb:
 		d = dstmt_vinfo