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-tree-ssa-sccvn.c-class-pass_fre-Add-may_iterate-pass.patch
744fd446c321f78f9a1ce4ef5f83df8dcfa44a9e

diff -Nurp a/gcc/passes.def b/gcc/passes.def
--- a/gcc/passes.def	2020-08-17 09:46:40.340000000 +0800
+++ b/gcc/passes.def	2020-08-17 10:09:10.808000000 +0800
@@ -83,7 +83,7 @@ along with GCC; see the file COPYING3.
 	  /* pass_build_ealias is a dummy pass that ensures that we
 	     execute TODO_rebuild_alias at this point.  */
 	  NEXT_PASS (pass_build_ealias);
-	  NEXT_PASS (pass_fre);
+	  NEXT_PASS (pass_fre, true /* may_iterate */);
 	  NEXT_PASS (pass_early_vrp);
 	  NEXT_PASS (pass_merge_phi);
           NEXT_PASS (pass_dse);
@@ -117,7 +117,7 @@ along with GCC; see the file COPYING3.
 	  NEXT_PASS (pass_oacc_kernels);
 	  PUSH_INSERT_PASSES_WITHIN (pass_oacc_kernels)
 	      NEXT_PASS (pass_ch);
-	      NEXT_PASS (pass_fre);
+	      NEXT_PASS (pass_fre, true /* may_iterate */);
 	      /* We use pass_lim to rewrite in-memory iteration and reduction
 		 variable accesses in loops into local variables accesses.  */
 	      NEXT_PASS (pass_lim);
@@ -201,7 +201,7 @@ along with GCC; see the file COPYING3.
 	 execute TODO_rebuild_alias at this point.  */
       NEXT_PASS (pass_build_alias);
       NEXT_PASS (pass_return_slot);
-      NEXT_PASS (pass_fre);
+      NEXT_PASS (pass_fre, true /* may_iterate */);
       NEXT_PASS (pass_merge_phi);
       NEXT_PASS (pass_thread_jumps);
       NEXT_PASS (pass_vrp, true /* warn_array_bounds_p */);
@@ -312,6 +312,7 @@ along with GCC; see the file COPYING3.
       NEXT_PASS (pass_strength_reduction);
       NEXT_PASS (pass_split_paths);
       NEXT_PASS (pass_tracer);
+      NEXT_PASS (pass_fre, false /* may_iterate */);
       NEXT_PASS (pass_thread_jumps);
       NEXT_PASS (pass_dominator, false /* may_peel_loop_headers_p */);
       NEXT_PASS (pass_strlen);
diff -Nurp a/gcc/testsuite/gcc.dg/tree-ssa/pr77445-2.c b/gcc/testsuite/gcc.dg/tree-ssa/pr77445-2.c
--- a/gcc/testsuite/gcc.dg/tree-ssa/pr77445-2.c	2020-08-17 09:46:41.332000000 +0800
+++ b/gcc/testsuite/gcc.dg/tree-ssa/pr77445-2.c	2020-08-17 10:09:10.808000000 +0800
@@ -125,7 +125,7 @@ enum STATES FMS( u8 **in , u32 *transiti
    jump threading opportunities.  Skip the later tests on aarch64.  */
 /* { dg-final { scan-tree-dump "Jumps threaded: 1\[1-9\]" "thread1" } } */
 /* { dg-final { scan-tree-dump-times "Invalid sum" 3 "thread1" } } */
-/* { dg-final { scan-tree-dump-not "not considered" "thread1" } } */
-/* { dg-final { scan-tree-dump-not "not considered" "thread2" } } */
-/* { dg-final { scan-tree-dump-not "not considered" "thread3" { target { ! aarch64*-*-* } } } } */
-/* { dg-final { scan-tree-dump-not "not considered" "thread4" { target { ! aarch64*-*-* } } } } */ 
+/* { dg-final { scan-tree-dump-not "optimizing for size" "thread1" } } */
+/* { dg-final { scan-tree-dump-not "optimizing for size" "thread2" } } */
+/* { dg-final { scan-tree-dump-not "optimizing for size" "thread3" { target { ! aarch64*-*-* } } } } */
+/* { dg-final { scan-tree-dump-not "optimizing for size" "thread4" { target { ! aarch64*-*-* } } } } */ 
diff -Nurp a/gcc/tree-ssa-sccvn.c b/gcc/tree-ssa-sccvn.c
--- a/gcc/tree-ssa-sccvn.c	2020-08-17 09:46:42.212000000 +0800
+++ b/gcc/tree-ssa-sccvn.c	2020-08-17 10:09:10.808000000 +0800
@@ -7232,14 +7232,24 @@ class pass_fre : public gimple_opt_pass
 {
 public:
   pass_fre (gcc::context *ctxt)
-    : gimple_opt_pass (pass_data_fre, ctxt)
+    : gimple_opt_pass (pass_data_fre, ctxt), may_iterate (true)
   {}
 
   /* opt_pass methods: */
   opt_pass * clone () { return new pass_fre (m_ctxt); }
-  virtual bool gate (function *) { return flag_tree_fre != 0; }
+  void set_pass_param (unsigned int n, bool param)
+    {
+      gcc_assert (n == 0);
+      may_iterate = param;
+    }
+  virtual bool gate (function *)
+    {
+      return flag_tree_fre != 0 && (may_iterate || optimize > 1);
+    }
   virtual unsigned int execute (function *);
 
+private:
+  bool may_iterate;
 }; // class pass_fre
 
 unsigned int
@@ -7248,15 +7258,16 @@ pass_fre::execute (function *fun)
   unsigned todo = 0;
 
   /* At -O[1g] use the cheap non-iterating mode.  */
+  bool iterate_p = may_iterate && (optimize > 1);
   calculate_dominance_info (CDI_DOMINATORS);
-  if (optimize > 1)
+  if (iterate_p)
     loop_optimizer_init (AVOID_CFG_MODIFICATIONS);
 
   default_vn_walk_kind = VN_WALKREWRITE;
-  todo = do_rpo_vn (fun, NULL, NULL, optimize > 1, true);
+  todo = do_rpo_vn (fun, NULL, NULL, iterate_p, true);
   free_rpo_vn ();
 
-  if (optimize > 1)
+  if (iterate_p)
     loop_optimizer_finalize ();
 
   return todo;