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-lra-Avoid-cycling-on-certain-subreg-reloads-PR96796.patch
6001db79c477b03eacc7e7049560921fb54b7845

diff -uprN a/gcc/lra-constraints.c b/gcc/lra-constraints.c
--- a/gcc/lra-constraints.c	2020-03-12 19:07:21.000000000 +0800
+++ b/gcc/lra-constraints.c	2020-09-08 10:02:52.308147305 +0800
@@ -235,12 +235,17 @@ get_reg_class (int regno)
    CL.  Use elimination first if REG is a hard register.  If REG is a
    reload pseudo created by this constraints pass, assume that it will
    be allocated a hard register from its allocno class, but allow that
-   class to be narrowed to CL if it is currently a superset of CL.
+   class to be narrowed to CL if it is currently a superset of CL and
+   if either:
+
+   - ALLOW_ALL_RELOAD_CLASS_CHANGES_P is true or
+   - the instruction we're processing is not a reload move.
 
    If NEW_CLASS is nonnull, set *NEW_CLASS to the new allocno class of
    REGNO (reg), or NO_REGS if no change in its class was needed.  */
 static bool
-in_class_p (rtx reg, enum reg_class cl, enum reg_class *new_class)
+in_class_p (rtx reg, enum reg_class cl, enum reg_class *new_class,
+	    bool allow_all_reload_class_changes_p = false)
 {
   enum reg_class rclass, common_class;
   machine_mode reg_mode;
@@ -265,7 +270,8 @@ in_class_p (rtx reg, enum reg_class cl,
 	 typically moves that have many alternatives, and restricting
 	 reload pseudos for one alternative may lead to situations
 	 where other reload pseudos are no longer allocatable.  */
-      || (INSN_UID (curr_insn) >= new_insn_uid_start
+      || (!allow_all_reload_class_changes_p
+	  && INSN_UID (curr_insn) >= new_insn_uid_start
 	  && curr_insn_set != NULL
 	  && ((OBJECT_P (SET_SRC (curr_insn_set))
 	       && ! CONSTANT_P (SET_SRC (curr_insn_set)))
@@ -557,13 +563,12 @@ init_curr_insn_input_reloads (void)
   curr_insn_input_reloads_num = 0;
 }
 
-/* Create a new pseudo using MODE, RCLASS, ORIGINAL or reuse already
-   created input reload pseudo (only if TYPE is not OP_OUT).  Don't
-   reuse pseudo if IN_SUBREG_P is true and the reused pseudo should be
-   wrapped up in SUBREG.  The result pseudo is returned through
-   RESULT_REG.  Return TRUE if we created a new pseudo, FALSE if we
-   reused the already created input reload pseudo.  Use TITLE to
-   describe new registers for debug purposes.  */
+/* Create a new pseudo using MODE, RCLASS, ORIGINAL or reuse an existing
+   reload pseudo.  Don't reuse an existing reload pseudo if IN_SUBREG_P
+   is true and the reused pseudo should be wrapped up in a SUBREG.
+   The result pseudo is returned through RESULT_REG.  Return TRUE if we
+   created a new pseudo, FALSE if we reused an existing reload pseudo.
+   Use TITLE to describe new registers for debug purposes.  */
 static bool
 get_reload_reg (enum op_type type, machine_mode mode, rtx original,
 		enum reg_class rclass, bool in_subreg_p,
@@ -575,6 +580,35 @@ get_reload_reg (enum op_type type, machi
 
   if (type == OP_OUT)
     {
+      /* Output reload registers tend to start out with a conservative
+	 choice of register class.  Usually this is ALL_REGS, although
+	 a target might narrow it (for performance reasons) through
+	 targetm.preferred_reload_class.  It's therefore quite common
+	 for a reload instruction to require a more restrictive class
+	 than the class that was originally assigned to the reload register.
+
+	 In these situations, it's more efficient to refine the choice
+	 of register class rather than create a second reload register.
+	 This also helps to avoid cycling for registers that are only
+	 used by reload instructions.  */
+      if (REG_P (original)
+	  && (int) REGNO (original) >= new_regno_start
+	  && INSN_UID (curr_insn) >= new_insn_uid_start
+	  && in_class_p (original, rclass, &new_class, true))
+	{
+	  unsigned int regno = REGNO (original);
+	  if (lra_dump_file != NULL)
+	    {
+	      fprintf (lra_dump_file, "	 Reuse r%d for output ", regno);
+	      dump_value_slim (lra_dump_file, original, 1);
+	    }
+	  if (new_class != lra_get_allocno_class (regno))
+	    lra_change_class (regno, new_class, ", change to", false);
+	  if (lra_dump_file != NULL)
+	    fprintf (lra_dump_file, "\n");
+	  *result_reg = original;
+	  return false;
+	}
       *result_reg
 	= lra_create_new_reg_with_unique_value (mode, original, rclass, title);
       return true;
diff -uprN a/gcc/testsuite/gcc.c-torture/compile/pr96796.c b/gcc/testsuite/gcc.c-torture/compile/pr96796.c
--- a/gcc/testsuite/gcc.c-torture/compile/pr96796.c	1970-01-01 08:00:00.000000000 +0800
+++ b/gcc/testsuite/gcc.c-torture/compile/pr96796.c	2020-09-08 09:59:40.077774393 +0800
@@ -0,0 +1,55 @@
+/* { dg-additional-options "-fcommon" } */
+
+struct S0 {
+  signed f0 : 8;
+  unsigned f1;
+  unsigned f4;
+};
+struct S1 {
+  long f3;
+  char f4;
+} g_3_4;
+
+int g_5, func_1_l_32, func_50___trans_tmp_31;
+static struct S0 g_144, g_834, g_1255, g_1261;
+
+int g_273[120] = {};
+int *g_555;
+char **g_979;
+static int g_1092_0;
+static int g_1193;
+int safe_mul_func_int16_t_s_s(int si1, int si2) { return si1 * si2; }
+static struct S0 *func_50();
+int func_1() { func_50(g_3_4, g_5, func_1_l_32, 8, 3); }
+void safe_div_func_int64_t_s_s(int *);
+void safe_mod_func_uint32_t_u_u(struct S0);
+struct S0 *func_50(int p_51, struct S0 p_52, struct S1 p_53, int p_54,
+                   int p_55) {
+  int __trans_tmp_30;
+  char __trans_tmp_22;
+  short __trans_tmp_19;
+  long l_985_1;
+  long l_1191[8];
+  safe_div_func_int64_t_s_s(g_273);
+  __builtin_printf((char*)g_1261.f4);
+  safe_mod_func_uint32_t_u_u(g_834);
+  g_144.f0 += 1;
+  for (;;) {
+    struct S1 l_1350 = {&l_1350};
+    for (; p_53.f3; p_53.f3 -= 1)
+      for (; g_1193 <= 2; g_1193 += 1) {
+        __trans_tmp_19 = safe_mul_func_int16_t_s_s(l_1191[l_985_1 + p_53.f3],
+                                                   p_55 % (**g_979 = 10));
+        __trans_tmp_22 = g_1255.f1 * p_53.f4;
+        __trans_tmp_30 = __trans_tmp_19 + __trans_tmp_22;
+        if (__trans_tmp_30)
+          g_1261.f0 = p_51;
+        else {
+          g_1255.f0 = p_53.f3;
+          int *l_1422 = g_834.f0 = g_144.f4 != (*l_1422)++ > 0 < 0 ^ 51;
+          g_555 = ~0;
+          g_1092_0 |= func_50___trans_tmp_31;
+        }
+      }
+  }
+}