PR/59994 gcc/m68k generates bad code for stack protector

Record the alignment of the stack canary before allocating space for
it in a stack frame.

Generate an assignment expression to initialise the canary instead of
emitting a move instruction.

Author: kalvisd

external/gpl3/gcc.old/dist/gcc/cfgexpand.cc

@@ -2067,8 +2067,11 @@ create_stack_guard (void)
 {
   tree guard = build_decl (DECL_SOURCE_LOCATION (current_function_decl),
 			   VAR_DECL, NULL, ptr_type_node);
+  machine_mode mode = TYPE_MODE (TREE_TYPE (guard));
   TREE_THIS_VOLATILE (guard) = 1;
   TREE_USED (guard) = 1;
+  // record the alignment requirements for this stack slot
+  record_alignment_for_reg_var (GET_MODE_ALIGNMENT (mode));
   expand_one_stack_var (guard);
   crtl->stack_protect_guard = guard;
 }
@@ -6544,11 +6547,13 @@ stack_protect_prologue (void)
   rtx x, y;
 
   crtl->stack_protect_guard_decl = guard_decl;
-  x = expand_normal (crtl->stack_protect_guard);
+  x = NULL_RTX;
 
   if (targetm.have_stack_protect_combined_set () && guard_decl)
     {
       gcc_assert (DECL_P (guard_decl));
+      if (x == NULL_RTX)
+        x = expand_normal (crtl->stack_protect_guard);
       y = DECL_RTL (guard_decl);
 
       /* Allow the target to compute address of Y and copy it to X without
@@ -6562,22 +6567,29 @@ stack_protect_prologue (void)
 	}
     }
 
-  if (guard_decl)
-    y = expand_normal (guard_decl);
-  else
-    y = const0_rtx;
-
   /* Allow the target to copy from Y to X without leaking Y into a
      register.  */
   if (targetm.have_stack_protect_set ())
-    if (rtx_insn *insn = targetm.gen_stack_protect_set (x, y))
-      {
-	emit_insn (insn);
-	return;
-      }
+    {
+      if (x == NULL_RTX)
+        x = expand_normal (crtl->stack_protect_guard);
+      if (guard_decl)
+        y = expand_normal (guard_decl);
+      else
+        y = const0_rtx;
+
+      if (rtx_insn *insn = targetm.gen_stack_protect_set (x, y))
+        {
+	  emit_insn (insn);
+	  return;
+	}
+    }
 
-  /* Otherwise do a straight move.  */
-  emit_move_insn (x, y);
+  /* Otherwise generate and expand an assignment expression.  This
+     generates code that works in the case where the stack is aligned
+     differently to the guard variable (e.g. on m68k). */
+  expand_assignment(crtl->stack_protect_guard,
+      crtl->stack_protect_guard_decl, false);
 }
 
 /* Translate the intermediate representation contained in the CFG