Add missing store in emission of asan_stack_free.

gcc/ChangeLog:

2020-05-19  Martin Liska  <mliska@suse.cz>

	PR sanitizer/94910
	* asan.c (asan_emit_stack_protection): Emit
	also **SavedFlagPtr(FakeStack, class_id) = 0 in order to release
	a stack frame.

(cherry picked from commit 8b6731e674c76cb48a417f2eef74ced92a17f469)

gcc/asan.c

@@ -1598,8 +1598,24 @@ asan_emit_stack_protection (rtx base, rtx pbase, unsigned int alignb,
       if (use_after_return_class < 5
 	  && can_store_by_pieces (sz, builtin_memset_read_str, &c,
 				  BITS_PER_UNIT, true))
-	store_by_pieces (shadow_mem, sz, builtin_memset_read_str, &c,
-			 BITS_PER_UNIT, true, RETURN_BEGIN);
+	{
+	  /* Emit:
+	       memset(ShadowBase, kAsanStackAfterReturnMagic, ShadowSize);
+	       **SavedFlagPtr(FakeStack, class_id) = 0
+	  */
+	  store_by_pieces (shadow_mem, sz, builtin_memset_read_str, &c,
+			   BITS_PER_UNIT, true, RETURN_BEGIN);
+
+	  unsigned HOST_WIDE_INT offset
+	    = (1 << (use_after_return_class + 6));
+	  offset -= GET_MODE_SIZE (ptr_mode);
+	  mem = gen_rtx_MEM (ptr_mode, base);
+	  mem = adjust_address (mem, ptr_mode, offset);
+	  rtx addr = gen_reg_rtx (ptr_mode);
+	  emit_move_insn (addr, mem);
+	  mem = gen_rtx_MEM (QImode, addr);
+	  emit_move_insn (mem, const0_rtx);
+	}
       else if (use_after_return_class >= 5
 	       || !set_storage_via_setmem (shadow_mem,
 					   GEN_INT (sz),