Index: gcc/expr.c
===================================================================
--- gcc/expr.c	(revision 205234)
+++ gcc/expr.c	(working copy)
@@ -4811,6 +4811,17 @@ expand_assignment (tree to, tree from, bool nontem
       if (TREE_CODE (to) == COMPONENT_REF
 	  && DECL_BIT_FIELD_TYPE (TREE_OPERAND (to, 1)))
 	get_bit_range (&bitregion_start, &bitregion_end, to, &bitpos, &offset);
+      /* The C++ memory model naturally applies to byte-aligned fields.
+	 However, if we do not have a DECL_BIT_FIELD_TYPE but BITPOS or
+	 BITSIZE are not byte-aligned, there is no need to limit the range
+	 we can access.  This can occur with packed structures in Ada.  */
+      else if (bitsize > 0
+	       && bitsize % BITS_PER_UNIT == 0
+	       && bitpos % BITS_PER_UNIT == 0)
+	{
+	  bitregion_start = bitpos;
+	  bitregion_end = bitpos + bitsize - 1;
+	}
 
       to_rtx = expand_expr (tem, NULL_RTX, VOIDmode, EXPAND_WRITE);
 
--- gcc/testsuite/gcc.dg/pr56997-4.c	(working copy)
+++ gcc/testsuite/gcc.dg/pr56997-4.c	(working copy)
@@ -0,0 +1,23 @@
+/* Test volatile access to unaligned field.  */
+/* { dg-do compile } */
+/* { dg-options "-fno-strict-volatile-bitfields -fdump-rtl-final" } */
+
+#define test_type unsigned short
+
+typedef struct s{
+ unsigned char Prefix[1];
+ volatile test_type Type;
+}__attribute((__packed__,__aligned__(4))) ss;
+
+extern volatile ss v;
+
+void
+foo (test_type u)
+{
+  v.Type = u;
+}
+
+/* The C++ memory model forbids data store race conditions outside the
+   unaligned data member, therefore only QI or HI access is allowed, no SI.  */
+/* { dg-final { scan-rtl-dump-not "mem/v(/.)*:SI" "final" } } */
+/* { dg-final { cleanup-rtl-dump "final" } } */