Index: doc/invoke.texi
===================================================================
--- doc/invoke.texi	(revision 173263)
+++ doc/invoke.texi	(working copy)
@@ -8886,6 +8886,11 @@ The maximum number of conditional stores
 if either vectorization (@option{-ftree-vectorize}) or if-conversion
 (@option{-ftree-loop-if-convert}) is disabled.  The default is 2.
 
+@item allow-store-data-races
+Allow optimizers to introduce new data races on stores.
+Set to 1 to allow, otherwise to 0.  This option is enabled by default
+unless implicitly set by the @option{-fmemory-model=} option.
+
 @end table
 @end table
 
Index: machmode.h
===================================================================
--- machmode.h	(revision 173263)
+++ machmode.h	(working copy)
@@ -248,7 +248,10 @@ extern enum machine_mode mode_for_vector
 
 /* Find the best mode to use to access a bit field.  */
 
-extern enum machine_mode get_best_mode (int, int, unsigned int,
+extern enum machine_mode get_best_mode (int, int,
+					unsigned HOST_WIDE_INT,
+					unsigned HOST_WIDE_INT,
+					unsigned int,
 					enum machine_mode, int);
 
 /* Determine alignment, 1<=result<=BIGGEST_ALIGNMENT.  */
Index: tree.h
===================================================================
--- tree.h	(revision 173263)
+++ tree.h	(working copy)
@@ -3156,6 +3156,10 @@ struct GTY(()) tree_parm_decl {
 #define DECL_THREAD_LOCAL_P(NODE) \
   (VAR_DECL_CHECK (NODE)->decl_with_vis.tls_model >= TLS_MODEL_REAL)
 
+/* Return true if a VAR_DECL is visible from another thread.  */
+#define DECL_THREAD_VISIBLE_P(NODE) \
+  (TREE_STATIC (NODE) && !DECL_THREAD_LOCAL_P (NODE))
+
 /* In a non-local VAR_DECL with static storage duration, true if the
    variable has an initialization priority.  If false, the variable
    will be initialized at the DEFAULT_INIT_PRIORITY.  */
Index: fold-const.c
===================================================================
--- fold-const.c	(revision 173263)
+++ fold-const.c	(working copy)
@@ -3409,7 +3409,7 @@ optimize_bit_field_compare (location_t l
       && flag_strict_volatile_bitfields > 0)
     nmode = lmode;
   else
-    nmode = get_best_mode (lbitsize, lbitpos,
+    nmode = get_best_mode (lbitsize, lbitpos, 0, 0,
 			   const_p ? TYPE_ALIGN (TREE_TYPE (linner))
 			   : MIN (TYPE_ALIGN (TREE_TYPE (linner)),
 				  TYPE_ALIGN (TREE_TYPE (rinner))),
@@ -5237,7 +5237,7 @@ fold_truthop (location_t loc, enum tree_
      to be relative to a field of that size.  */
   first_bit = MIN (ll_bitpos, rl_bitpos);
   end_bit = MAX (ll_bitpos + ll_bitsize, rl_bitpos + rl_bitsize);
-  lnmode = get_best_mode (end_bit - first_bit, first_bit,
+  lnmode = get_best_mode (end_bit - first_bit, first_bit, 0, 0,
 			  TYPE_ALIGN (TREE_TYPE (ll_inner)), word_mode,
 			  volatilep);
   if (lnmode == VOIDmode)
@@ -5302,7 +5302,7 @@ fold_truthop (location_t loc, enum tree_
 
       first_bit = MIN (lr_bitpos, rr_bitpos);
       end_bit = MAX (lr_bitpos + lr_bitsize, rr_bitpos + rr_bitsize);
-      rnmode = get_best_mode (end_bit - first_bit, first_bit,
+      rnmode = get_best_mode (end_bit - first_bit, first_bit, 0, 0,
 			      TYPE_ALIGN (TREE_TYPE (lr_inner)), word_mode,
 			      volatilep);
       if (rnmode == VOIDmode)
Index: params.h
===================================================================
--- params.h	(revision 173263)
+++ params.h	(working copy)
@@ -206,4 +206,6 @@ extern void init_param_values (int *para
   PARAM_VALUE (PARAM_MIN_NONDEBUG_INSN_UID)
 #define MAX_STORES_TO_SINK \
   PARAM_VALUE (PARAM_MAX_STORES_TO_SINK)
+#define ALLOW_STORE_DATA_RACES \
+  PARAM_VALUE (PARAM_ALLOW_STORE_DATA_RACES)
 #endif /* ! GCC_PARAMS_H */
Index: testsuite/gcc.dg/20110509.c
===================================================================
--- testsuite/gcc.dg/20110509.c	(revision 0)
+++ testsuite/gcc.dg/20110509.c	(revision 0)
@@ -0,0 +1,18 @@
+/* { dg-do compile { target i?86-*-* x86_64-*-* } } */
+/* { dg-options "-O2 --param allow-store-data-races=0" } */
+
+/* Test that we don't store past VAR.A.  */
+
+struct S
+{
+  volatile unsigned int a : 4;
+  unsigned char b;
+  unsigned int c : 6;
+} var;
+
+void set_a()
+{
+  var.a = 12;
+}
+
+/* { dg-final { scan-assembler-not "movl.*, var" } } */
Index: testsuite/gcc.dg/20110509-2.c
===================================================================
--- testsuite/gcc.dg/20110509-2.c	(revision 0)
+++ testsuite/gcc.dg/20110509-2.c	(revision 0)
@@ -0,0 +1,19 @@
+/* { dg-do compile { target i?86-*-* x86_64-*-* } } */
+/* { dg-options "-O2 --param allow-store-data-races=0" } */
+
+/* Test that we don't store past VAR.K.  */
+
+struct S
+{
+  volatile int i;
+  volatile int j: 32;
+  volatile int k: 15;
+  volatile char c[2];
+} var;
+
+void setit()
+{
+  var.k = 13;
+}
+
+/* { dg-final { scan-assembler-not "movl.*, var" } } */
Index: ifcvt.c
===================================================================
--- ifcvt.c	(revision 173263)
+++ ifcvt.c	(working copy)
@@ -885,7 +885,7 @@ noce_emit_move_insn (rtx x, rtx y)
 		}
 
 	      gcc_assert (start < (MEM_P (op) ? BITS_PER_UNIT : BITS_PER_WORD));
-	      store_bit_field (op, size, start, GET_MODE (x), y);
+	      store_bit_field (op, size, start, 0, 0, GET_MODE (x), y);
 	      return;
 	    }
 
@@ -939,7 +939,8 @@ noce_emit_move_insn (rtx x, rtx y)
   inner = XEXP (outer, 0);
   outmode = GET_MODE (outer);
   bitpos = SUBREG_BYTE (outer) * BITS_PER_UNIT;
-  store_bit_field (inner, GET_MODE_BITSIZE (outmode), bitpos, outmode, y);
+  store_bit_field (inner, GET_MODE_BITSIZE (outmode), bitpos,
+		   0, 0, outmode, y);
 }
 
 /* Return sequence of instructions generated by if conversion.  This
Index: expr.c
===================================================================
--- expr.c	(revision 173263)
+++ expr.c	(working copy)
@@ -54,6 +54,7 @@ along with GCC; see the file COPYING3.  
 #include "diagnostic.h"
 #include "ssaexpand.h"
 #include "target-globals.h"
+#include "params.h"
 
 /* Decide whether a function's arguments should be processed
    from first to last or from last to first.
@@ -142,7 +143,9 @@ static void store_constructor_field (rtx
 				     HOST_WIDE_INT, enum machine_mode,
 				     tree, tree, int, alias_set_type);
 static void store_constructor (tree, rtx, int, HOST_WIDE_INT);
-static rtx store_field (rtx, HOST_WIDE_INT, HOST_WIDE_INT, enum machine_mode,
+static rtx store_field (rtx, HOST_WIDE_INT, HOST_WIDE_INT,
+			unsigned HOST_WIDE_INT, unsigned HOST_WIDE_INT,
+			enum machine_mode,
 			tree, tree, alias_set_type, bool);
 
 static unsigned HOST_WIDE_INT highest_pow2_factor_for_target (const_tree, const_tree);
@@ -2063,7 +2066,7 @@ emit_group_store (rtx orig_dst, rtx src,
 	emit_move_insn (adjust_address (dest, mode, bytepos), tmps[i]);
       else
 	store_bit_field (dest, bytelen * BITS_PER_UNIT, bytepos * BITS_PER_UNIT,
-			 mode, tmps[i]);
+			 0, 0, mode, tmps[i]);
     }
 
   /* Copy from the pseudo into the (probable) hard reg.  */
@@ -2157,7 +2160,7 @@ copy_blkmode_from_reg (rtx tgtblk, rtx s
 
       /* Use xbitpos for the source extraction (right justified) and
 	 bitpos for the destination store (left justified).  */
-      store_bit_field (dst, bitsize, bitpos % BITS_PER_WORD, copy_mode,
+      store_bit_field (dst, bitsize, bitpos % BITS_PER_WORD, 0, 0, copy_mode,
 		       extract_bit_field (src, bitsize,
 					  xbitpos % BITS_PER_WORD, 1, false,
 					  NULL_RTX, copy_mode, copy_mode));
@@ -2794,7 +2797,7 @@ write_complex_part (rtx cplx, rtx val, b
 	gcc_assert (MEM_P (cplx) && ibitsize < BITS_PER_WORD);
     }
 
-  store_bit_field (cplx, ibitsize, imag_p ? ibitsize : 0, imode, val);
+  store_bit_field (cplx, ibitsize, imag_p ? ibitsize : 0, 0, 0, imode, val);
 }
 
 /* Extract one of the components of the complex value CPLX.  Extract the
@@ -3929,6 +3932,8 @@ get_subtarget (rtx x)
 static bool
 optimize_bitfield_assignment_op (unsigned HOST_WIDE_INT bitsize,
 				 unsigned HOST_WIDE_INT bitpos,
+				 unsigned HOST_WIDE_INT bitregion_start,
+				 unsigned HOST_WIDE_INT bitregion_end,
 				 enum machine_mode mode1, rtx str_rtx,
 				 tree to, tree src)
 {
@@ -3990,6 +3995,7 @@ optimize_bitfield_assignment_op (unsigne
       if (str_bitsize == 0 || str_bitsize > BITS_PER_WORD)
 	str_mode = word_mode;
       str_mode = get_best_mode (bitsize, bitpos,
+				bitregion_start, bitregion_end,
 				MEM_ALIGN (str_rtx), str_mode, 0);
       if (str_mode == VOIDmode)
 	return false;
@@ -4098,6 +4104,111 @@ optimize_bitfield_assignment_op (unsigne
   return false;
 }
 
+/* In the C++ memory model, consecutive bit fields in a structure are
+   considered one memory location.
+
+   Given a COMPONENT_REF, this function returns the bit range of
+   consecutive bits in which this COMPONENT_REF belongs in.  The
+   values are returned in *BITSTART and *BITEND.  If either the C++
+   memory model is not in activated, or this memory access is not
+   thread visible, 0 is returned in *BITSTART and *BITEND.
+
+   EXP is the COMPONENT_REF.
+   INNERDECL is the actual object being referenced.
+   BITPOS is the position in bits where the bit starts within the structure.
+   BITSIZE is size in bits of the field being referenced in EXP.
+
+   For example, while storing into FOO.A here...
+
+      struct {
+        BIT 0:
+          unsigned int a : 4;
+	  unsigned int b : 1;
+	BIT 8:
+	  unsigned char c;
+	  unsigned int d : 6;
+      } foo;
+
+   ...we are not allowed to store past <b>, so for the layout above, a
+   range of 0..7 (because no one cares if we store into the
+   padding).  */
+
+static void
+get_bit_range (unsigned HOST_WIDE_INT *bitstart,
+	       unsigned HOST_WIDE_INT *bitend,
+	       tree exp, tree innerdecl,
+	       HOST_WIDE_INT bitpos, HOST_WIDE_INT bitsize)
+{
+  tree field, record_type, fld;
+  bool found_field = false;
+  bool prev_field_is_bitfield;
+
+  gcc_assert (TREE_CODE (exp) == COMPONENT_REF);
+
+  /* If other threads can't see this value, no need to restrict stores.  */
+  if (ALLOW_STORE_DATA_RACES
+      || !DECL_THREAD_VISIBLE_P (innerdecl))
+    {
+      *bitstart = *bitend = 0;
+      return;
+    }
+
+  /* Bit field we're storing into.  */
+  field = TREE_OPERAND (exp, 1);
+  record_type = DECL_FIELD_CONTEXT (field);
+
+  /* Count the contiguous bitfields for the memory location that
+     contains FIELD.  */
+  *bitstart = 0;
+  prev_field_is_bitfield = true;
+  for (fld = TYPE_FIELDS (record_type); fld; fld = DECL_CHAIN (fld))
+    {
+      tree t, offset;
+      enum machine_mode mode;
+      int unsignedp, volatilep;
+
+      if (TREE_CODE (fld) != FIELD_DECL)
+	continue;
+
+      t = build3 (COMPONENT_REF, TREE_TYPE (exp),
+		  unshare_expr (TREE_OPERAND (exp, 0)),
+		  fld, NULL_TREE);
+      get_inner_reference (t, &bitsize, &bitpos, &offset,
+			   &mode, &unsignedp, &volatilep, true);
+
+      if (field == fld)
+	found_field = true;
+
+      if (DECL_BIT_FIELD_TYPE (fld) && bitsize > 0)
+	{
+	  if (prev_field_is_bitfield == false)
+	    {
+	      *bitstart = bitpos;
+	      prev_field_is_bitfield = true;
+	    }
+	}
+      else
+	{
+	  prev_field_is_bitfield = false;
+	  if (found_field)
+	    break;
+	}
+    }
+  gcc_assert (found_field);
+
+  if (fld)
+    {
+      /* We found the end of the bit field sequence.  Include the
+	 padding up to the next field and be done.  */
+      *bitend = bitpos - 1;
+    }
+  else
+    {
+      /* If this is the last element in the structure, include the padding
+	 at the end of structure.  */
+      *bitend = TREE_INT_CST_LOW (TYPE_SIZE (record_type));
+    }
+}
 
 /* Expand an assignment that stores the value of FROM into TO.  If NONTEMPORAL
    is true, try generating a nontemporal store.  */
@@ -4197,6 +4308,8 @@ expand_assignment (tree to, tree from, b
     {
       enum machine_mode mode1;
       HOST_WIDE_INT bitsize, bitpos;
+      unsigned HOST_WIDE_INT bitregion_start = 0;
+      unsigned HOST_WIDE_INT bitregion_end = 0;
       tree offset;
       int unsignedp;
       int volatilep = 0;
@@ -4206,6 +4319,11 @@ expand_assignment (tree to, tree from, b
       tem = get_inner_reference (to, &bitsize, &bitpos, &offset, &mode1,
 				 &unsignedp, &volatilep, true);
 
+      if (TREE_CODE (to) == COMPONENT_REF
+	  && DECL_BIT_FIELD_TYPE (TREE_OPERAND (to, 1)))
+	get_bit_range (&bitregion_start, &bitregion_end,
+		       to, tem, bitpos, bitsize);
+
       /* If we are going to use store_bit_field and extract_bit_field,
 	 make sure to_rtx will be safe for multiple use.  */
 
@@ -4287,11 +4405,14 @@ expand_assignment (tree to, tree from, b
 				 nontemporal);
 	  else if (bitpos + bitsize <= mode_bitsize / 2)
 	    result = store_field (XEXP (to_rtx, 0), bitsize, bitpos,
+				  bitregion_start, bitregion_end,
 				  mode1, from, TREE_TYPE (tem),
 				  get_alias_set (to), nontemporal);
 	  else if (bitpos >= mode_bitsize / 2)
 	    result = store_field (XEXP (to_rtx, 1), bitsize,
-				  bitpos - mode_bitsize / 2, mode1, from,
+				  bitpos - mode_bitsize / 2,
+				  bitregion_start, bitregion_end,
+				  mode1, from,
 				  TREE_TYPE (tem), get_alias_set (to),
 				  nontemporal);
 	  else if (bitpos == 0 && bitsize == mode_bitsize)
@@ -4312,7 +4433,9 @@ expand_assignment (tree to, tree from, b
 					    0);
 	      write_complex_part (temp, XEXP (to_rtx, 0), false);
 	      write_complex_part (temp, XEXP (to_rtx, 1), true);
-	      result = store_field (temp, bitsize, bitpos, mode1, from,
+	      result = store_field (temp, bitsize, bitpos,
+				    bitregion_start, bitregion_end,
+				    mode1, from,
 				    TREE_TYPE (tem), get_alias_set (to),
 				    nontemporal);
 	      emit_move_insn (XEXP (to_rtx, 0), read_complex_part (temp, false));
@@ -4337,11 +4460,15 @@ expand_assignment (tree to, tree from, b
 		MEM_KEEP_ALIAS_SET_P (to_rtx) = 1;
 	    }
 
-	  if (optimize_bitfield_assignment_op (bitsize, bitpos, mode1,
+	  if (optimize_bitfield_assignment_op (bitsize, bitpos,
+					       bitregion_start, bitregion_end,
+					       mode1,
 					       to_rtx, to, from))
 	    result = NULL;
 	  else
-	    result = store_field (to_rtx, bitsize, bitpos, mode1, from,
+	    result = store_field (to_rtx, bitsize, bitpos,
+				  bitregion_start, bitregion_end,
+				  mode1, from,
 				  TREE_TYPE (tem), get_alias_set (to),
 				  nontemporal);
 	}
@@ -4734,7 +4861,7 @@ store_expr (tree exp, rtx target, int ca
 			      : BLOCK_OP_NORMAL));
 	  else if (GET_MODE (target) == BLKmode)
 	    store_bit_field (target, INTVAL (expr_size (exp)) * BITS_PER_UNIT,
-			     0, GET_MODE (temp), temp);
+			     0, 0, 0, GET_MODE (temp), temp);
 	  else
 	    convert_move (target, temp, unsignedp);
 	}
@@ -5177,7 +5304,8 @@ store_constructor_field (rtx target, uns
       store_constructor (exp, target, cleared, bitsize / BITS_PER_UNIT);
     }
   else
-    store_field (target, bitsize, bitpos, mode, exp, type, alias_set, false);
+    store_field (target, bitsize, bitpos, 0, 0, mode, exp, type, alias_set,
+		 false);
 }
 
 /* Store the value of constructor EXP into the rtx TARGET.
@@ -5751,6 +5879,11 @@ store_constructor (tree exp, rtx target,
    BITSIZE bits, starting BITPOS bits from the start of TARGET.
    If MODE is VOIDmode, it means that we are storing into a bit-field.
 
+   BITREGION_START is bitpos of the first bitfield in this region.
+   BITREGION_END is the bitpos of the ending bitfield in this region.
+   These two fields are 0, if the C++ memory model does not apply,
+   or we are not interested in keeping track of bitfield regions.
+
    Always return const0_rtx unless we have something particular to
    return.
 
@@ -5764,6 +5897,8 @@ store_constructor (tree exp, rtx target,
 
 static rtx
 store_field (rtx target, HOST_WIDE_INT bitsize, HOST_WIDE_INT bitpos,
+	     unsigned HOST_WIDE_INT bitregion_start,
+	     unsigned HOST_WIDE_INT bitregion_end,
 	     enum machine_mode mode, tree exp, tree type,
 	     alias_set_type alias_set, bool nontemporal)
 {
@@ -5796,8 +5931,9 @@ store_field (rtx target, HOST_WIDE_INT b
       if (bitsize != (HOST_WIDE_INT) GET_MODE_BITSIZE (GET_MODE (target)))
 	emit_move_insn (object, target);
 
-      store_field (blk_object, bitsize, bitpos, mode, exp, type, alias_set,
-		   nontemporal);
+      store_field (blk_object, bitsize, bitpos,
+		   bitregion_start, bitregion_end,
+		   mode, exp, type, alias_set, nontemporal);
 
       emit_move_insn (target, object);
 
@@ -5911,7 +6047,9 @@ store_field (rtx target, HOST_WIDE_INT b
 	}
 
       /* Store the value in the bitfield.  */
-      store_bit_field (target, bitsize, bitpos, mode, temp);
+      store_bit_field (target, bitsize, bitpos,
+		       bitregion_start, bitregion_end,
+		       mode, temp);
 
       return const0_rtx;
     }
@@ -7323,7 +7461,7 @@ expand_expr_real_2 (sepops ops, rtx targ
 						    (treeop0))
 				 * BITS_PER_UNIT),
 				(HOST_WIDE_INT) GET_MODE_BITSIZE (mode)),
-			   0, TYPE_MODE (valtype), treeop0,
+			   0, 0, 0, TYPE_MODE (valtype), treeop0,
 			   type, 0, false);
 	    }
 
Index: expr.h
===================================================================
--- expr.h	(revision 173263)
+++ expr.h	(working copy)
@@ -665,7 +665,10 @@ extern enum machine_mode
 mode_for_extraction (enum extraction_pattern, int);
 
 extern void store_bit_field (rtx, unsigned HOST_WIDE_INT,
-			     unsigned HOST_WIDE_INT, enum machine_mode, rtx);
+			     unsigned HOST_WIDE_INT,
+			     unsigned HOST_WIDE_INT,
+			     unsigned HOST_WIDE_INT,
+			     enum machine_mode, rtx);
 extern rtx extract_bit_field (rtx, unsigned HOST_WIDE_INT,
 			      unsigned HOST_WIDE_INT, int, bool, rtx,
 			      enum machine_mode, enum machine_mode);
Index: stor-layout.c
===================================================================
--- stor-layout.c	(revision 173263)
+++ stor-layout.c	(working copy)
@@ -2428,6 +2428,13 @@ fixup_unsigned_type (tree type)
 /* Find the best machine mode to use when referencing a bit field of length
    BITSIZE bits starting at BITPOS.
 
+   BITREGION_START is the bit position of the first bit in this
+   sequence of bit fields.  BITREGION_END is the last bit in this
+   sequence.  If these two fields are non-zero, we should restrict the
+   memory access to a maximum sized chunk of
+   BITREGION_END - BITREGION_START + 1.  Otherwise, we are allowed to touch
+   any adjacent non bit-fields.
+
    The underlying object is known to be aligned to a boundary of ALIGN bits.
    If LARGEST_MODE is not VOIDmode, it means that we should not use a mode
    larger than LARGEST_MODE (usually SImode).
@@ -2445,11 +2452,23 @@ fixup_unsigned_type (tree type)
    decide which of the above modes should be used.  */
 
 enum machine_mode
-get_best_mode (int bitsize, int bitpos, unsigned int align,
+get_best_mode (int bitsize, int bitpos,
+	       unsigned HOST_WIDE_INT bitregion_start,
+	       unsigned HOST_WIDE_INT bitregion_end,
+	       unsigned int align,
 	       enum machine_mode largest_mode, int volatilep)
 {
   enum machine_mode mode;
   unsigned int unit = 0;
+  unsigned HOST_WIDE_INT maxbits;
+
+  /* If unset, no restriction.  */
+  if (!bitregion_end)
+    maxbits = 0;
+  else if ((unsigned) bitpos < bitregion_start)
+    maxbits = bitregion_end - bitregion_start + 1;
+  else
+    maxbits = bitregion_end - bitpos + 1;
 
   /* Find the narrowest integer mode that contains the bit field.  */
   for (mode = GET_CLASS_NARROWEST_MODE (MODE_INT); mode != VOIDmode;
@@ -2484,6 +2503,7 @@ get_best_mode (int bitsize, int bitpos, 
 	  if (bitpos / unit == (bitpos + bitsize - 1) / unit
 	      && unit <= BITS_PER_WORD
 	      && unit <= MIN (align, BIGGEST_ALIGNMENT)
+	      && (!maxbits || unit <= maxbits)
 	      && (largest_mode == VOIDmode
 		  || unit <= GET_MODE_BITSIZE (largest_mode)))
 	    wide_mode = tmode;
Index: calls.c
===================================================================
--- calls.c	(revision 173263)
+++ calls.c	(working copy)
@@ -909,8 +909,8 @@ store_unaligned_arguments_into_pseudos (
 	    emit_move_insn (reg, const0_rtx);
 
 	    bytes -= bitsize / BITS_PER_UNIT;
-	    store_bit_field (reg, bitsize, endian_correction, word_mode,
-			     word);
+	    store_bit_field (reg, bitsize, endian_correction, 0, 0,
+			     word_mode, word);
 	  }
       }
 }
Index: expmed.c
===================================================================
--- expmed.c	(revision 173263)
+++ expmed.c	(working copy)
@@ -47,9 +47,15 @@ struct target_expmed *this_target_expmed
 
 static void store_fixed_bit_field (rtx, unsigned HOST_WIDE_INT,
 				   unsigned HOST_WIDE_INT,
-				   unsigned HOST_WIDE_INT, rtx);
+				   unsigned HOST_WIDE_INT,
+				   unsigned HOST_WIDE_INT,
+				   unsigned HOST_WIDE_INT,
+				   rtx);
 static void store_split_bit_field (rtx, unsigned HOST_WIDE_INT,
-				   unsigned HOST_WIDE_INT, rtx);
+				   unsigned HOST_WIDE_INT,
+				   unsigned HOST_WIDE_INT,
+				   unsigned HOST_WIDE_INT,
+				   rtx);
 static rtx extract_fixed_bit_field (enum machine_mode, rtx,
 				    unsigned HOST_WIDE_INT,
 				    unsigned HOST_WIDE_INT,
@@ -333,7 +339,10 @@ mode_for_extraction (enum extraction_pat
 
 static bool
 store_bit_field_1 (rtx str_rtx, unsigned HOST_WIDE_INT bitsize,
-		   unsigned HOST_WIDE_INT bitnum, enum machine_mode fieldmode,
+		   unsigned HOST_WIDE_INT bitnum,
+		   unsigned HOST_WIDE_INT bitregion_start,
+		   unsigned HOST_WIDE_INT bitregion_end,
+		   enum machine_mode fieldmode,
 		   rtx value, bool fallback_p)
 {
   unsigned int unit
@@ -547,7 +556,9 @@ store_bit_field_1 (rtx str_rtx, unsigned
 
 	  if (!store_bit_field_1 (op0, MIN (BITS_PER_WORD,
 					    bitsize - i * BITS_PER_WORD),
-				  bitnum + bit_offset, word_mode,
+				  bitnum + bit_offset,
+				  bitregion_start, bitregion_end,
+				  word_mode,
 				  value_word, fallback_p))
 	    {
 	      delete_insns_since (last);
@@ -711,6 +722,12 @@ store_bit_field_1 (rtx str_rtx, unsigned
   if (HAVE_insv && MEM_P (op0))
     {
       enum machine_mode bestmode;
+      unsigned HOST_WIDE_INT maxbits;
+
+      if (bitnum < bitregion_start)
+	maxbits = bitregion_end - bitregion_start + 1;
+      else
+	maxbits = bitregion_end - bitnum + 1;
 
       /* Get the mode to use for inserting into this field.  If OP0 is
 	 BLKmode, get the smallest mode consistent with the alignment. If
@@ -718,9 +735,12 @@ store_bit_field_1 (rtx str_rtx, unsigned
 	 mode. Otherwise, use the smallest mode containing the field.  */
 
       if (GET_MODE (op0) == BLKmode
+	  || (bitregion_end && GET_MODE_BITSIZE (GET_MODE (op0)) > maxbits)
 	  || (op_mode != MAX_MACHINE_MODE
 	      && GET_MODE_SIZE (GET_MODE (op0)) > GET_MODE_SIZE (op_mode)))
-	bestmode = get_best_mode (bitsize, bitnum, MEM_ALIGN (op0),
+	bestmode = get_best_mode (bitsize, bitnum,
+				  bitregion_start, bitregion_end,
+				  MEM_ALIGN (op0),
 				  (op_mode == MAX_MACHINE_MODE
 				   ? VOIDmode : op_mode),
 				  MEM_VOLATILE_P (op0));
@@ -749,6 +769,7 @@ store_bit_field_1 (rtx str_rtx, unsigned
 	     the unit.  */
 	  tempreg = copy_to_reg (xop0);
 	  if (store_bit_field_1 (tempreg, bitsize, xbitpos,
+				 bitregion_start, bitregion_end,
 				 fieldmode, orig_value, false))
 	    {
 	      emit_move_insn (xop0, tempreg);
@@ -761,21 +782,33 @@ store_bit_field_1 (rtx str_rtx, unsigned
   if (!fallback_p)
     return false;
 
-  store_fixed_bit_field (op0, offset, bitsize, bitpos, value);
+  store_fixed_bit_field (op0, offset, bitsize, bitpos,
+			 bitregion_start, bitregion_end, value);
   return true;
 }
 
 /* Generate code to store value from rtx VALUE
    into a bit-field within structure STR_RTX
    containing BITSIZE bits starting at bit BITNUM.
+
+   BITREGION_START is bitpos of the first bitfield in this region.
+   BITREGION_END is the bitpos of the ending bitfield in this region.
+   These two fields are 0, if the C++ memory model does not apply,
+   or we are not interested in keeping track of bitfield regions.
+
    FIELDMODE is the machine-mode of the FIELD_DECL node for this field.  */
 
 void
 store_bit_field (rtx str_rtx, unsigned HOST_WIDE_INT bitsize,
-		 unsigned HOST_WIDE_INT bitnum, enum machine_mode fieldmode,
+		 unsigned HOST_WIDE_INT bitnum,
+		 unsigned HOST_WIDE_INT bitregion_start,
+		 unsigned HOST_WIDE_INT bitregion_end,
+		 enum machine_mode fieldmode,
 		 rtx value)
 {
-  if (!store_bit_field_1 (str_rtx, bitsize, bitnum, fieldmode, value, true))
+  if (!store_bit_field_1 (str_rtx, bitsize, bitnum,
+			  bitregion_start, bitregion_end,
+			  fieldmode, value, true))
     gcc_unreachable ();
 }
 
@@ -791,7 +824,10 @@ store_bit_field (rtx str_rtx, unsigned H
 static void
 store_fixed_bit_field (rtx op0, unsigned HOST_WIDE_INT offset,
 		       unsigned HOST_WIDE_INT bitsize,
-		       unsigned HOST_WIDE_INT bitpos, rtx value)
+		       unsigned HOST_WIDE_INT bitpos,
+		       unsigned HOST_WIDE_INT bitregion_start,
+		       unsigned HOST_WIDE_INT bitregion_end,
+		       rtx value)
 {
   enum machine_mode mode;
   unsigned int total_bits = BITS_PER_WORD;
@@ -812,12 +848,23 @@ store_fixed_bit_field (rtx op0, unsigned
       /* Special treatment for a bit field split across two registers.  */
       if (bitsize + bitpos > BITS_PER_WORD)
 	{
-	  store_split_bit_field (op0, bitsize, bitpos, value);
+	  store_split_bit_field (op0, bitsize, bitpos,
+				 bitregion_start, bitregion_end,
+				 value);
 	  return;
 	}
     }
   else
     {
+      unsigned HOST_WIDE_INT maxbits;
+
+      if (!bitregion_end)
+	maxbits = 0;
+      else if (bitpos + offset * BITS_PER_UNIT < bitregion_start)
+	maxbits = bitregion_end - bitregion_start + 1;
+      else
+	maxbits = bitregion_end - (bitpos + offset * BITS_PER_UNIT) + 1;
+
       /* Get the proper mode to use for this field.  We want a mode that
 	 includes the entire field.  If such a mode would be larger than
 	 a word, we won't be doing the extraction the normal way.
@@ -830,10 +877,12 @@ store_fixed_bit_field (rtx op0, unsigned
 
       if (MEM_VOLATILE_P (op0)
           && GET_MODE_BITSIZE (GET_MODE (op0)) > 0
+	  && GET_MODE_BITSIZE (GET_MODE (op0)) <= maxbits
 	  && flag_strict_volatile_bitfields > 0)
 	mode = GET_MODE (op0);
       else
 	mode = get_best_mode (bitsize, bitpos + offset * BITS_PER_UNIT,
+			      bitregion_start, bitregion_end,
 			      MEM_ALIGN (op0), mode, MEM_VOLATILE_P (op0));
 
       if (mode == VOIDmode)
@@ -841,7 +890,7 @@ store_fixed_bit_field (rtx op0, unsigned
 	  /* The only way this should occur is if the field spans word
 	     boundaries.  */
 	  store_split_bit_field (op0, bitsize, bitpos + offset * BITS_PER_UNIT,
-				 value);
+				 bitregion_start, bitregion_end, value);
 	  return;
 	}
 
@@ -961,7 +1010,10 @@ store_fixed_bit_field (rtx op0, unsigned
 
 static void
 store_split_bit_field (rtx op0, unsigned HOST_WIDE_INT bitsize,
-		       unsigned HOST_WIDE_INT bitpos, rtx value)
+		       unsigned HOST_WIDE_INT bitpos,
+		       unsigned HOST_WIDE_INT bitregion_start,
+		       unsigned HOST_WIDE_INT bitregion_end,
+		       rtx value)
 {
   unsigned int unit;
   unsigned int bitsdone = 0;
@@ -1076,7 +1128,7 @@ store_split_bit_field (rtx op0, unsigned
 	 it is just an out-of-bounds access.  Ignore it.  */
       if (word != const0_rtx)
 	store_fixed_bit_field (word, offset * unit / BITS_PER_UNIT, thissize,
-			       thispos, part);
+			       thispos, bitregion_start, bitregion_end, part);
       bitsdone += thissize;
     }
 }
@@ -1520,7 +1572,7 @@ extract_bit_field_1 (rtx str_rtx, unsign
       if (GET_MODE (op0) == BLKmode
 	  || (ext_mode != MAX_MACHINE_MODE
 	      && GET_MODE_SIZE (GET_MODE (op0)) > GET_MODE_SIZE (ext_mode)))
-	bestmode = get_best_mode (bitsize, bitnum, MEM_ALIGN (op0),
+	bestmode = get_best_mode (bitsize, bitnum, 0, 0, MEM_ALIGN (op0),
 				  (ext_mode == MAX_MACHINE_MODE
 				   ? VOIDmode : ext_mode),
 				  MEM_VOLATILE_P (op0));
@@ -1646,7 +1698,7 @@ extract_fixed_bit_field (enum machine_mo
 	    mode = tmode;
 	}
       else
-	mode = get_best_mode (bitsize, bitpos + offset * BITS_PER_UNIT,
+	mode = get_best_mode (bitsize, bitpos + offset * BITS_PER_UNIT, 0, 0,
 			      MEM_ALIGN (op0), word_mode, MEM_VOLATILE_P (op0));
 
       if (mode == VOIDmode)
Index: Makefile.in
===================================================================
--- Makefile.in	(revision 173263)
+++ Makefile.in	(working copy)
@@ -2916,7 +2916,7 @@ expr.o : expr.c $(CONFIG_H) $(SYSTEM_H) 
    typeclass.h hard-reg-set.h toplev.h $(DIAGNOSTIC_CORE_H) hard-reg-set.h $(EXCEPT_H) \
    reload.h langhooks.h intl.h $(TM_P_H) $(TARGET_H) \
    tree-iterator.h gt-expr.h $(MACHMODE_H) $(TIMEVAR_H) $(TREE_FLOW_H) \
-   $(TREE_PASS_H) $(DF_H) $(DIAGNOSTIC_H) vecprim.h $(SSAEXPAND_H)
+   $(TREE_PASS_H) $(DF_H) $(DIAGNOSTIC_H) vecprim.h $(SSAEXPAND_H) $(PARAMS_H)
 dojump.o : dojump.c $(CONFIG_H) $(SYSTEM_H) coretypes.h $(TM_H) $(RTL_H) $(TREE_H) \
    $(FLAGS_H) $(FUNCTION_H) $(EXPR_H) $(OPTABS_H) $(INSN_ATTR_H) insn-config.h \
    langhooks.h $(GGC_H) gt-dojump.h vecprim.h $(BASIC_BLOCK_H) output.h
Index: stmt.c
===================================================================
--- stmt.c	(revision 173263)
+++ stmt.c	(working copy)
@@ -1758,7 +1758,8 @@ expand_return (tree retval)
 
 	  /* Use bitpos for the source extraction (left justified) and
 	     xbitpos for the destination store (right justified).  */
-	  store_bit_field (dst, bitsize, xbitpos % BITS_PER_WORD, word_mode,
+	  store_bit_field (dst, bitsize, xbitpos % BITS_PER_WORD,
+			   0, 0, word_mode,
 			   extract_bit_field (src, bitsize,
 					      bitpos % BITS_PER_WORD, 1, false,
 					      NULL_RTX, word_mode, word_mode));
Index: params.def
===================================================================
--- params.def	(revision 173263)
+++ params.def	(working copy)
@@ -884,6 +884,13 @@ DEFPARAM (PARAM_MAX_STORES_TO_SINK,
           "Maximum number of conditional store pairs that can be sunk",
           2, 0, 0)
 
+/* Data race flags for C++0x memory model compliance.  */
+
+DEFPARAM (PARAM_ALLOW_STORE_DATA_RACES,
+	  "allow-store-data-races",
+	  "Allow new data races on stores to be introduced",
+	  1, 0, 1)
+
 
 /*
 Local variables:
