Patchwork [C++0x] contiguous bitfields race implementation

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Submitter Aldy Hernandez
Date Aug. 15, 2011, 5:53 p.m.
Message ID <4E495D11.5000306@redhat.com>
Download mbox | patch
Permalink /patch/110084/
State New
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Comments

Aldy Hernandez - Aug. 15, 2011, 5:53 p.m.
> Some comments.
>
>        /* If we have a bit-field with a bitsize>  0... */
>        if (DECL_BIT_FIELD_TYPE (fld)
> 	&&  tree_low_cst (DECL_SIZE (fld), 1)>  0)
>
> I think we can check bitsize != 0, thus
>
> &&  !integer_zerop (DECL_SIZE (fld))

Done

> 	  /* Short-circuit out if we have the max bits allowed.  */
> 	  if (cumulative_bitsize>= MAX_FIXED_MODE_SIZE)
> 	    {
> 	      *maxbits = MAX_FIXED_MODE_SIZE;
> 	      /* Calculate byte offset to the beginning of the bit region.  */
> 	      gcc_assert (start_bitpos % BITS_PER_UNIT == 0);
> 	      *offset = fold_build2 (PLUS_EXPR, TREE_TYPE (start_offset),
> 				     start_offset,
> 				     build_int_cst (integer_type_node,
> 						    start_bitpos / BITS_PER_UNIT));
> 	      return;
>
> apart from the *offset calculation being redundant, *offset + maxbits
> may not include the referenced field.  How do you plan to find
> an "optimal" window for such access? (*)

Actually offset is always needed because we use it in store_bit_field() 
to adjust the memory reference up to the bit region.  However... I have 
removed the MAX_FIXED_MODE_SIZE constrain all throughout.  See 
explanation below.

>        if (!DECL_BIT_FIELD_TYPE (fld)
>           || integer_zerop (DECL_SIZE (fld)))
>          break;
>
> we know that we'll eventually find field.

Good catch.  Done.

>    /* If we found the end of the bit field sequence, include the
>       padding up to the next field...  */
>    if (fld)
>      {
>
> could be a non-FIELD_DECL, you have to skip those first.

I can't find an example of a non-FIELD_DECL here for the life of me. 
I've run numerous tests and can't trigger one.  Do you have one in mind 
so I can handle it?

> Note that another complication comes to my mind now - the offset
> field of a COMPONENT_REF is used to specify a variable offset
> and has to be used, if present, instead of DECL_FIELD_OFFSET.
> Thus your building of COMPONENT_REFs to then pass them to
> get_inner_reference is broken.  As you are in generic code and not
> in the C++ frontend I believe you have to properly handle this case
> (may I suggest to, at the start of the function, simply return a
> minimum byte-aligned blob for the case that there is a variable
> offset to the bitfield?)

Done.

BTW, where would this happen?  I'd like an actual test to stick in 
there.  Is this for non C/C++?

>    /* ...otherwise, this is the last element in the structure.  */
>    else
>      {
>        /* Include the padding at the end of structure.  */
>        *maxbits = TREE_INT_CST_LOW (TYPE_SIZE (record_type))
> 	- TREE_INT_CST_LOW (start_bitpos_direct_parent);
>        if (*maxbits>  MAX_FIXED_MODE_SIZE)
> 	*maxbits = MAX_FIXED_MODE_SIZE;
>      }
>
> with Andrews answer this is invalid.  You can (and should) at most do
>
>    else
>      *maxbits = (*maxbits + BITS_PER_UNIT - 1)&  ~(BITS_PER_UNIT - 1);

I have done this, but I have also removed the constraint to 
MAX_FIXED_MODE_SIZE.  See below.

> There is still the general issue of packed bitfields which will probably
> make the issue of the computed group not covering all of field more
> prominent (esp. if you limit to MAX_FIXED_MODE_SIZE - consider
> struct __attribute__((packed)) { long long : 1; long long a : 64; char
> c; } where
> a does not fit in a DImode mem but crosses it.  Why constrain
> *maxbits to MAX_FIXED_MODE_SIZE at all?  Shoudln't the *offset,
> *maxbits pair just constrain what the caller does, not force it to actually
> use an access covering that full range (does it?)?

Well, I have removed the MAX_FIXED_MODE_SIZE restriction, since every 
offset adjustment in the bit field machinery will need a corresponding 
MAXBITS adjustment.  We can end up reducing MAXBITS into negative 
territory.  So we really have to keep track of the actual number of 
bits.  So, we've converged, just for different reasons :).

I have bootstrapped the compiler with the bitfield restrictions in 
place.  This unearthed a few corner cases, which we are now handling 
correctly with this revision.  So, please take a look at the entire 
patch, as there are small changes throughout.

I am including both the entire patch, and the get_bit_range() function 
separately, to make it easier to review.

Thanks.
/* In the C++ memory model, consecutive non-zero bit fields in a
   structure are considered one memory location.

   Given a COMPONENT_REF, this function calculates the byte offset
   from the containing object to the start of the contiguous bit
   region containing the field in question.  This byte offset is
   returned in *OFFSET.

   The maximum number of bits that can be addressed while storing into
   the COMPONENT_REF is returned in *MAXBITS.  This number is the
   number of bits in the contiguous bit region, including any
   padding.  */

static void
get_bit_range (tree exp, tree *offset, HOST_WIDE_INT *maxbits)
{
  tree field, record_type, fld;
  bool prev_field_is_bitfield;
  tree start_offset;
  tree start_bitpos_direct_parent = NULL_TREE;
  HOST_WIDE_INT start_bitpos;
  HOST_WIDE_INT cumulative_bitsize = 0;
  /* First field of the bitfield group containing the bitfield we are
     referencing.  */
  tree bitregion_start;

  HOST_WIDE_INT tbitsize;
  enum machine_mode tmode;
  int tunsignedp, tvolatilep;

  gcc_assert (TREE_CODE (exp) == COMPONENT_REF);

  /* Be as conservative as possible on variable offsets.  */
  if (TREE_OPERAND (exp, 2)
      && !host_integerp (TREE_OPERAND (exp, 2), 1))
    {
      *offset = TREE_OPERAND (exp, 2);
      *maxbits = BITS_PER_UNIT;
      return;
    }

  /* Bit field we're storing into.  */
  field = TREE_OPERAND (exp, 1);
  record_type = DECL_FIELD_CONTEXT (field);

  /* Find the bitfield group containing the field in question, and set
     BITREGION_START to the start of the group.  */
  prev_field_is_bitfield = false;
  bitregion_start = NULL_TREE;
  for (fld = TYPE_FIELDS (record_type); fld; fld = DECL_CHAIN (fld))
    {
      if (TREE_CODE (fld) != FIELD_DECL)
	continue;

      /* If we have a non-zero bit-field.  */
      if (DECL_BIT_FIELD_TYPE (fld)
	  && !integer_zerop (DECL_SIZE (fld)))
	{
	  if (!prev_field_is_bitfield)
	    {
	      bitregion_start = fld;
	      prev_field_is_bitfield = true;
	    }
	}
      else
	prev_field_is_bitfield = false;
      if (fld == field)
	break;
    }
  gcc_assert (bitregion_start);
  gcc_assert (fld);

  /* Save the starting position of the bitregion.  */
  get_inner_reference (build3 (COMPONENT_REF,
			       TREE_TYPE (exp),
			       TREE_OPERAND (exp, 0),
			       bitregion_start, NULL_TREE),
		       &tbitsize, &start_bitpos, &start_offset,
		       &tmode, &tunsignedp, &tvolatilep, true);

  if (!start_offset)
    start_offset = size_zero_node;
  /* Calculate byte offset to the beginning of the bit region.  */
  /* OFFSET = START_OFFSET + (START_BITPOS / BITS_PER_UNIT) */
  gcc_assert (start_bitpos % BITS_PER_UNIT == 0);
  *offset = fold_build2 (PLUS_EXPR, TREE_TYPE (start_offset),
			 start_offset,
			 build_int_cst (integer_type_node,
					start_bitpos / BITS_PER_UNIT));

  /* Save the bit offset of the current structure.  */
  start_bitpos_direct_parent = DECL_FIELD_BIT_OFFSET (bitregion_start);

  /* Count the bitsize of the bitregion containing the field in question.  */
  cumulative_bitsize = 0;
  for (fld = bitregion_start; fld; fld = DECL_CHAIN (fld))
    {
      if (TREE_CODE (fld) != FIELD_DECL)
	continue;

      if (!DECL_BIT_FIELD_TYPE (fld)
	  || integer_zerop (DECL_SIZE (fld)))
	break;

      cumulative_bitsize += tree_low_cst (DECL_SIZE (fld), 1);
    }

  /* If we found the end of the bit field sequence, include the
     padding up to the next field...  */
  if (fld)
    {
      tree end_offset, maxbits_tree;
      HOST_WIDE_INT end_bitpos;

      /* Calculate bitpos and offset of the next field.  */
      get_inner_reference (build3 (COMPONENT_REF,
				   TREE_TYPE (exp),
				   TREE_OPERAND (exp, 0),
				   fld, NULL_TREE),
			   &tbitsize, &end_bitpos, &end_offset,
			   &tmode, &tunsignedp, &tvolatilep, true);
      gcc_assert (end_bitpos % BITS_PER_UNIT == 0);

      if (end_offset)
	{
	  tree type = TREE_TYPE (end_offset), end;

	  /* Calculate byte offset to the end of the bit region.  */
	  end = fold_build2 (PLUS_EXPR, type,
			     end_offset,
			     build_int_cst (type,
					    end_bitpos / BITS_PER_UNIT));
	  maxbits_tree = fold_build2 (MINUS_EXPR, type, end, *offset);
	}
      else
	maxbits_tree = build_int_cst (integer_type_node,
				      end_bitpos - start_bitpos);

      *maxbits = TREE_INT_CST_LOW (maxbits_tree);
    }
  /* ...otherwise, this is the last element in the structure.  */
  else
    {
      /* Include the padding at the end of structure.  */
      *maxbits = TREE_INT_CST_LOW (TYPE_SIZE (record_type))
	- TREE_INT_CST_LOW (start_bitpos_direct_parent);
      /* Round up to the next byte.  */
      *maxbits = (*maxbits + BITS_PER_UNIT - 1) & ~(BITS_PER_UNIT - 1);
    }
}
* machmode.h (get_best_mode): Remove 2 arguments.
	* fold-const.c (optimize_bit_field_compare): Same.
	(fold_truthop): Same.
	* expr.c (store_field): Change argument types in prototype.
	(emit_group_store): Change argument types to store_bit_field call.
	(copy_blkmode_from_reg): Same.
	(write_complex_part): Same.
	(optimize_bitfield_assignment_op): Change argument types.
	Change arguments to get_best_mode.
	(get_bit_range): Rewrite.
	(expand_assignment): Adjust new call to get_bit_range.
	Adjust bitregion_offset when to_rtx is changed.
	Adjust calls to store_field with new argument types.
	(store_field): New argument types.
	Adjust calls to store_bit_field with new arguments.
	* expr.h (store_bit_field): Change argument types.
	* stor-layout.c (get_best_mode): Remove use of bitregion* arguments.
	* expmed.c (store_bit_field_1): Change argument types.
	Do not calculate maxbits.
	Adjust bitregion_maxbits if offset changes.
	(store_bit_field): Change argument types.
	Adjust address taking into account bitregion_offset.
	(store_fixed_bit_field): Change argument types.
	Do not calculate maxbits.
	(store_split_bit_field): Change argument types.
	(extract_bit_field_1): Adjust arguments to get_best_mode.
	(extract_fixed_bit_field): Same.

Patch

Index: machmode.h
===================================================================
--- machmode.h	(revision 176891)
+++ machmode.h	(working copy)
@@ -249,8 +249,6 @@  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 HOST_WIDE_INT,
-					unsigned HOST_WIDE_INT,
 					unsigned int,
 					enum machine_mode, int);
 
Index: fold-const.c
===================================================================
--- fold-const.c	(revision 176891)
+++ fold-const.c	(working copy)
@@ -3394,7 +3394,7 @@  optimize_bit_field_compare (location_t l
       && flag_strict_volatile_bitfields > 0)
     nmode = lmode;
   else
-    nmode = get_best_mode (lbitsize, lbitpos, 0, 0,
+    nmode = get_best_mode (lbitsize, lbitpos,
 			   const_p ? TYPE_ALIGN (TREE_TYPE (linner))
 			   : MIN (TYPE_ALIGN (TREE_TYPE (linner)),
 				  TYPE_ALIGN (TREE_TYPE (rinner))),
@@ -5221,7 +5221,7 @@  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, 0, 0,
+  lnmode = get_best_mode (end_bit - first_bit, first_bit,
 			  TYPE_ALIGN (TREE_TYPE (ll_inner)), word_mode,
 			  volatilep);
   if (lnmode == VOIDmode)
@@ -5286,7 +5286,7 @@  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, 0, 0,
+      rnmode = get_best_mode (end_bit - first_bit, first_bit,
 			      TYPE_ALIGN (TREE_TYPE (lr_inner)), word_mode,
 			      volatilep);
       if (rnmode == VOIDmode)
Index: testsuite/c-c++-common/cxxbitfields-6.c
===================================================================
--- testsuite/c-c++-common/cxxbitfields-6.c	(revision 0)
+++ testsuite/c-c++-common/cxxbitfields-6.c	(revision 0)
@@ -0,0 +1,19 @@ 
+/* { dg-do compile { target i?86-*-* x86_64-*-* } } */
+/* { dg-options "-O2 --param allow-store-data-races=0" } */
+
+struct bits
+{
+  char a;
+  int b:7;
+  int :0;
+  volatile int c:7;
+  unsigned char d;
+} x;
+
+/* Store into <c> should not clobber <d>.  */
+void update_c(struct bits *p, int val) 
+{
+    p -> c = val;
+}
+
+/* { dg-final { scan-assembler "movb" } } */
Index: testsuite/c-c++-common/cxxbitfields-8.c
===================================================================
--- testsuite/c-c++-common/cxxbitfields-8.c	(revision 0)
+++ testsuite/c-c++-common/cxxbitfields-8.c	(revision 0)
@@ -0,0 +1,29 @@ 
+/* { dg-do compile { target x86_64-*-* } } */
+/* { dg-options "-O --param allow-store-data-races=0" } */
+
+struct bits {
+  /* Make sure the bit position of the bitfield is larger than what
+     can be represented in an unsigned HOST_WIDE_INT, to force
+     get_inner_reference() to return something in POFFSET.  */
+      
+  struct {
+    int some_padding[1<<30];
+    char more_padding;
+  } pad[1<<29];
+
+  struct {
+    volatile char bitfield :1;
+  } x;
+  char b;
+};
+
+struct bits *p;
+
+/* Test that the store into <bitfield> is not done with something
+   wider than a byte move.  */
+void foo()
+{
+  p->x.bitfield = 1;
+}
+
+/* { dg-final { scan-assembler "movb" } } */
Index: testsuite/c-c++-common/cxxbitfields-7.c
===================================================================
--- testsuite/c-c++-common/cxxbitfields-7.c	(revision 0)
+++ testsuite/c-c++-common/cxxbitfields-7.c	(revision 0)
@@ -0,0 +1,19 @@ 
+/* { dg-do compile { target i?86-*-* x86_64-*-* } } */
+/* { dg-options "-O2 --param allow-store-data-races=0" } */
+
+struct bits
+{
+  int some_padding;
+  struct {
+    volatile char bitfield :1;
+  } x;
+  char b;
+};
+
+/* Store into <bitfield> should not clobber <b>.  */
+void update(struct bits *p)
+{
+    p->x.bitfield = 1;
+}
+
+/* { dg-final { scan-assembler "movb" } } */
Index: ifcvt.c
===================================================================
--- ifcvt.c	(revision 176891)
+++ ifcvt.c	(working copy)
@@ -885,7 +885,8 @@  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, 0, 0, GET_MODE (x), y);
+	      store_bit_field (op, size, start, integer_zero_node, 0,
+			       GET_MODE (x), y);
 	      return;
 	    }
 
@@ -940,7 +941,7 @@  noce_emit_move_insn (rtx x, rtx y)
   outmode = GET_MODE (outer);
   bitpos = SUBREG_BYTE (outer) * BITS_PER_UNIT;
   store_bit_field (inner, GET_MODE_BITSIZE (outmode), bitpos,
-		   0, 0, outmode, y);
+		   integer_zero_node, 0, outmode, y);
 }
 
 /* Return sequence of instructions generated by if conversion.  This
Index: expr.c
===================================================================
--- expr.c	(revision 176891)
+++ expr.c	(working copy)
@@ -145,7 +145,7 @@  static void store_constructor_field (rtx
 				     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,
-			unsigned HOST_WIDE_INT, unsigned HOST_WIDE_INT,
+			tree, HOST_WIDE_INT,
 			enum machine_mode,
 			tree, tree, alias_set_type, bool);
 
@@ -2077,7 +2077,7 @@  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,
-			 0, 0, mode, tmps[i]);
+			 integer_zero_node, 0, mode, tmps[i]);
     }
 
   /* Copy from the pseudo into the (probable) hard reg.  */
@@ -2171,7 +2171,8 @@  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, 0, 0, copy_mode,
+      store_bit_field (dst, bitsize, bitpos % BITS_PER_WORD,
+		       integer_zero_node, 0, copy_mode,
 		       extract_bit_field (src, bitsize,
 					  xbitpos % BITS_PER_WORD, 1, false,
 					  NULL_RTX, copy_mode, copy_mode));
@@ -2808,7 +2809,8 @@  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, 0, 0, imode, val);
+  store_bit_field (cplx, ibitsize, imag_p ? ibitsize : 0,
+		   integer_zero_node, 0, imode, val);
 }
 
 /* Extract one of the components of the complex value CPLX.  Extract the
@@ -3943,8 +3945,7 @@  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,
+				 HOST_WIDE_INT bitregion_maxbits,
 				 enum machine_mode mode1, rtx str_rtx,
 				 tree to, tree src)
 {
@@ -4005,8 +4006,9 @@  optimize_bitfield_assignment_op (unsigne
 
       if (str_bitsize == 0 || str_bitsize > BITS_PER_WORD)
 	str_mode = word_mode;
+      if (bitregion_maxbits && bitregion_maxbits < GET_MODE_BITSIZE (str_mode))
+	str_mode = smallest_mode_for_size (bitregion_maxbits, MODE_INT);
       str_mode = get_best_mode (bitsize, bitpos,
-				bitregion_start, bitregion_end,
 				MEM_ALIGN (str_rtx), str_mode, 0);
       if (str_mode == VOIDmode)
 	return false;
@@ -4115,57 +4117,44 @@  optimize_bitfield_assignment_op (unsigne
   return false;
 }
 
-/* In the C++ memory model, consecutive bit fields in a structure are
-   considered one memory location.
+/* In the C++ memory model, consecutive non-zero 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 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).  */
+   Given a COMPONENT_REF, this function calculates the byte offset
+   from the containing object to the start of the contiguous bit
+   region containing the field in question.  This byte offset is
+   returned in *OFFSET.
+
+   The maximum number of bits that can be addressed while storing into
+   the COMPONENT_REF is returned in *MAXBITS.  This number is the
+   number of bits in the contiguous bit region, including any
+   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)
+get_bit_range (tree exp, tree *offset, HOST_WIDE_INT *maxbits)
 {
   tree field, record_type, fld;
-  bool found_field = false;
   bool prev_field_is_bitfield;
+  tree start_offset;
+  tree start_bitpos_direct_parent = NULL_TREE;
+  HOST_WIDE_INT start_bitpos;
+  HOST_WIDE_INT cumulative_bitsize = 0;
+  /* First field of the bitfield group containing the bitfield we are
+     referencing.  */
+  tree bitregion_start;
+
+  HOST_WIDE_INT tbitsize;
+  enum machine_mode tmode;
+  int tunsignedp, tvolatilep;
 
   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
-      || ((TREE_CODE (innerdecl) == MEM_REF
-	   || TREE_CODE (innerdecl) == TARGET_MEM_REF)
-	  && !ptr_deref_may_alias_global_p (TREE_OPERAND (innerdecl, 0)))
-      || (DECL_P (innerdecl)
-	  && (DECL_THREAD_LOCAL_P (innerdecl)
-	      || !TREE_STATIC (innerdecl))))
+  /* Be as conservative as possible on variable offsets.  */
+  if (TREE_OPERAND (exp, 2)
+      && !host_integerp (TREE_OPERAND (exp, 2), 1))
     {
-      *bitstart = *bitend = 0;
+      *offset = TREE_OPERAND (exp, 2);
+      *maxbits = BITS_PER_UNIT;
       return;
     }
 
@@ -4173,56 +4162,109 @@  get_bit_range (unsigned HOST_WIDE_INT *b
   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;
+  /* Find the bitfield group containing the field in question, and set
+     BITREGION_START to the start of the group.  */
+  prev_field_is_bitfield = false;
+  bitregion_start = NULL_TREE;
   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 we have a non-zero bit-field.  */
+      if (DECL_BIT_FIELD_TYPE (fld)
+	  && !integer_zerop (DECL_SIZE (fld)))
 	{
-	  if (prev_field_is_bitfield == false)
+	  if (!prev_field_is_bitfield)
 	    {
-	      *bitstart = bitpos;
+	      bitregion_start = fld;
 	      prev_field_is_bitfield = true;
 	    }
 	}
       else
-	{
-	  prev_field_is_bitfield = false;
-	  if (found_field)
-	    break;
-	}
+	prev_field_is_bitfield = false;
+      if (fld == field)
+	break;
     }
-  gcc_assert (found_field);
+  gcc_assert (bitregion_start);
+  gcc_assert (fld);
 
+  /* Save the starting position of the bitregion.  */
+  get_inner_reference (build3 (COMPONENT_REF,
+			       TREE_TYPE (exp),
+			       TREE_OPERAND (exp, 0),
+			       bitregion_start, NULL_TREE),
+		       &tbitsize, &start_bitpos, &start_offset,
+		       &tmode, &tunsignedp, &tvolatilep, true);
+
+  if (!start_offset)
+    start_offset = size_zero_node;
+  /* Calculate byte offset to the beginning of the bit region.  */
+  /* OFFSET = START_OFFSET + (START_BITPOS / BITS_PER_UNIT) */
+  gcc_assert (start_bitpos % BITS_PER_UNIT == 0);
+  *offset = fold_build2 (PLUS_EXPR, TREE_TYPE (start_offset),
+			 start_offset,
+			 build_int_cst (integer_type_node,
+					start_bitpos / BITS_PER_UNIT));
+
+  /* Save the bit offset of the current structure.  */
+  start_bitpos_direct_parent = DECL_FIELD_BIT_OFFSET (bitregion_start);
+
+  /* Count the bitsize of the bitregion containing the field in question.  */
+  cumulative_bitsize = 0;
+  for (fld = bitregion_start; fld; fld = DECL_CHAIN (fld))
+    {
+      if (TREE_CODE (fld) != FIELD_DECL)
+	continue;
+
+      if (!DECL_BIT_FIELD_TYPE (fld)
+	  || integer_zerop (DECL_SIZE (fld)))
+	break;
+
+      cumulative_bitsize += tree_low_cst (DECL_SIZE (fld), 1);
+    }
+
+  /* If we found the end of the bit field sequence, include the
+     padding up to the next 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;
+      tree end_offset, maxbits_tree;
+      HOST_WIDE_INT end_bitpos;
+
+      /* Calculate bitpos and offset of the next field.  */
+      get_inner_reference (build3 (COMPONENT_REF,
+				   TREE_TYPE (exp),
+				   TREE_OPERAND (exp, 0),
+				   fld, NULL_TREE),
+			   &tbitsize, &end_bitpos, &end_offset,
+			   &tmode, &tunsignedp, &tvolatilep, true);
+      gcc_assert (end_bitpos % BITS_PER_UNIT == 0);
+
+      if (end_offset)
+	{
+	  tree type = TREE_TYPE (end_offset), end;
+
+	  /* Calculate byte offset to the end of the bit region.  */
+	  end = fold_build2 (PLUS_EXPR, type,
+			     end_offset,
+			     build_int_cst (type,
+					    end_bitpos / BITS_PER_UNIT));
+	  maxbits_tree = fold_build2 (MINUS_EXPR, type, end, *offset);
+	}
+      else
+	maxbits_tree = build_int_cst (integer_type_node,
+				      end_bitpos - start_bitpos);
+
+      *maxbits = TREE_INT_CST_LOW (maxbits_tree);
     }
+  /* ...otherwise, this is the last element in the structure.  */
   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)) - 1;
+      /* Include the padding at the end of structure.  */
+      *maxbits = TREE_INT_CST_LOW (TYPE_SIZE (record_type))
+	- TREE_INT_CST_LOW (start_bitpos_direct_parent);
+      /* Round up to the next byte.  */
+      *maxbits = (*maxbits + BITS_PER_UNIT - 1) & ~(BITS_PER_UNIT - 1);
     }
 }
 
@@ -4324,12 +4366,14 @@  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;
       tree tem;
+      tree bitregion_offset = size_zero_node;
+      /* Set to 0 for the special case where there is no restriction
+	 in play.  */
+      HOST_WIDE_INT bitregion_maxbits = 0;
 
       push_temp_slots ();
       tem = get_inner_reference (to, &bitsize, &bitpos, &offset, &mode1,
@@ -4337,8 +4381,26 @@  expand_assignment (tree to, tree from, b
 
       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 other threads can't see this value, no need to
+	     restrict stores.  */
+	  if (ALLOW_STORE_DATA_RACES
+	      || ((TREE_CODE (tem) == MEM_REF
+		   || TREE_CODE (tem) == TARGET_MEM_REF)
+		  && !ptr_deref_may_alias_global_p (TREE_OPERAND (tem, 0)))
+	      || TREE_CODE (tem) == RESULT_DECL
+	      || (DECL_P (tem)
+		  && (DECL_THREAD_LOCAL_P (tem)
+		      || !TREE_STATIC (tem))))
+	    {
+	      bitregion_offset = size_zero_node;
+	      /* Set to 0 for the special case where there is no
+		 restriction in play.  */
+	      bitregion_maxbits = 0;
+	    }
+	  else
+	    get_bit_range (to, &bitregion_offset, &bitregion_maxbits);
+	}
 
       /* If we are going to use store_bit_field and extract_bit_field,
 	 make sure to_rtx will be safe for multiple use.  */
@@ -4388,12 +4450,19 @@  expand_assignment (tree to, tree from, b
 	      && MEM_ALIGN (to_rtx) == GET_MODE_ALIGNMENT (mode1))
 	    {
 	      to_rtx = adjust_address (to_rtx, mode1, bitpos / BITS_PER_UNIT);
+	      bitregion_offset = fold_build2 (MINUS_EXPR, integer_type_node,
+					      bitregion_offset,
+					      build_int_cst (integer_type_node,
+							     bitpos / BITS_PER_UNIT));
 	      bitpos = 0;
 	    }
 
 	  to_rtx = offset_address (to_rtx, offset_rtx,
 				   highest_pow2_factor_for_target (to,
 				   				   offset));
+	  bitregion_offset = fold_build2 (MINUS_EXPR, integer_type_node,
+					  bitregion_offset,
+					  offset);
 	}
 
       /* No action is needed if the target is not a memory and the field
@@ -4421,13 +4490,13 @@  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,
+				  bitregion_offset, bitregion_maxbits,
 				  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,
-				  bitregion_start, bitregion_end,
+				  bitregion_offset, bitregion_maxbits,
 				  mode1, from,
 				  TREE_TYPE (tem), get_alias_set (to),
 				  nontemporal);
@@ -4450,7 +4519,7 @@  expand_assignment (tree to, tree from, b
 	      write_complex_part (temp, XEXP (to_rtx, 0), false);
 	      write_complex_part (temp, XEXP (to_rtx, 1), true);
 	      result = store_field (temp, bitsize, bitpos,
-				    bitregion_start, bitregion_end,
+				    bitregion_offset, bitregion_maxbits,
 				    mode1, from,
 				    TREE_TYPE (tem), get_alias_set (to),
 				    nontemporal);
@@ -4477,13 +4546,13 @@  expand_assignment (tree to, tree from, b
 	    }
 
 	  if (optimize_bitfield_assignment_op (bitsize, bitpos,
-					       bitregion_start, bitregion_end,
+					       bitregion_maxbits,
 					       mode1,
 					       to_rtx, to, from))
 	    result = NULL;
 	  else
 	    result = store_field (to_rtx, bitsize, bitpos,
-				  bitregion_start, bitregion_end,
+				  bitregion_offset, bitregion_maxbits,
 				  mode1, from,
 				  TREE_TYPE (tem), get_alias_set (to),
 				  nontemporal);
@@ -4877,7 +4946,7 @@  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, 0, 0, GET_MODE (temp), temp);
+			     0, integer_zero_node, 0, GET_MODE (temp), temp);
 	  else
 	    convert_move (target, temp, unsignedp);
 	}
@@ -5342,8 +5411,8 @@  store_constructor_field (rtx target, uns
       store_constructor (exp, target, cleared, bitsize / BITS_PER_UNIT);
     }
   else
-    store_field (target, bitsize, bitpos, 0, 0, mode, exp, type, alias_set,
-		 false);
+    store_field (target, bitsize, bitpos, integer_zero_node, 0, mode, exp,
+		 type, alias_set, false);
 }
 
 /* Store the value of constructor EXP into the rtx TARGET.
@@ -5917,10 +5986,10 @@  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.
+   BITREGION_OFFSET is the byte offset from the beginning of the
+   containing object to the start of the bit region.
+   BITREGION_MAXBITS is the size in bits of the largest mode that can
+   be used to set the bit-field in question.
 
    Always return const0_rtx unless we have something particular to
    return.
@@ -5935,8 +6004,8 @@  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,
+	     tree bitregion_offset,
+	     HOST_WIDE_INT bitregion_maxbits,
 	     enum machine_mode mode, tree exp, tree type,
 	     alias_set_type alias_set, bool nontemporal)
 {
@@ -5970,7 +6039,7 @@  store_field (rtx target, HOST_WIDE_INT b
 	emit_move_insn (object, target);
 
       store_field (blk_object, bitsize, bitpos,
-		   bitregion_start, bitregion_end,
+		   bitregion_offset, bitregion_maxbits,
 		   mode, exp, type, alias_set, nontemporal);
 
       emit_move_insn (target, object);
@@ -6086,7 +6155,7 @@  store_field (rtx target, HOST_WIDE_INT b
 
       /* Store the value in the bitfield.  */
       store_bit_field (target, bitsize, bitpos,
-		       bitregion_start, bitregion_end,
+		       bitregion_offset, bitregion_maxbits,
 		       mode, temp);
 
       return const0_rtx;
Index: expr.h
===================================================================
--- expr.h	(revision 176891)
+++ expr.h	(working copy)
@@ -666,8 +666,8 @@  mode_for_extraction (enum extraction_pat
 
 extern void store_bit_field (rtx, unsigned HOST_WIDE_INT,
 			     unsigned HOST_WIDE_INT,
-			     unsigned HOST_WIDE_INT,
-			     unsigned HOST_WIDE_INT,
+			     tree,
+			     HOST_WIDE_INT,
 			     enum machine_mode, rtx);
 extern rtx extract_bit_field (rtx, unsigned HOST_WIDE_INT,
 			      unsigned HOST_WIDE_INT, int, bool, rtx,
Index: stor-layout.c
===================================================================
--- stor-layout.c	(revision 176891)
+++ stor-layout.c	(working copy)
@@ -2361,13 +2361,6 @@  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).
@@ -2386,20 +2379,11 @@  fixup_unsigned_type (tree type)
 
 enum machine_mode
 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 = MAX_FIXED_MODE_SIZE;
-  else
-    maxbits = (bitregion_end - bitregion_start) % align + 1;
 
   /* Find the narrowest integer mode that contains the bit field.  */
   for (mode = GET_CLASS_NARROWEST_MODE (MODE_INT); mode != VOIDmode;
@@ -2436,7 +2420,6 @@  get_best_mode (int bitsize, int bitpos,
 	      && bitpos / unit == (bitpos + bitsize - 1) / unit
 	      && unit <= BITS_PER_WORD
 	      && unit <= MIN (align, BIGGEST_ALIGNMENT)
-	      && unit <= maxbits
 	      && (largest_mode == VOIDmode
 		  || unit <= GET_MODE_BITSIZE (largest_mode)))
 	    wide_mode = tmode;
Index: expmed.c
===================================================================
--- expmed.c	(revision 176891)
+++ expmed.c	(working copy)
@@ -48,13 +48,11 @@  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,
-				   unsigned HOST_WIDE_INT,
-				   unsigned HOST_WIDE_INT,
+				   HOST_WIDE_INT,
 				   rtx);
 static void store_split_bit_field (rtx, unsigned HOST_WIDE_INT,
 				   unsigned HOST_WIDE_INT,
-				   unsigned HOST_WIDE_INT,
-				   unsigned HOST_WIDE_INT,
+				   HOST_WIDE_INT,
 				   rtx);
 static rtx extract_fixed_bit_field (enum machine_mode, rtx,
 				    unsigned HOST_WIDE_INT,
@@ -340,8 +338,7 @@  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,
-		   unsigned HOST_WIDE_INT bitregion_start,
-		   unsigned HOST_WIDE_INT bitregion_end,
+		   HOST_WIDE_INT bitregion_maxbits,
 		   enum machine_mode fieldmode,
 		   rtx value, bool fallback_p)
 {
@@ -558,7 +555,7 @@  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,
-				  bitregion_start, bitregion_end,
+				  bitregion_maxbits,
 				  word_mode,
 				  value_word, fallback_p))
 	    {
@@ -722,10 +719,6 @@  store_bit_field_1 (rtx str_rtx, unsigned
   if (HAVE_insv && MEM_P (op0))
     {
       enum machine_mode bestmode;
-      unsigned HOST_WIDE_INT maxbits = MAX_FIXED_MODE_SIZE;
-
-      if (bitregion_end)
-	maxbits = bitregion_end - bitregion_start + 1;
 
       /* Get the mode to use for inserting into this field.  If OP0 is
 	 BLKmode, get the smallest mode consistent with the alignment. If
@@ -733,15 +726,19 @@  store_bit_field_1 (rtx str_rtx, unsigned
 	 mode. Otherwise, use the smallest mode containing the field.  */
 
       if (GET_MODE (op0) == BLKmode
-	  || GET_MODE_BITSIZE (GET_MODE (op0)) > maxbits
+	  || (bitregion_maxbits
+	      && GET_MODE_BITSIZE (GET_MODE (op0)) > bitregion_maxbits)
 	  || (op_mode != MAX_MACHINE_MODE
 	      && GET_MODE_SIZE (GET_MODE (op0)) > GET_MODE_SIZE (op_mode)))
-	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));
+	{
+	  bestmode = (op_mode == MAX_MACHINE_MODE ? VOIDmode : op_mode);
+	  if (bitregion_maxbits && bitregion_maxbits < GET_MODE_SIZE (op_mode))
+	    bestmode = smallest_mode_for_size (bitregion_maxbits, MODE_INT);
+	  bestmode = get_best_mode  (bitsize, bitnum,
+				     MEM_ALIGN (op0),
+				     bestmode,
+				     MEM_VOLATILE_P (op0));
+	}
       else
 	bestmode = GET_MODE (op0);
 
@@ -752,6 +749,7 @@  store_bit_field_1 (rtx str_rtx, unsigned
 	{
 	  rtx last, tempreg, xop0;
 	  unsigned HOST_WIDE_INT xoffset, xbitpos;
+	  HOST_WIDE_INT xmaxbits = bitregion_maxbits;
 
 	  last = get_last_insn ();
 
@@ -762,12 +760,13 @@  store_bit_field_1 (rtx str_rtx, unsigned
 	  xoffset = (bitnum / unit) * GET_MODE_SIZE (bestmode);
 	  xbitpos = bitnum % unit;
 	  xop0 = adjust_address (op0, bestmode, xoffset);
+	  if (xmaxbits)
+	    xmaxbits -= xoffset * BITS_PER_UNIT;
 
 	  /* Fetch that unit, store the bitfield in it, then store
 	     the unit.  */
 	  tempreg = copy_to_reg (xop0);
-	  if (store_bit_field_1 (tempreg, bitsize, xbitpos,
-				 bitregion_start, bitregion_end,
+	  if (store_bit_field_1 (tempreg, bitsize, xbitpos, xmaxbits,
 				 fieldmode, orig_value, false))
 	    {
 	      emit_move_insn (xop0, tempreg);
@@ -780,8 +779,10 @@  store_bit_field_1 (rtx str_rtx, unsigned
   if (!fallback_p)
     return false;
 
+  if (bitregion_maxbits)
+    bitregion_maxbits -= offset * BITS_PER_UNIT;
   store_fixed_bit_field (op0, offset, bitsize, bitpos,
-			 bitregion_start, bitregion_end, value);
+			 bitregion_maxbits, value);
   return true;
 }
 
@@ -789,18 +790,17 @@  store_bit_field_1 (rtx str_rtx, unsigned
    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.
+   BITREGION_OFFSET is the byte offset STR_RTX to the start of the bit
+   region.  BITREGION_MAXBITS is the number of bits of the largest
+   mode that can be used to set the bit-field in question.
 
    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,
-		 unsigned HOST_WIDE_INT bitregion_start,
-		 unsigned HOST_WIDE_INT bitregion_end,
+		 tree bitregion_offset,
+		 HOST_WIDE_INT bitregion_maxbits,
 		 enum machine_mode fieldmode,
 		 rtx value)
 {
@@ -808,30 +808,29 @@  store_bit_field (rtx str_rtx, unsigned H
      bit region.  Adjust the address to start at the beginning of the
      bit region.  */
   if (MEM_P (str_rtx)
-      && bitregion_start > 0)
+      && bitregion_maxbits
+      && !integer_zerop (bitregion_offset))
     {
-      enum machine_mode bestmode;
-      enum machine_mode op_mode;
-      unsigned HOST_WIDE_INT offset;
+      HOST_WIDE_INT offset;
 
-      op_mode = mode_for_extraction (EP_insv, 3);
-      if (op_mode == MAX_MACHINE_MODE)
-	op_mode = VOIDmode;
-
-      offset = bitregion_start / BITS_PER_UNIT;
-      bitnum -= bitregion_start;
-      bitregion_end -= bitregion_start;
-      bitregion_start = 0;
-      bestmode = get_best_mode (bitsize, bitnum,
-				bitregion_start, bitregion_end,
-				MEM_ALIGN (str_rtx),
-				op_mode,
-				MEM_VOLATILE_P (str_rtx));
-      str_rtx = adjust_address (str_rtx, bestmode, offset);
+      if (host_integerp (bitregion_offset, 1))
+	{
+	  /* Adjust the bit position accordingly.  */
+	  offset = tree_low_cst (bitregion_offset, 1);
+	  bitnum -= offset * BITS_PER_UNIT;
+	  /* Adjust the actual address.  */
+	  str_rtx = adjust_address (str_rtx, GET_MODE (str_rtx), offset);
+	}
+      else
+	{
+	  /* Handle variable length offsets.  */
+	  str_rtx = offset_address (str_rtx,
+				    expand_normal (bitregion_offset), 1);
+	}
+      bitregion_offset = integer_zero_node;
     }
 
-  if (!store_bit_field_1 (str_rtx, bitsize, bitnum,
-			  bitregion_start, bitregion_end,
+  if (!store_bit_field_1 (str_rtx, bitsize, bitnum, bitregion_maxbits,
 			  fieldmode, value, true))
     gcc_unreachable ();
 }
@@ -849,8 +848,7 @@  static void
 store_fixed_bit_field (rtx op0, unsigned HOST_WIDE_INT offset,
 		       unsigned HOST_WIDE_INT bitsize,
 		       unsigned HOST_WIDE_INT bitpos,
-		       unsigned HOST_WIDE_INT bitregion_start,
-		       unsigned HOST_WIDE_INT bitregion_end,
+		       HOST_WIDE_INT bitregion_maxbits,
 		       rtx value)
 {
   enum machine_mode mode;
@@ -872,19 +870,12 @@  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,
-				 bitregion_start, bitregion_end,
-				 value);
+	  store_split_bit_field (op0, bitsize, bitpos, bitregion_maxbits, value);
 	  return;
 	}
     }
   else
     {
-      unsigned HOST_WIDE_INT maxbits = MAX_FIXED_MODE_SIZE;
-
-      if (bitregion_end)
-	maxbits = bitregion_end - bitregion_start + 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.
@@ -897,20 +888,26 @@  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
+	  && (!bitregion_maxbits
+	      || GET_MODE_BITSIZE (GET_MODE (op0)) <= bitregion_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 (bitregion_maxbits && bitregion_maxbits < GET_MODE_BITSIZE (mode))
+	    mode = smallest_mode_for_size (bitregion_maxbits, MODE_INT);
+	  mode = get_best_mode (bitsize, bitpos + offset * BITS_PER_UNIT,
+				MEM_ALIGN (op0), mode, MEM_VOLATILE_P (op0));
+	}
 
       if (mode == VOIDmode)
 	{
+	  if (bitregion_maxbits)
+	    bitregion_maxbits -= offset * BITS_PER_UNIT;
 	  /* The only way this should occur is if the field spans word
 	     boundaries.  */
 	  store_split_bit_field (op0, bitsize, bitpos + offset * BITS_PER_UNIT,
-				 bitregion_start, bitregion_end, value);
+				 bitregion_maxbits, value);
 	  return;
 	}
 
@@ -1031,8 +1028,7 @@  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,
-		       unsigned HOST_WIDE_INT bitregion_start,
-		       unsigned HOST_WIDE_INT bitregion_end,
+		       HOST_WIDE_INT bitregion_maxbits,
 		       rtx value)
 {
   unsigned int unit;
@@ -1147,8 +1143,13 @@  store_split_bit_field (rtx op0, unsigned
 	 store_fixed_bit_field wants offset in bytes.  If WORD is const0_rtx,
 	 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, bitregion_start, bitregion_end, part);
+	{
+	  HOST_WIDE_INT xmaxbits = bitregion_maxbits;
+	  if (bitregion_maxbits)
+	    xmaxbits -= offset * unit / BITS_PER_UNIT;
+	  store_fixed_bit_field (word, offset * unit / BITS_PER_UNIT, thissize,
+				 thispos, xmaxbits, part);
+	}
       bitsdone += thissize;
     }
 }
@@ -1588,7 +1589,7 @@  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, 0, 0, MEM_ALIGN (op0),
+	bestmode = get_best_mode (bitsize, bitnum, MEM_ALIGN (op0),
 				  (ext_mode == MAX_MACHINE_MODE
 				   ? VOIDmode : ext_mode),
 				  MEM_VOLATILE_P (op0));
@@ -1714,7 +1715,7 @@  extract_fixed_bit_field (enum machine_mo
 	    mode = tmode;
 	}
       else
-	mode = get_best_mode (bitsize, bitpos + offset * BITS_PER_UNIT, 0, 0,
+	mode = get_best_mode (bitsize, bitpos + offset * BITS_PER_UNIT,
 			      MEM_ALIGN (op0), word_mode, MEM_VOLATILE_P (op0));
 
       if (mode == VOIDmode)