Patchwork patch: honor volatile bitfield types

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Submitter DJ Delorie
Date June 11, 2010, 10:04 p.m.
Message ID <201006112204.o5BM4wsk031264@greed.delorie.com>
Download mbox | patch
Permalink /patch/55374/
State New
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Comments

DJ Delorie - June 11, 2010, 10:04 p.m.
Ok, updated documentation, and the default value for the flag is -1.
The checks are all "flag > 0" so it can be left at -1 or changed to
some other default, but the targets can see if it's still unspecified
and change it if they wish.

2010-06-11  DJ Delorie  <dj@redhat.com>

	* common.opt (-fhonor-volatile-bitfield-types): new.
	* doc/invoke.texi: Document it.
	* fold-const.c (optimize_bit_field_compare): For volatile
	bitfields, use the field's type to determine the mode, not the
	field's size.
	* expr.c (expand_assignment): Likewise.
	(get_inner_reference): Likewise.
	(expand_expr_real_1): Likewise.
	* expmed.c (store_fixed_bit_field): Likewise.
	(extract_bit_field_1): Likewise.
	(extract_fixed_bit_field): Likewise.

	* gcc.target/i386/volatile-bitfields-1.c: New.
	* gcc.target/i386/volatile-bitfields-2.c: New.
Mark Mitchell - June 12, 2010, 12:46 a.m.
DJ Delorie wrote:

> Ok, updated documentation, and the default value for the flag is -1.
> The checks are all "flag > 0" so it can be left at -1 or changed to
> some other default, but the targets can see if it's still unspecified
> and change it if they wish.

Thanks, both for that and for improving the documentation.

I'm still not very happy with the option name.  I don't think GCC
normally dishonors bitfield types. :-)  But, I guess I can't really
think of something I like a lot better.

I also don't think "strict alignment" is a very oft-used term outside of
GCC's internals.  I'd prefer to explicitly say what that means.

I think these warnings:

> +		warning (0, "mis-aligned access required for structure member");

should mention the fact that the bit-field is volatile and what the
consequences are.  Something like:

  using mis-aligned load to or store from field 'x' of 'struct a'
because multiple aligned loads/stores would be required and the field is
volatile

I think it really needs to be detailed at this level so that people have
some idea what's going on.  And, the field name should definitely be
mentioned if at all possible.

The patch is OK with changes in that spirit.

Thanks,
DJ Delorie - June 12, 2010, 1:15 a.m.
> I'm still not very happy with the option name.

I'm open to suggestions :-)

> I don't think GCC normally dishonors bitfield types. :-)

The problem is, it *does* in that specific case.  Choosing an option
name that identifies the actual change in functionality without
confusing the user is tricky.

>   using mis-aligned load to or store from field 'x' of 'struct a'
> because multiple aligned loads/stores would be required and the field is
> volatile

That's not a warning, that's a paragraph :-)
Mark Mitchell - June 12, 2010, 1:17 a.m.
DJ Delorie wrote:

>>   using mis-aligned load to or store from field 'x' of 'struct a'
>> because multiple aligned loads/stores would be required and the field is
>> volatile
> 
> That's not a warning, that's a paragraph :-)

This warning needs to be a paragraph; no phrase is going to be
meaningful to anyone that doesn't understand the compiler internals.
You can use the technique we use elsewhere where more explanation is
given the first time the warning is emitted.

Thanks,
Richard Guenther - June 12, 2010, 8:14 a.m.
On Sat, Jun 12, 2010 at 3:15 AM, DJ Delorie <dj@redhat.com> wrote:
>
>> I'm still not very happy with the option name.
>
> I'm open to suggestions :-)

-fstrict-volatile-bitfields

>> I don't think GCC normally dishonors bitfield types. :-)
>
> The problem is, it *does* in that specific case.  Choosing an option
> name that identifies the actual change in functionality without
> confusing the user is tricky.
>
>>   using mis-aligned load to or store from field 'x' of 'struct a'
>> because multiple aligned loads/stores would be required and the field is
>> volatile
>
> That's not a warning, that's a paragraph :-)
>
DJ Delorie - June 15, 2010, 1:20 a.m.
> And, the field name should definitely be mentioned if at all
> possible.

Sigh, not possible without major code changes.  This is the problem I
ran into writing the patch itself - the decls are all long since lost
by the time we get to these routines, and some of the paths that lead
to them make it very difficult to keep track of them (if they're
available at all).

How's this for the message?


	      if (bitsize == total_bits)
		warning (0, "mis-aligned access used for structure member");
	      else
		warning (0, "mis-aligned access used for structure bitfield");

	      if (! warned_about_misalignment)
		{
		  warned_about_misalignment = true;
		  warning (0, "volatile objects require a single access to preserve their"
			   " volatility, but this member spans multiple type-sized locations."
			   " Normally the compiler would use multiple accesses for such fields"
			   " to avoid mis-aligned accesses.  This code may fail at runtime")
		}
Mark Mitchell - June 15, 2010, 2:32 a.m.
DJ Delorie wrote:
>> And, the field name should definitely be mentioned if at all
>> possible.
> 
> Sigh, not possible without major code changes. 

OK.

> 		  warning (0, "volatile objects require a single access to preserve their"
> 			   " volatility, but this member spans multiple type-sized locations."
> 			   " Normally the compiler would use multiple accesses for such fields"
> 			   " to avoid mis-aligned accesses.  This code may fail at runtime")

I think that can be made more precise.

use of 'volatile' objects should result in a single access to memory,
but this field cannot be accessed with a single aligned X-bit access.
Therefore, the compiler will use an unaligned access which may cause a
run-time fault.

Thanks,
DJ Delorie - June 15, 2010, 2:39 a.m.
I like that.

What do you think of Richard's suggested option name?

	-fstrict-volatile-bitfields
Mark Mitchell - June 15, 2010, 2:40 a.m.
DJ Delorie wrote:
> I like that.
> 
> What do you think of Richard's suggested option name?
> 
> 	-fstrict-volatile-bitfields

Works for me!
Manuel López-Ibáñez - June 15, 2010, 7:18 a.m.
On 15 June 2010 03:20, DJ Delorie <dj@redhat.com> wrote:
>
>> And, the field name should definitely be mentioned if at all
>> possible.
>
> Sigh, not possible without major code changes.  This is the problem I
> ran into writing the patch itself - the decls are all long since lost
> by the time we get to these routines, and some of the paths that lead
> to them make it very difficult to keep track of them (if they're
> available at all).
>
> How's this for the message?
>
>
>              if (bitsize == total_bits)
>                warning (0, "mis-aligned access used for structure member");
>              else
>                warning (0, "mis-aligned access used for structure bitfield");
>
>              if (! warned_about_misalignment)
>                {
>                  warned_about_misalignment = true;
>                  warning (0, "volatile objects require a single access to preserve their"
>                           " volatility, but this member spans multiple type-sized locations."
>                           " Normally the compiler would use multiple accesses for such fields"
>                           " to avoid mis-aligned accesses.  This code may fail at runtime")
>                }
>

You should use "inform" for such follow-up notes. Since the warning is
only enabled by fstrict-volatile-fields, you can use
OPT_fstrict_volatile_fields instead of 0. This way the option will be
shown alongside the warning.

I guess it is impossible to get a precise location here. I still would
prefer to use warning_at (input_location to remind us that this should
be fixed in the future.

Cheers,

Manuel.
Richard Henderson - June 15, 2010, 4:43 p.m.
On 06/14/2010 06:20 PM, DJ Delorie wrote:
>> And, the field name should definitely be mentioned if at all
>> possible.
> 
> Sigh, not possible without major code changes.  This is the problem I
> ran into writing the patch itself - the decls are all long since lost
> by the time we get to these routines, and some of the paths that lead
> to them make it very difficult to keep track of them (if they're
> available at all).
> 
> How's this for the message?
> 
> 
> 	      if (bitsize == total_bits)
> 		warning (0, "mis-aligned access used for structure member");
> 	      else
> 		warning (0, "mis-aligned access used for structure bitfield");
> 
> 	      if (! warned_about_misalignment)
> 		{
> 		  warned_about_misalignment = true;
> 		  warning (0, "volatile objects require a single access to preserve their"
> 			   " volatility, but this member spans multiple type-sized locations."
> 			   " Normally the compiler would use multiple accesses for such fields"
> 			   " to avoid mis-aligned accesses.  This code may fail at runtime")
> 		}

Is this second message more approriate to "info" than "warning"?


r~
DJ Delorie - June 15, 2010, 4:54 p.m.
> Is this second message more approriate to "info" than "warning"?

Yes, but I wanted to keep them in sync wrt enabling/disabling
diagnostics by type.
Manuel López-Ibáñez - June 15, 2010, 4:57 p.m.
On 15 June 2010 18:54, DJ Delorie <dj@redhat.com> wrote:
>
>> Is this second message more approriate to "info" than "warning"?
>
> Yes, but I wanted to keep them in sync wrt enabling/disabling
> diagnostics by type.

What does it mean that?

Enabling/disabling how?

Cheers,

Manuel.
Richard Henderson - June 15, 2010, 7:11 p.m.
On 06/15/2010 09:54 AM, DJ Delorie wrote:
>> Is this second message more approriate to "info" than "warning"?
> 
> Yes, but I wanted to keep them in sync wrt enabling/disabling
> diagnostics by type.

You mean e.g. "-w" disabling warnings?  I suppose that does 
require a bit of extra logic.  Though that would be easier
if warning returned a value indicating if the warning was
actually emitted...

r~
Manuel López-Ibáñez - June 15, 2010, 7:17 p.m.
On 15 June 2010 21:11, Richard Henderson <rth@redhat.com> wrote:
> On 06/15/2010 09:54 AM, DJ Delorie wrote:
>>> Is this second message more approriate to "info" than "warning"?
>>
>> Yes, but I wanted to keep them in sync wrt enabling/disabling
>> diagnostics by type.
>
> You mean e.g. "-w" disabling warnings?  I suppose that does
> require a bit of extra logic.  Though that would be easier
> if warning returned a value indicating if the warning was
> actually emitted...

/* A warning at LOCATION.  Use this for code which is correct according to the
   relevant language specification but is likely to be buggy anyway.
   Returns true if the warning was printed, false if it was inhibited.  */

bool
warning_at (location_t location, int opt, const char *gmsgid, ...)


The same goes for pedwarn and permerror. This has been implemented
years ago for precisely this case.

Cheers,

Manuel.

Patch

Index: doc/invoke.texi
===================================================================
--- doc/invoke.texi	(revision 160589)
+++ doc/invoke.texi	(working copy)
@@ -17706,12 +17706,38 @@  be thrown between DSOs must be explicitl
 visibility so that the @samp{type_info} nodes will be unified between
 the DSOs.
 
 An overview of these techniques, their benefits and how to use them
 is at @w{@uref{http://gcc.gnu.org/wiki/Visibility}}.
 
+@item -fhonor-volatile-bitfield-types
+This option should be used if accesses to volatile bitfields (or other
+structure fields, although the compiler usually honors those types
+anyway) should use a single access in a mode of the same size as the
+container's type, aligned to a natural alignment if possible.  For
+example, targets with memory-mapped peripheral registers might require
+all such accesses to be 16 bits wide; with this flag the user could
+declare all peripheral bitfields as ``unsigned short'' (assuming short
+is 16 bits on these targets) to force GCC to use 16 bit accesses
+instead of, perhaps, a more efficient 32 bit access.
+
+If this option is disabled, the compiler will use the most efficient
+instruction.  In the previous example, that might be a 32-bit load
+instruction, even though that will access bytes that do not contain
+any portion of the bitfield, or memory-mapped registers unrelated to
+the one being updated.
+
+If the target requires strict alignment, and honoring the container
+type would require violating this alignment, a warning is issued.
+However, the access happens as the user requested, under the
+assumption that the user knows something about the target hardware
+that GCC is unaware of.
+
+The default value of this option is determined by the application binary
+interface for the target processor.
+
 @end table
 
 @c man end
 
 @node Environment Variables
 @section Environment Variables Affecting GCC
Index: fold-const.c
===================================================================
--- fold-const.c	(revision 160589)
+++ fold-const.c	(working copy)
@@ -3460,17 +3460,22 @@  optimize_bit_field_compare (location_t l
 	 || TREE_CODE (rinner) == PLACEHOLDER_EXPR)
        return 0;
    }
 
   /* See if we can find a mode to refer to this field.  We should be able to,
      but fail if we can't.  */
-  nmode = get_best_mode (lbitsize, lbitpos,
-			 const_p ? TYPE_ALIGN (TREE_TYPE (linner))
-			 : MIN (TYPE_ALIGN (TREE_TYPE (linner)),
-				TYPE_ALIGN (TREE_TYPE (rinner))),
-			 word_mode, lvolatilep || rvolatilep);
+  if (lvolatilep
+      && GET_MODE_BITSIZE (lmode) > 0
+      && flag_honor_volatile_bitfield_types > 0)
+    nmode = lmode;
+  else
+    nmode = get_best_mode (lbitsize, lbitpos,
+			   const_p ? TYPE_ALIGN (TREE_TYPE (linner))
+			   : MIN (TYPE_ALIGN (TREE_TYPE (linner)),
+				  TYPE_ALIGN (TREE_TYPE (rinner))),
+			   word_mode, lvolatilep || rvolatilep);
   if (nmode == VOIDmode)
     return 0;
 
   /* Set signed and unsigned types of the precision of this mode for the
      shifts below.  */
   signed_type = lang_hooks.types.type_for_mode (nmode, 0);
Index: testsuite/gcc.target/i386/volatile-bitfields-2.c
===================================================================
--- testsuite/gcc.target/i386/volatile-bitfields-2.c	(revision 0)
+++ testsuite/gcc.target/i386/volatile-bitfields-2.c	(revision 0)
@@ -0,0 +1,17 @@ 
+/* { dg-do compile } */
+/* { dg-options "-O2 -fno-honor-volatile-bitfield-types" } */
+
+typedef struct {
+  char a:1;
+  char b:7;
+  int c;
+} BitStruct;
+
+volatile BitStruct bits;
+
+int foo ()
+{
+  return bits.b;
+}
+
+/* { dg-final { scan-assembler "movl.*bits" } } */
Index: testsuite/gcc.target/i386/volatile-bitfields-1.c
===================================================================
--- testsuite/gcc.target/i386/volatile-bitfields-1.c	(revision 0)
+++ testsuite/gcc.target/i386/volatile-bitfields-1.c	(revision 0)
@@ -0,0 +1,17 @@ 
+/* { dg-do compile } */
+/* { dg-options "-O2 -fhonor-volatile-bitfield-types" } */
+
+typedef struct {
+  char a:1;
+  char b:7;
+  int c;
+} BitStruct;
+
+volatile BitStruct bits;
+
+int foo ()
+{
+  return bits.b;
+}
+
+/* { dg-final { scan-assembler "movzbl.*bits" } } */
Index: expr.c
===================================================================
--- expr.c	(revision 160589)
+++ expr.c	(working copy)
@@ -4226,12 +4226,19 @@  expand_assignment (tree to, tree from, b
 
       /* If we are going to use store_bit_field and extract_bit_field,
 	 make sure to_rtx will be safe for multiple use.  */
 
       to_rtx = expand_normal (tem);
 
+      /* If the bitfield is volatile, we want to access it in the
+	 field's mode, not the computed mode.  */
+      if (volatilep
+	  && GET_CODE (to_rtx) == MEM
+	  && flag_honor_volatile_bitfield_types > 0)
+	to_rtx = adjust_address (to_rtx, mode1, 0);
+ 
       if (offset != 0)
 	{
 	  enum machine_mode address_mode;
 	  rtx offset_rtx;
 
 	  if (!MEM_P (to_rtx))
@@ -5987,12 +5994,18 @@  get_inner_reference (tree exp, HOST_WIDE
       tree field = TREE_OPERAND (exp, 1);
       size_tree = DECL_SIZE (field);
       if (!DECL_BIT_FIELD (field))
 	mode = DECL_MODE (field);
       else if (DECL_MODE (field) == BLKmode)
 	blkmode_bitfield = true;
+      else if (TREE_THIS_VOLATILE (exp)
+	       && flag_honor_volatile_bitfield_types > 0)
+	/* Volatile bitfields should be accessed in the mode of the
+	     field's type, not the mode computed based on the bit
+	     size.  */
+	mode = TYPE_MODE (DECL_BIT_FIELD_TYPE (field));
 
       *punsignedp = DECL_UNSIGNED (field);
     }
   else if (TREE_CODE (exp) == BIT_FIELD_REF)
     {
       size_tree = TREE_OPERAND (exp, 1);
@@ -8963,12 +8976,20 @@  expand_expr_real_1 (tree exp, rtx target
 			 VOIDmode,
 			 (modifier == EXPAND_INITIALIZER
 			  || modifier == EXPAND_CONST_ADDRESS
 			  || modifier == EXPAND_STACK_PARM)
 			 ? modifier : EXPAND_NORMAL);
 
+
+	/* If the bitfield is volatile, we want to access it in the
+	   field's mode, not the computed mode.  */
+	if (volatilep
+	    && GET_CODE (op0) == MEM
+	    && flag_honor_volatile_bitfield_types > 0)
+	  op0 = adjust_address (op0, mode1, 0);
+
 	mode2
 	  = CONSTANT_P (op0) ? TYPE_MODE (TREE_TYPE (tem)) : GET_MODE (op0);
 
 	/* If we have either an offset, a BLKmode result, or a reference
 	   outside the underlying object, we must force it to memory.
 	   Such a case can occur in Ada if we have unchecked conversion
@@ -9088,12 +9109,15 @@  expand_expr_real_1 (tree exp, rtx target
 	    || REG_P (op0) || GET_CODE (op0) == SUBREG
 	    || (mode1 != BLKmode && ! direct_load[(int) mode1]
 		&& GET_MODE_CLASS (mode) != MODE_COMPLEX_INT
 		&& GET_MODE_CLASS (mode) != MODE_COMPLEX_FLOAT
 		&& modifier != EXPAND_CONST_ADDRESS
 		&& modifier != EXPAND_INITIALIZER)
+	    /* If the field is volatile, we always want an aligned
+	       access.  */
+	    || (volatilep && flag_honor_volatile_bitfield_types > 0)
 	    /* If the field isn't aligned enough to fetch as a memref,
 	       fetch it as a bit field.  */
 	    || (mode1 != BLKmode
 		&& (((TYPE_ALIGN (TREE_TYPE (tem)) < GET_MODE_ALIGNMENT (mode)
 		      || (bitpos % GET_MODE_ALIGNMENT (mode) != 0)
 		      || (MEM_P (op0)
Index: expmed.c
===================================================================
--- expmed.c	(revision 160589)
+++ expmed.c	(working copy)
@@ -900,14 +900,20 @@  store_fixed_bit_field (rtx op0, unsigned
 	 We don't want a mode bigger than the destination.  */
 
       mode = GET_MODE (op0);
       if (GET_MODE_BITSIZE (mode) == 0
 	  || GET_MODE_BITSIZE (mode) > GET_MODE_BITSIZE (word_mode))
 	mode = word_mode;
-      mode = get_best_mode (bitsize, bitpos + offset * BITS_PER_UNIT,
-			    MEM_ALIGN (op0), mode, MEM_VOLATILE_P (op0));
+
+      if (MEM_VOLATILE_P (op0)
+          && GET_MODE_BITSIZE (GET_MODE (op0)) > 0
+	  && flag_honor_volatile_bitfield_types > 0)
+	mode = GET_MODE (op0);
+      else
+	mode = get_best_mode (bitsize, bitpos + offset * BITS_PER_UNIT,
+			      MEM_ALIGN (op0), mode, MEM_VOLATILE_P (op0));
 
       if (mode == VOIDmode)
 	{
 	  /* The only way this should occur is if the field spans word
 	     boundaries.  */
 	  store_split_bit_field (op0, bitsize, bitpos + offset * BITS_PER_UNIT,
@@ -1374,12 +1380,20 @@  extract_bit_field_1 (rtx str_rtx, unsign
      we want a mode based on the size, so we must avoid calling it for FP
      modes.  */
   mode1  = (SCALAR_INT_MODE_P (tmode)
 	    ? mode_for_size (bitsize, GET_MODE_CLASS (tmode), 0)
 	    : mode);
 
+  /* If the bitfield is volatile, we need to make sure the access
+     remains on a type-aligned boundary.  */
+  if (GET_CODE (op0) == MEM
+      && MEM_VOLATILE_P (op0)
+      && GET_MODE_BITSIZE (GET_MODE (op0)) > 0
+      && flag_honor_volatile_bitfield_types > 0)
+    goto no_subreg_mode_swap;
+
   if (((bitsize >= BITS_PER_WORD && bitsize == GET_MODE_BITSIZE (mode)
 	&& bitpos % BITS_PER_WORD == 0)
        || (mode1 != BLKmode
 	   /* ??? The big endian test here is wrong.  This is correct
 	      if the value is in a register, and if mode_for_size is not
 	      the same mode as op0.  This causes us to get unnecessarily
@@ -1726,14 +1740,25 @@  extract_fixed_bit_field (enum machine_mo
   else
     {
       /* 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.  */
 
-      mode = get_best_mode (bitsize, bitpos + offset * BITS_PER_UNIT,
-			    MEM_ALIGN (op0), word_mode, MEM_VOLATILE_P (op0));
+      if (MEM_VOLATILE_P (op0)
+	  && flag_honor_volatile_bitfield_types > 0)
+	{
+	  if (GET_MODE_BITSIZE (GET_MODE (op0)) > 0)
+	    mode = GET_MODE (op0);
+	  else if (target && GET_MODE_BITSIZE (GET_MODE (target)) > 0)
+	    mode = GET_MODE (target);
+	  else
+	    mode = tmode;
+	}
+      else
+	mode = get_best_mode (bitsize, bitpos + offset * BITS_PER_UNIT,
+			      MEM_ALIGN (op0), word_mode, MEM_VOLATILE_P (op0));
 
       if (mode == VOIDmode)
 	/* The only way this should occur is if the field spans word
 	   boundaries.  */
 	return extract_split_bit_field (op0, bitsize,
 					bitpos + offset * BITS_PER_UNIT,
@@ -1748,18 +1773,41 @@  extract_fixed_bit_field (enum machine_mo
 	{
 	  offset += (bitpos / total_bits) * (total_bits / BITS_PER_UNIT);
 	  bitpos -= ((bitpos / total_bits) * (total_bits / BITS_PER_UNIT)
 		     * BITS_PER_UNIT);
 	}
 
-      /* Get ref to an aligned byte, halfword, or word containing the field.
-	 Adjust BITPOS to be position within a word,
-	 and OFFSET to be the offset of that word.
-	 Then alter OP0 to refer to that word.  */
-      bitpos += (offset % (total_bits / BITS_PER_UNIT)) * BITS_PER_UNIT;
-      offset -= (offset % (total_bits / BITS_PER_UNIT));
+      /* If we're accessing a volatile MEM, we can't do the next
+	 alignment step if it results in a multi-word access where we
+	 otherwise wouldn't have one.  So, check for that case
+	 here.  */
+      if (MEM_P (op0)
+	  && MEM_VOLATILE_P (op0)
+	  && flag_honor_volatile_bitfield_types > 0
+	  && bitpos + bitsize <= total_bits
+	  && bitpos + bitsize + (offset % (total_bits / BITS_PER_UNIT)) * BITS_PER_UNIT > total_bits)
+	{
+	  if (STRICT_ALIGNMENT)
+	    {
+	      if (bitsize == total_bits)
+		warning (0, "mis-aligned access required for structure member");
+	      else
+		warning (0, "mis-aligned access required for structure bitfield");
+	    }
+	}
+      else
+	{
+
+	  /* Get ref to an aligned byte, halfword, or word containing the field.
+	     Adjust BITPOS to be position within a word,
+	     and OFFSET to be the offset of that word.
+	     Then alter OP0 to refer to that word.  */
+	  bitpos += (offset % (total_bits / BITS_PER_UNIT)) * BITS_PER_UNIT;
+	  offset -= (offset % (total_bits / BITS_PER_UNIT));
+	}
+
       op0 = adjust_address (op0, mode, offset);
     }
 
   mode = GET_MODE (op0);
 
   if (BYTES_BIG_ENDIAN)
Index: common.opt
===================================================================
--- common.opt	(revision 160589)
+++ common.opt	(working copy)
@@ -622,12 +622,16 @@  Common Report Var(flag_loop_interchange)
 Enable Loop Interchange transformation
 
 floop-block
 Common Report Var(flag_loop_block) Optimization
 Enable Loop Blocking transformation
 
+fhonor-volatile-bitfield-types
+Common Report Var(flag_honor_volatile_bitfield_types) Init(-1)
+Force bitfield accesses to match their type width
+
 fguess-branch-probability
 Common Report Var(flag_guess_branch_prob) Optimization
 Enable guessing of branch probabilities
 
 ; Nonzero means ignore `#ident' directives.  0 means handle them.
 ; Generate position-independent code for executables if possible