common.opt (-fstrict-volatile-bitfields): new.

* common.opt (-fstrict-volatile-bitfields): 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.

From-SVN: r160865

1 files changed, 74 insertions, 10 deletions

diff --git a/gcc/expmed.c b/gcc/expmed.c
index 017e208..a06137e 100644
--- a/gcc/expmed.c
+++ b/gcc/expmed.c
@@ -903,8 +903,14 @@ store_fixed_bit_field (rtx op0, unsigned HOST_WIDE_INT offset,
       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_strict_volatile_bitfields > 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)
 	{
@@ -1377,6 +1383,14 @@ extract_bit_field_1 (rtx str_rtx, unsigned HOST_WIDE_INT bitsize,
 	    ? 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_strict_volatile_bitfields > 0)
+    goto no_subreg_mode_swap;
+
   if (((bitsize >= BITS_PER_WORD && bitsize == GET_MODE_BITSIZE (mode)
 	&& bitpos % BITS_PER_WORD == 0)
        || (mode1 != BLKmode
@@ -1729,8 +1743,19 @@ extract_fixed_bit_field (enum machine_mode tmode, rtx op0,
 	 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_strict_volatile_bitfields > 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
@@ -1751,12 +1776,51 @@ extract_fixed_bit_field (enum machine_mode tmode, rtx op0,
 		     * 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_strict_volatile_bitfields > 0
+	  && bitpos + bitsize <= total_bits
+	  && bitpos + bitsize + (offset % (total_bits / BITS_PER_UNIT)) * BITS_PER_UNIT > total_bits)
+	{
+	  if (STRICT_ALIGNMENT)
+	    {
+	      static bool informed_about_misalignment = false;
+	      bool warned;
+
+	      if (bitsize == total_bits)
+		warned = warning_at (input_location, OPT_fstrict_volatile_bitfields,
+				     "mis-aligned access used for structure member");
+	      else
+		warned = warning_at (input_location, OPT_fstrict_volatile_bitfields,
+				     "mis-aligned access used for structure bitfield");
+
+	      if (! informed_about_misalignment && warned)
+		{
+		  informed_about_misalignment = true;
+		  inform (input_location,
+			  "When a volatile object spans multiple type-sized locations,"
+			  " the compiler must choose between using a single mis-aligned access to"
+			  " preserve the volatility, or using multiple aligned accesses to avoid"
+			  " runtime faults.  This code may fail at runtime if the hardware does"
+			  " not allow this access.");
+		}
+	    }
+	}
+      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);
     }