re PR middle-end/71310 (Bitfields cause load hit store with smaller store and larger load)

2016-06-14  Richard Biener  <rguenther@suse.de>

	PR middle-end/71310
	PR bootstrap/71510
	* expr.h (get_bit_range): Declare.
	* expr.c (get_bit_range): Export.
	* fold-const.c (optimize_bit_field_compare): Use get_bit_range and
	word_mode again to constrain the bitfield access.

From-SVN: r237426

gcc/ChangeLog

 2016-06-14  Richard Biener  <rguenther@suse.de>
 
+	PR middle-end/71310
+	PR bootstrap/71510
+	* expr.h (get_bit_range): Declare.
+	* expr.c (get_bit_range): Export.
+	* fold-const.c (optimize_bit_field_compare): Use get_bit_range and
+	word_mode again to constrain the bitfield access.
+
+2016-06-14  Richard Biener  <rguenther@suse.de>
+
 	PR tree-optimization/71521
 	* tree-vrp.c (extract_range_from_binary_expr_1): Guard
 	division int_const_binop against zero divisor.

gcc/expr.c

@@ -4795,7 +4795,7 @@ optimize_bitfield_assignment_op (unsigned HOST_WIDE_INT bitsize,
    If the access does not need to be restricted, 0 is returned in both
    *BITSTART and *BITEND.  */
 
-static void
+void
 get_bit_range (unsigned HOST_WIDE_INT *bitstart,
 	       unsigned HOST_WIDE_INT *bitend,
 	       tree exp,

gcc/expr.h

@@ -242,6 +242,10 @@ extern rtx push_block (rtx, int, int);
 extern bool emit_push_insn (rtx, machine_mode, tree, rtx, unsigned int,
 			    int, rtx, int, rtx, rtx, int, rtx, bool);
 
+/* Extract the accessible bit-range from a COMPONENT_REF.  */
+extern void get_bit_range (unsigned HOST_WIDE_INT *, unsigned HOST_WIDE_INT *,
+			   tree, HOST_WIDE_INT *, tree *);
+
 /* Expand an assignment that stores the value of FROM into TO.  */
 extern void expand_assignment (tree, tree, bool);
 

gcc/fold-const.c

@@ -3902,24 +3902,23 @@ optimize_bit_field_compare (location_t loc, enum tree_code code,
        return 0;
    }
 
-  /* Don't use a larger mode for reading the bit field than we will
-     use in other places accessing the bit field.  */
-  machine_mode largest_mode = word_mode;
+  /* Honor the C++ memory model and mimic what RTL expansion does.  */
+  unsigned HOST_WIDE_INT bitstart = 0;
+  unsigned HOST_WIDE_INT bitend = 0;
   if (TREE_CODE (lhs) == COMPONENT_REF)
     {
-      tree field = TREE_OPERAND (lhs, 1);
-      tree repr = DECL_BIT_FIELD_REPRESENTATIVE (field);
-      if (repr)
-	largest_mode = DECL_MODE (repr);
+      get_bit_range (&bitstart, &bitend, lhs, &lbitpos, &offset);
+      if (offset != NULL_TREE)
+	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, 0, 0,
+  nmode = get_best_mode (lbitsize, lbitpos, bitstart, bitend,
 			 const_p ? TYPE_ALIGN (TREE_TYPE (linner))
 			 : MIN (TYPE_ALIGN (TREE_TYPE (linner)),
 				TYPE_ALIGN (TREE_TYPE (rinner))),
-			 largest_mode, false);
+			 word_mode, false);
   if (nmode == VOIDmode)
     return 0;