This patch fixes a bug in the bswap pass.

This patch fixes a bug in the bswap pass.  In big-endian BIT_FIELD_REF uses
big-endian bit numbering so we need to adjust the bit position.
The existing version could potentially generate incorrect code however GCC
doesn't emit a BIT_FIELD_REF to access the low byte in a register, so the
symbolic number never matches in big-endian.

    gcc/
	* tree-ssa-math-opts.c (find_bswap_or_nop_1): Adjust bitnumbering
	for big-endian BIT_FIELD_REF.

From-SVN: r237822

gcc/ChangeLog

+2016-06-28  Wilco Dijkstra  <wdijkstr@arm.com>
+
+	* tree-ssa-math-opts.c (find_bswap_or_nop_1): Adjust bitnumbering
+	for big-endian BIT_FIELD_REF.
+
 2016-06-28  Pat Haugen  <pthaugen@us.ibm.com>
 
 	* config/rs6000/rs6000.md ('type' attribute): Add htmsimple/dfp types.

gcc/tree-ssa-math-opts.c

@@ -2307,6 +2307,10 @@ find_bswap_or_nop_1 (gimple *stmt, struct symbolic_number *n, int limit)
 	  && bitsize % BITS_PER_UNIT == 0
 	  && init_symbolic_number (n, TREE_OPERAND (rhs1, 0)))
 	{
+	  /* Handle big-endian bit numbering in BIT_FIELD_REF.  */
+	  if (BYTES_BIG_ENDIAN)
+	    bitpos = TYPE_PRECISION (n->type) - bitpos - bitsize;
+
 	  /* Shift.  */
 	  if (!do_shift_rotate (RSHIFT_EXPR, n, bitpos))
 	    return NULL;