Robustify REAL_MODE_FORMAT

Make sure that REAL_MODE_FORMAT aborts if it is passed an invalid mode,
rather than stepping beyond the bounds of an array.  It turned out that
some code was passing non-float modes to the real.h routines.

Tested on x86_64-linux-gnu, arm-linux-gnueabi and aarch64-linux-gnu.

gcc/
	* real.h (REAL_MODE_FORMAT): Abort if the mode isn't a
	SCALAR_FLOAT_MODE_P.

From-SVN: r229579

gcc/ChangeLog

+2015-10-30  Richard Sandiford  <richard.sandiford@arm.com>
+
+	* real.h (REAL_MODE_FORMAT): Abort if the mode isn't a
+	SCALAR_FLOAT_MODE_P.
+
 2015-10-30  Alan Lawrence  <alan.lawrence@arm.com>
 
 	* tree-sra.c (scalarizable_type_p): Comment variable-length arrays.

gcc/real.h

@@ -167,7 +167,9 @@ extern const struct real_format *
   (real_format_for_mode[DECIMAL_FLOAT_MODE_P (MODE)			\
 			? (((MODE) - MIN_MODE_DECIMAL_FLOAT)		\
 			   + (MAX_MODE_FLOAT - MIN_MODE_FLOAT + 1))	\
-			: ((MODE) - MIN_MODE_FLOAT)])
+			: GET_MODE_CLASS (MODE) == MODE_FLOAT		\
+			? ((MODE) - MIN_MODE_FLOAT)			\
+			: (gcc_unreachable (), 0)])
 
 #define FLOAT_MODE_FORMAT(MODE) \
   (REAL_MODE_FORMAT (SCALAR_FLOAT_MODE_P (MODE)? (MODE) \