[PATCH 01/11] rt2x00: Calculate register offset during compile time

[Ivo van Doorn <ivdoorn@gmail.com>]

By using __ffs() the register offsets were always calculated
at run-time which all FIELD32/FIELD16 definitions were builtin
constants. This means we can heavily optimize the register handling
by allowing GCC to do all the work during compilation.

Add some compile_ffs() macros to perform the calculation at
compile time. After this each rt2x00 module size is reduced
by ~2500 bytes. And the stack size of several functions is reduced
as well which further limits the number of rt2x00 results in
'make checkstack'.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
---
 drivers/net/wireless/rt2x00/rt2x00reg.h |   55 +++++++++++++++++++++++--------
 1 files changed, 41 insertions(+), 14 deletions(-)

diff --git a/drivers/net/wireless/rt2x00/rt2x00reg.h b/drivers/net/wireless/rt2x00/rt2x00reg.h
index 3f255df..03e846f 100644
--- a/drivers/net/wireless/rt2x00/rt2x00reg.h
+++ b/drivers/net/wireless/rt2x00/rt2x00reg.h
@@ -130,40 +130,67 @@ struct rt2x00_field32 {
 
 /*
  * Power of two check, this will check
- * if the mask that has been given contains
- * and contiguous set of bits.
+ * if the mask that has been given contains and contiguous set of bits.
+ * Note that we cannot use the is_power_of_2() function since this
+ * check must be done at compile-time.
  */
 #define is_power_of_two(x)	( !((x) & ((x)-1)) )
 #define low_bit_mask(x)		( ((x)-1) & ~(x) )
 #define is_valid_mask(x)	is_power_of_two(1 + (x) + low_bit_mask(x))
 
+/*
+ * Macro's to find first set bit in a variable.
+ * These macro's behaves the same as the __ffs() function with
+ * the most important difference that this is done during
+ * compile-time rather then run-time.
+ */
+#define compile_ffs2(__x) \
+	((__x) & 0x1) ? 0 : 1
+
+#define compile_ffs4(__x) \
+	((__x) & 0x3) ? compile_ffs2(__x) : compile_ffs2(__x >> 2) + 2
+
+#define compile_ffs8(__x) \
+	((__x) & 0xf) ? compile_ffs4(__x) : compile_ffs4(__x >> 4) + 4
+
+#define compile_ffs16(__x) \
+	((__x) & 0xff) ? compile_ffs8(__x) : compile_ffs8(__x >> 8) + 8
+
+#define compile_ffs32(__x) \
+	((__x) & 0xffff) ? compile_ffs16(__x) : compile_ffs16(__x >> 16) + 16
+
+/*
+ * This macro will check the requirements for the FIELD{8,16,32} macros
+ * The mask should be a constant non-zero contiguous set of bits which
+ * does not exceed the given typelimit.
+ */
+#define FIELD_CHECK(__mask, __type)			\
+	BUILD_BUG_ON(!__builtin_constant_p(__mask) ||	\
+		     !(__mask) ||			\
+		     !is_valid_mask(__mask) ||		\
+		     (__mask) != (__type)(__mask))	\
+
 #define FIELD8(__mask)				\
 ({						\
-	BUILD_BUG_ON(!(__mask) ||		\
-		     !is_valid_mask(__mask) ||	\
-		     (__mask) != (u8)(__mask));	\
+	FIELD_CHECK(__mask, u8);		\
 	(struct rt2x00_field8) {		\
-		__ffs(__mask), (__mask)		\
+		compile_ffs8(__mask), (__mask)	\
 	};					\
 })
 
 #define FIELD16(__mask)				\
 ({						\
-	BUILD_BUG_ON(!(__mask) ||		\
-		     !is_valid_mask(__mask) ||	\
-		     (__mask) != (u16)(__mask));\
+	FIELD_CHECK(__mask, u16);		\
 	(struct rt2x00_field16) {		\
-		__ffs(__mask), (__mask)		\
+		compile_ffs16(__mask), (__mask)	\
 	};					\
 })
 
 #define FIELD32(__mask)				\
 ({						\
-	BUILD_BUG_ON(!(__mask) ||		\
-		     !is_valid_mask(__mask) ||	\
-		     (__mask) != (u32)(__mask));\
+	FIELD_CHECK(__mask, u32);		\
 	(struct rt2x00_field32) {		\
-		__ffs(__mask), (__mask)		\
+		compile_ffs32(__mask), (__mask)	\
 	};					\
 })
 
-- 
1.5.5.3