Re: [PATCH 01/11 v3] rt2x00: Calculate register offset during compile time

[Harvey Harrison <harvey.harrison@gmail.com>]

On Tue, 2008-06-03 at 22:45 +0200, Ivo van Doorn wrote:
> 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.
>=20
> 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'.
> +/*
> + * 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 const_ffs8(__x) ( \
	BUILD_BUG_ON(!__builtin_constant_p(__x));	\
	__builtin_choose_expr((__x) & 0x01, 0,		\
	=EF=BB=BF__builtin_choose_expr((__x) & 0x02, 1,		\
	=EF=BB=BF__builtin_choose_expr((__x) & 0x04, 2,		\
	=EF=BB=BF__builtin_choose_expr((__x) & 0x08, 3,		\
	=EF=BB=BF__builtin_choose_expr((__x) & 0x10, 4,		\
	=EF=BB=BF__builtin_choose_expr((__x) & 0x20, 5,		\
	=EF=BB=BF__builtin_choose_expr((__x) & 0x40, 6,		\
	=EF=BB=BF__builtin_choose_expr((__x) & 0x80, 7,		\
	8)))))))); )

#define const_ffs16(__x) ( \
	__builtin_choose_expr((__x) & 0xff,	\
 	const_ffs8(__x),			\
	const_ffs8((__x) >> 8) + 8); )

=EF=BB=BF
#define const_ffs32(__x) ( \
	__builtin_choose_expr((__x) & 0xffff,	\
	const_ffs16(__x),			\
	const_ffs16((__x) >> 16) + 16); )


Just a thought.

Harvey

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