When should we use likely() / unlikely() / get_unaligned() ?

[David Woodhouse]

There seems to be no coherent answer to the above questions. On some
architectures likely() might bypass dynamic branch prediction, so we
shouldn't use it unless there's at _least_ a 95% probability; on others
it may simply affect code ordering and we gain a tiny benefit from it if
the probabilities aren't precisely 50/50.

Likewise for using get_unaligned() to inline the alignment fixups --
what is the ratio between the costs of inlining the fixup and of
potentially taking the exception? If the pointer is expected to be
unaligned 25% of the time, should we use get_unaligned? What if it's
50%? 75%?

These are all very arch-specific, and sometimes compiler-specific. The
likely()/unlikely()/get_unaligned() functions as they currently stand
make little sense.

I think we need to include a probability, in order for use of these
functions in _generic_ code to make any sense. So we replace
likely(condition) with probable(condition, percentage), and likewise
with get_unaligned()...


#define ARCH_PROBABILITY_HIGH 75    // These actually defined by arch code
#define ARCH_PROBABILITY_LOW 25
#define ARCH_ALIGNMENT_COST_THRESHOLD 50

#define probable(condition, pc)					\
	(__builtin_constant_p(pc)?				\
	 (((pc) > ARCH_PROBABILITY_HIGH)?			\
	      __builtin_expect((condition),1):			\
	      (((pc) < ARCH_PROBABILITY_LOW)?			\
	           __builtin_expect((condition),0):		\
		  (condition)))					\
	 :(condition))


#define get_unaligned_p(ptr, pc)						\
	((__builtin_constant_p(pc)&&(pc) < ARCH_ALIGNMENT_COST_THRESHOLD)?	\
	 (*(ptr)):get_unaligned(ptr))


In fact some uClinux architectures _cannot_ fix up alignment, and would
set ARCH_ALIGNMENT_COST_THRESHOLD to zero. 

-- 
dwmw2

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