likely and unlikely
Ivan Voras
ivoras at freebsd.org
Fri Mar 12 12:18:40 UTC 2010
While looking into branch prediction in gcc I found this post describing
the "likely" and "unlikely" macros in Linux:
http://kerneltrap.org/node/4705
In short, they are wrappers for the gcc-specific __builtin_expect for
static branch prediction.
Grepping around I see it is defined and used locally in several places:
amd64/include/xen/xen-os.h:#define likely(x) __builtin_expect((x),1)
amd64/include/xen/xen-os.h:#define unlikely(x) __builtin_expect((x),0)
dev/nxge/xge-osdep.h:#define xge_os_unlikely(x) (x)
gnu/fs/xfs/FreeBSD/xfs_compat.h:#define likely(x) __builtin_expect((x), 1)
gnu/fs/xfs/FreeBSD/xfs_compat.h:#define unlikely(x) __builtin_expect((x), 0)
i386/include/xen/xen-os.h:#define likely(x) __builtin_expect((x),1)
i386/include/xen/xen-os.h:#define unlikely(x) __builtin_expect((x),0)
sun4v/include/cpu.h:#define likely(x) __builtin_expect((x),1)
sun4v/include/cpu.h:#define unlikely(x) __builtin_expect((x),0)
Wouldn't it be more convenient to have a single global definition of
them, under #ifdef __GNUC__ for example in sys/stddef.h ?
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