Tim Peters wrote:
> if (normal)
> do normal stuff
> else do exceptional stuff
>
> Most dumb compilers on platforms that care use a "forward branches probably
> aren't taken, backward branches probably are" heuristic for setting
> branch-prediction hints in the machine code; and on platforms that don't
> care it's usually faster to fall through than to change the program counter
> anyway.
I seem to remember someone saying that GCC generated better code for:
if (exceptional) {
do exceptional things
break / return / goto
}
do normal things
Is GCC in the dumb category? Also, the Linux is starting to use this
set of macros more often:
/* Somewhere in the middle of the GCC 2.96 development cycle, we
* implemented a mechanism by which the user can annotate likely
* branch directions and expect the blocks to be reordered
* appropriately. Define __builtin_expect to nothing for earlier
* compilers. */
#if __GNUC__ == 2 && __GNUC_MINOR__ < 96
#define __builtin_expect(x, expected_value) (x)
#endif
#define likely(x) __builtin_expect((x),1)
#define unlikely(x) __builtin_expect((x),0)
For example:
if (likely(normal))
do normal stuff
else do exceptional stuff
I don't have GCC >= 2.96 otherwise I would have tried sprinkling some of
those macros in ceval and testing the effect.
Neil
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