The other day on kernelnewbies, the able Robert Day wondered whether or not anyone had quantified the effects of the likely() and unlikely() macros scattered all over the Linux kernel.
He received no less than 5 replies telling him how the macros worked. (If you are curious, this is the best explanation) - but not a single piece of measurement or other characterization.
LXR shows over 3500 uses of those macros in the kernel, and nobody has any data for any scenario? Wowzers.
Doing before/after benchmarks with those macros changed to nops would be an interesting project. Could use the usual suspects of linux kernel performance enhancements to test (lmbench, compile test, some kind of network load generator, etc..)
Comments with pointers to data would be cool.