contrib/timerwheel: probable bug on 32-bit, use __builtin_ffs()

Simply always defining BITS_PER_LONG as 64 seems like it's almost
certainly wrong on 32-bit platforms and could potentially result in
incorrect results.

fls and, e.g., __builtin_ffs() return the same answer for any given
input, making it seem like the name fls (find last set) is a misnomer
and ffs (find first set, starting from the lsb) is the more accurate
name.

Using __builtin_ffs() causes the compiler (in intel) to emit code
with the bsf (bit scan forward) insn, which is approx 3x faster than
the code in ffs(), at least on the machine I tried it on. (Even so,
it takes 10M+ iterations for the speed difference to be measurable.
Choosing the "faster" implementation seems like a no-brainer, even
if there may not be any significant gain by doing so.)

Change-Id: I1616dda1a5b76f208ba737a713877c1673131e33
Signed-off-by: Kaleb S. KEITHLEY <kkeithle@redhat.com>
Reviewed-on: https://review.gluster.org/17142
Smoke: Gluster Build System <jenkins@build.gluster.org>
NetBSD-regression: NetBSD Build System <jenkins@build.gluster.org>
CentOS-regression: Gluster Build System <jenkins@build.gluster.org>
Reviewed-by: Niels de Vos <ndevos@redhat.com>
Reviewed-by: Jeff Darcy <jeff@pl.atyp.us>

0 files changed, 0 insertions, 0 deletions