andy/glusterfs
1
0
Fork 0
Code Issues Pull Requests Releases Wiki Activity

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

Browse Source 
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>
This commit is contained in:
Kaleb S. KEITHLEY 2017-04-30 18:59:17 -04:00 committed by Jeff Darcy
parent 73fcf3a874
commit c92b8347ae
1 changed files with 12 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

17
contrib/timer-wheel/find_last_bit.c
View File

@ -15,15 +15,22 @@
*/
/**
* @find_last_bit
* optimized implementation of find last bit in
* @find_first_bit
* optimized implementation of find first bit in
*/
#ifndef BITS_PER_LONG
#ifdef __LP64__
#define BITS_PER_LONG 64
#else
#define BITS_PER_LONG 32
#endif
#endif
static inline int fls(int x)
#if defined(__GNUC__) || defined(__clang__)
#define ffs(p) __builtin_ffs(p)
#else
static inline int ffs(int x)
{
int r = 32;
@ -51,7 +58,7 @@ static inline int fls(int x)
}
return r;
}
#endif
unsigned long gf_tw_find_last_bit(const unsigned long *addr, unsigned long size)
{
@ -73,7 +80,7 @@ unsigned long gf_tw_find_last_bit(const unsigned long *addr, unsigned long size)
tmp = addr[--words];
if (tmp) {
found:
return words * BITS_PER_LONG + fls(tmp);
return words * BITS_PER_LONG + ffs(tmp);
}
}