diff options
| author | Matthew Wilcox <[email protected]> | 2017-02-24 15:00:58 -0800 |
|---|---|---|
| committer | Linus Torvalds <[email protected]> | 2017-02-24 17:46:57 -0800 |
| commit | e4afd2e5567fc5d59988025f7528f9b4794d86a5 (patch) | |
| tree | d00cf7c2409d3fdeb4dbb81ec04d6cb9c0e7965d | |
| parent | 55ded9551f9a64f2872df77a954d4c30f8958e82 (diff) | |
lib/find_bit.c: micro-optimise find_next_*_bit
This saves 32 bytes on my x86-64 build, mostly due to alignment
considerations and sharing more code between find_next_bit and
find_next_zero_bit, but it does save a couple of instructions.
There's really two parts to this commit:
- First, the first half of the test: (!nbits || start >= nbits) is
trivially a subset of the second half, since nbits and start are both
unsigned
- Second, while looking at the disassembly, I noticed that GCC was
predicting the branch taken. Since this is a failure case, it's
clearly the less likely of the two branches, so add an unlikely() to
override GCC's heuristics.
[[email protected]: v2]
Link: http://lkml.kernel.org/r/[email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Matthew Wilcox <[email protected]>
Acked-by: Yury Norov <[email protected]>
Acked-by: Rasmus Villemoes <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
| -rw-r--r-- | lib/find_bit.c | 4 | ||||
| -rw-r--r-- | tools/lib/find_bit.c | 2 |
2 files changed, 3 insertions, 3 deletions
diff --git a/lib/find_bit.c b/lib/find_bit.c index 18072ea9c20e..6ed74f78380c 100644 --- a/lib/find_bit.c +++ b/lib/find_bit.c @@ -33,7 +33,7 @@ static unsigned long _find_next_bit(const unsigned long *addr, { unsigned long tmp; - if (!nbits || start >= nbits) + if (unlikely(start >= nbits)) return nbits; tmp = addr[start / BITS_PER_LONG] ^ invert; @@ -151,7 +151,7 @@ static unsigned long _find_next_bit_le(const unsigned long *addr, { unsigned long tmp; - if (!nbits || start >= nbits) + if (unlikely(start >= nbits)) return nbits; tmp = addr[start / BITS_PER_LONG] ^ invert; diff --git a/tools/lib/find_bit.c b/tools/lib/find_bit.c index 6d8b8f22cf55..42c15f906aac 100644 --- a/tools/lib/find_bit.c +++ b/tools/lib/find_bit.c @@ -34,7 +34,7 @@ static unsigned long _find_next_bit(const unsigned long *addr, { unsigned long tmp; - if (!nbits || start >= nbits) + if (unlikely(start >= nbits)) return nbits; tmp = addr[start / BITS_PER_LONG] ^ invert; |