From: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx> Subject: 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. [mawilcox@xxxxxxxxxxxxx: v2] Link: http://lkml.kernel.org/r/1483709016-1834-1-git-send-email-mawilcox@xxxxxxxxxxxxxxxxx Link: http://lkml.kernel.org/r/1483709016-1834-1-git-send-email-mawilcox@xxxxxxxxxxxxxxxxx Signed-off-by: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx> Acked-by: Yury Norov <ynorov@xxxxxxxxxxxxxxxxxx> Acked-by: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- lib/find_bit.c | 4 ++-- tools/lib/find_bit.c | 2 +- 2 files changed, 3 insertions(+), 3 deletions(-) diff -puN lib/find_bit.c~find_bit-micro-optimise-find_next__bit lib/find_bit.c --- a/lib/find_bit.c~find_bit-micro-optimise-find_next__bit +++ a/lib/find_bit.c @@ -33,7 +33,7 @@ static unsigned long _find_next_bit(cons { 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(c { unsigned long tmp; - if (!nbits || start >= nbits) + if (unlikely(start >= nbits)) return nbits; tmp = addr[start / BITS_PER_LONG] ^ invert; diff -puN tools/lib/find_bit.c~find_bit-micro-optimise-find_next__bit tools/lib/find_bit.c --- a/tools/lib/find_bit.c~find_bit-micro-optimise-find_next__bit +++ a/tools/lib/find_bit.c @@ -34,7 +34,7 @@ static unsigned long _find_next_bit(cons { unsigned long tmp; - if (!nbits || start >= nbits) + if (unlikely(start >= nbits)) return nbits; tmp = addr[start / BITS_PER_LONG] ^ invert; _ -- To unsubscribe from this list: send the line "unsubscribe mm-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
