Messages in this thread Patch in this message | | | From | Sergey Senozhatsky <> | Subject | [PATCH] bsearch: micro-optimize pivot position calculation | Date | Thu, 8 Jun 2017 00:04:57 +0900 |
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There is a slightly faster way (in terms of the number of instructions being used) to calculate the position of a middle element, preserving integer overflow safeness.
./scripts/bloat-o-meter lib/bsearch.o.old lib/bsearch.o.new add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-24 (-24) function old new delta bsearch 122 98 -24
TEST
INT array of size 100001, elements [0..100000]. gcc 7.1, Os, x86_64.
a) bsearch() of existing key "100001 - 2":
BASE ====
$ perf stat ./a.out
Performance counter stats for './a.out':
619.445196 task-clock:u (msec) # 0.999 CPUs utilized 0 context-switches:u # 0.000 K/sec 0 cpu-migrations:u # 0.000 K/sec 133 page-faults:u # 0.215 K/sec 1,949,517,279 cycles:u # 3.147 GHz (83.06%) 181,017,938 stalled-cycles-frontend:u # 9.29% frontend cycles idle (83.05%) 82,959,265 stalled-cycles-backend:u # 4.26% backend cycles idle (67.02%) 4,355,706,383 instructions:u # 2.23 insn per cycle # 0.04 stalled cycles per insn (83.54%) 1,051,539,242 branches:u # 1697.550 M/sec (83.54%) 15,263,381 branch-misses:u # 1.45% of all branches (83.43%)
0.620082548 seconds time elapsed
PATCHED =======
$ perf stat ./a.out
Performance counter stats for './a.out':
475.097316 task-clock:u (msec) # 0.999 CPUs utilized 0 context-switches:u # 0.000 K/sec 0 cpu-migrations:u # 0.000 K/sec 135 page-faults:u # 0.284 K/sec 1,487,467,717 cycles:u # 3.131 GHz (82.95%) 186,537,162 stalled-cycles-frontend:u # 12.54% frontend cycles idle (82.93%) 28,797,869 stalled-cycles-backend:u # 1.94% backend cycles idle (67.10%) 3,807,564,203 instructions:u # 2.56 insn per cycle # 0.05 stalled cycles per insn (83.57%) 1,049,344,291 branches:u # 2208.693 M/sec (83.60%) 5,485 branch-misses:u # 0.00% of all branches (83.58%)
0.475760235 seconds time elapsed
b) bsearch() of un-existing key "100001 + 2":
BASE ====
$ perf stat ./a.out
Performance counter stats for './a.out':
499.244480 task-clock:u (msec) # 0.999 CPUs utilized 0 context-switches:u # 0.000 K/sec 0 cpu-migrations:u # 0.000 K/sec 132 page-faults:u # 0.264 K/sec 1,571,194,855 cycles:u # 3.147 GHz (83.18%) 13,450,980 stalled-cycles-frontend:u # 0.86% frontend cycles idle (83.18%) 21,256,072 stalled-cycles-backend:u # 1.35% backend cycles idle (66.78%) 4,171,197,909 instructions:u # 2.65 insn per cycle # 0.01 stalled cycles per insn (83.68%) 1,009,175,281 branches:u # 2021.405 M/sec (83.79%) 3,122 branch-misses:u # 0.00% of all branches (83.37%)
0.499871144 seconds time elapsed
PATCHED =======
$ perf stat ./a.out
Performance counter stats for './a.out':
399.023499 task-clock:u (msec) # 0.998 CPUs utilized 0 context-switches:u # 0.000 K/sec 0 cpu-migrations:u # 0.000 K/sec 134 page-faults:u # 0.336 K/sec 1,245,793,991 cycles:u # 3.122 GHz (83.39%) 11,529,273 stalled-cycles-frontend:u # 0.93% frontend cycles idle (83.46%) 12,116,311 stalled-cycles-backend:u # 0.97% backend cycles idle (66.92%) 3,679,710,005 instructions:u # 2.95 insn per cycle # 0.00 stalled cycles per insn (83.47%) 1,009,792,625 branches:u # 2530.660 M/sec (83.46%) 2,590 branch-misses:u # 0.00% of all branches (83.12%)
0.399733539 seconds time elapsed
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com> --- lib/bsearch.c | 22 ++++++++++++---------- 1 file changed, 12 insertions(+), 10 deletions(-)
diff --git a/lib/bsearch.c b/lib/bsearch.c index e33c179089db..18b445b010c3 100644 --- a/lib/bsearch.c +++ b/lib/bsearch.c @@ -33,19 +33,21 @@ void *bsearch(const void *key, const void *base, size_t num, size_t size, int (*cmp)(const void *key, const void *elt)) { - size_t start = 0, end = num; + const char *pivot; int result; - while (start < end) { - size_t mid = start + (end - start) / 2; + while (num > 0) { + pivot = base + (num >> 1) * size; + result = cmp(key, pivot); - result = cmp(key, base + mid * size); - if (result < 0) - end = mid; - else if (result > 0) - start = mid + 1; - else - return (void *)base + mid * size; + if (result == 0) + return (void *)pivot; + + if (result > 0) { + base = pivot + size; + num--; + } + num >>= 1; } return NULL; -- 2.13.1
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