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authorSergey Senozhatsky <sergey.senozhatsky@gmail.com>2017-07-10 15:52:01 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2017-07-10 16:32:35 -0700
commit166a0f780a8fdb67f232c85e9905ba84f7247da9 (patch)
tree110c049bfc9be26c3515a7af4f12c0a5b40194e1
parenta94c33dd1f677d16c4f1a162b4b3e9eba1b07c24 (diff)
lib/bsearch.c: micro-optimize pivot position calculation
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 Link: http://lkml.kernel.org/r/20170607150457.5905-1-sergey.senozhatsky@gmail.com Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com> Cc: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r--lib/bsearch.c22
1 files 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;