Написал на коленке немножко кода для замеров. Меряет в тиках процессора (ну, во всяком случае гугл так говорит).
Методика: берем switch и else if каждого по сотне проверок. Замеряем время отработки цикла из 100000 итераций отдельно для if else, отдельно для switch. И это все замеряем по каждому числу, тоесть обходим каждую ветку ветвления. Выводим результаты по каждому ветвлению и усредненные в конце.
Ну, по коду думается мне будет понятнее...
Для не желающих листать эту простыню — вывод: собирайте с оптимизацией — в этом случае абсолютно без разницы что использовать
Код:
gate ~ # cat ./switch-if.cpp
#include <iostream>
#include <stdlib.h>
using namespace std;
#include <stdint.h>
static inline unsigned long long tick()
{
unsigned long long d;
__asm__ __volatile__ ("rdtsc" : "=A" (d) );
return d;
}
#define SOME return
#define ELSIF(_x) else if (num == _x) { SOME; }
void test_if(int num)
{
if (num == 0) { SOME; }
ELSIF(1) ELSIF(2) ELSIF(3) ELSIF(4) ELSIF(5) ELSIF(6) ELSIF(7) ELSIF(8) ELSIF(9) ELSIF(10)
ELSIF(11) ELSIF(12) ELSIF(13) ELSIF(14) ELSIF(15) ELSIF(16) ELSIF(17) ELSIF(18) ELSIF(19) ELSIF(20)
ELSIF(21) ELSIF(22) ELSIF(23) ELSIF(24) ELSIF(25) ELSIF(26) ELSIF(27) ELSIF(28) ELSIF(29) ELSIF(30)
ELSIF(31) ELSIF(32) ELSIF(33) ELSIF(34) ELSIF(35) ELSIF(36) ELSIF(37) ELSIF(38) ELSIF(39) ELSIF(40)
ELSIF(41) ELSIF(42) ELSIF(43) ELSIF(44) ELSIF(45) ELSIF(46) ELSIF(47) ELSIF(48) ELSIF(49) ELSIF(50)
ELSIF(51) ELSIF(52) ELSIF(53) ELSIF(54) ELSIF(55) ELSIF(56) ELSIF(57) ELSIF(58) ELSIF(59) ELSIF(60)
ELSIF(61) ELSIF(62) ELSIF(63) ELSIF(64) ELSIF(65) ELSIF(66) ELSIF(67) ELSIF(68) ELSIF(69) ELSIF(70)
ELSIF(71) ELSIF(72) ELSIF(73) ELSIF(74) ELSIF(75) ELSIF(76) ELSIF(77) ELSIF(78) ELSIF(79) ELSIF(80)
ELSIF(81) ELSIF(82) ELSIF(83) ELSIF(84) ELSIF(85) ELSIF(86) ELSIF(87) ELSIF(88) ELSIF(89) ELSIF(90)
ELSIF(91) ELSIF(92) ELSIF(93) ELSIF(94) ELSIF(95) ELSIF(96) ELSIF(97) ELSIF(98) ELSIF(99)
}
#define SCASE(_x) case _x: { SOME; } break;
void test_switch(int num)
{
switch (num)
{
SCASE(0)
SCASE(1) SCASE(2) SCASE(3) SCASE(4) SCASE(5) SCASE(6) SCASE(7) SCASE(8) SCASE(9) SCASE(10)
SCASE(11) SCASE(12) SCASE(13) SCASE(14) SCASE(15) SCASE(16) SCASE(17) SCASE(18) SCASE(19) SCASE(20)
SCASE(21) SCASE(22) SCASE(23) SCASE(24) SCASE(25) SCASE(26) SCASE(27) SCASE(28) SCASE(29) SCASE(30)
SCASE(31) SCASE(32) SCASE(33) SCASE(34) SCASE(35) SCASE(36) SCASE(37) SCASE(38) SCASE(39) SCASE(40)
SCASE(41) SCASE(42) SCASE(43) SCASE(44) SCASE(45) SCASE(46) SCASE(47) SCASE(48) SCASE(49) SCASE(50)
SCASE(51) SCASE(52) SCASE(53) SCASE(54) SCASE(55) SCASE(56) SCASE(57) SCASE(58) SCASE(59) SCASE(60)
SCASE(61) SCASE(62) SCASE(63) SCASE(64) SCASE(65) SCASE(66) SCASE(67) SCASE(68) SCASE(69) SCASE(70)
SCASE(71) SCASE(72) SCASE(73) SCASE(74) SCASE(75) SCASE(76) SCASE(77) SCASE(78) SCASE(79) SCASE(80)
SCASE(81) SCASE(82) SCASE(83) SCASE(84) SCASE(85) SCASE(86) SCASE(87) SCASE(88) SCASE(89) SCASE(90)
SCASE(91) SCASE(92) SCASE(93) SCASE(94) SCASE(95) SCASE(96) SCASE(97) SCASE(98) SCASE(99)
}
}
struct STicksData
{
unsigned long long ticks_start, ticks_if, ticks_case;
};
#define CYCLE for (unsigned int i = 0; i < 100000; i++)
STicksData test(int num)
{
STicksData data;
unsigned long long ticks_start = 0, ticks_if = 0, ticks_case = 0;
data.ticks_start = tick();
CYCLE { test_if (num); }
data.ticks_if = tick();
CYCLE { test_switch(num); }
data.ticks_case = tick();
cout << "Number:" << num << endl;
cout << "if: " << (data.ticks_if - data.ticks_start) << endl;
cout << "case: " << (data.ticks_case - data.ticks_if ) << endl;
// cout << "Total: " << (data.ticks_case - data.ticks_start) << endl;
cout << "-------------------------------------" << endl;
return data;
}
// -----------------------------------------------------------------------------------------------------------------------------
#define TESTS_COUNT 100
int main ()
{
STicksData allTicks[TESTS_COUNT];
for (int i = 0; i<TESTS_COUNT; i++)
{
allTicks[i] = test(i);
}
unsigned long long average_if = 0, average_switch = 0, average_total = 0;
for (int i = 0; i<TESTS_COUNT; i++)
{
average_if += (allTicks[i].ticks_if - allTicks[i].ticks_start);
average_switch += (allTicks[i].ticks_case - allTicks[i].ticks_if );
// average_total += (allTicks[i].ticks_case - allTicks[i].ticks_start);
}
cout << "Average ticks" << endl;
cout << "if: " << (average_if /TESTS_COUNT) << endl;
cout << "case: " << (average_switch/TESTS_COUNT) << endl;
// cout << "Total: " << (average_total /TESTS_COUNT) << endl;
return 0;
}
В результатах конечно простыня еще та...
Результаты неоптимизированной сборки:
gate ~ # g++ switch-if.cpp -o si; ./si
Number:0
if: 2434620
case: 2642925
-------------------------------------
Number:1
if: 2088030
case: 2672880
-------------------------------------
Number:2
if: 2221920
case: 2781540
-------------------------------------
Number:3
if: 2421030
case: 2737995
-------------------------------------
Number:4
if: 2621880
case: 2812230
-------------------------------------
Number:5
if: 2822580
case: 2724525
-------------------------------------
Number:6
if: 3030675
case: 2719425
-------------------------------------
Number:7
if: 3156915
case: 2714835
-------------------------------------
Number:8
if: 3438315
case: 2716065
-------------------------------------
Number:9
if: 3638130
case: 2713380
-------------------------------------
Number:10
if: 3832155
case: 2716230
-------------------------------------
Number:11
if: 4246065
case: 2628285
-------------------------------------
Number:12
if: 4452555
case: 2628375
-------------------------------------
Number:13
if: 4457835
case: 2716320
-------------------------------------
Number:14
if: 4687230
case: 2627640
-------------------------------------
Number:15
if: 5183325
case: 2627520
-------------------------------------
Number:16
if: 5231145
case: 2625900
-------------------------------------
Number:17
if: 5642625
case: 2600220
-------------------------------------
Number:18
if: 5947050
case: 2600235
-------------------------------------
Number:19
if: 6234570
case: 2600235
-------------------------------------
Number:20
if: 6528075
case: 2625795
-------------------------------------
Number:21
if: 6482460
case: 2625150
-------------------------------------
Number:22
if: 6846480
case: 2632905
-------------------------------------
Number:23
if: 6941340
case: 2626965
-------------------------------------
Number:24
if: 7362780
case: 2625120
-------------------------------------
Number:25
if: 7341525
case: 2624685
-------------------------------------
Number:26
if: 7762650
case: 2625045
-------------------------------------
Number:27
if: 7940040
case: 2623950
-------------------------------------
Number:28
if: 8163450
case: 2624775
-------------------------------------
Number:29
if: 8169825
case: 2624970
-------------------------------------
Number:30
if: 8566215
case: 2600235
-------------------------------------
Number:31
if: 8739090
case: 2624505
-------------------------------------
Number:32
if: 8981820
case: 2629395
-------------------------------------
Number:33
if: 9057780
case: 2634000
-------------------------------------
Number:34
if: 9332310
case: 2601630
-------------------------------------
Number:35
if: 9412215
case: 2600235
-------------------------------------
Number:36
if: 9776640
case: 2624415
-------------------------------------
Number:37
if: 9779775
case: 2625495
-------------------------------------
Number:38
if: 10179045
case: 2623605
-------------------------------------
Number:39
if: 10201335
case: 2627055
-------------------------------------
Number:40
if: 10572450
case: 2626125
-------------------------------------
Number:41
if: 10625220
case: 2624205
-------------------------------------
Number:42
if: 10915875
case: 2625105
-------------------------------------
Number:43
if: 11056425
case: 2627700
-------------------------------------
Number:44
if: 11360070
case: 2625075
-------------------------------------
Number:45
if: 11415120
case: 2626635
-------------------------------------
Number:46
if: 11805510
case: 2649015
-------------------------------------
Number:47
if: 11999310
case: 2627490
-------------------------------------
Number:48
if: 12197250
case: 2626650
-------------------------------------
Number:49
if: 12147510
case: 2628675
-------------------------------------
Number:50
if: 12508350
case: 2625495
-------------------------------------
Number:51
if: 12546900
case: 2626950
-------------------------------------
Number:52
if: 12904680
case: 2624940
-------------------------------------
Number:53
if: 13033860
case: 2600235
-------------------------------------
Number:54
if: 13378335
case: 2627565
-------------------------------------
Number:55
if: 13298010
case: 2626185
-------------------------------------
Number:56
if: 13750980
case: 2625945
-------------------------------------
Number:57
if: 13661265
case: 2625705
-------------------------------------
Number:58
if: 14171475
case: 2626275
-------------------------------------
Number:59
if: 14284815
case: 2625645
-------------------------------------
Number:60
if: 14571810
case: 2627130
-------------------------------------
Number:61
if: 14485335
case: 2600235
-------------------------------------
Number:62
if: 14965695
case: 2623515
-------------------------------------
Number:63
if: 15233610
case: 2600235
-------------------------------------
Number:64
if: 15269625
case: 2600235
-------------------------------------
Number:65
if: 23596770
case: 2625780
-------------------------------------
Number:66
if: 15780885
case: 2626020
-------------------------------------
Number:67
if: 15831420
case: 2624625
-------------------------------------
Number:68
if: 16075050
case: 2625225
-------------------------------------
Number:69
if: 16204530
case: 2624565
-------------------------------------
Number:70
if: 16496790
case: 2625240
-------------------------------------
Number:71
if: 16668495
case: 2623065
-------------------------------------
Number:72
if: 16877895
case: 2625285
-------------------------------------
Number:73
if: 17105505
case: 2630115
-------------------------------------
Number:74
if: 17321640
case: 2624715
-------------------------------------
Number:75
if: 17563830
case: 2622690
-------------------------------------
Number:76
if: 17705850
case: 2623290
-------------------------------------
Number:77
if: 17793165
case: 2625090
-------------------------------------
Number:78
if: 18034365
case: 2625600
-------------------------------------
Number:79
if: 18388860
case: 2624595
-------------------------------------
Number:80
if: 18570120
case: 2601345
-------------------------------------
Number:81
if: 18635235
case: 2601525
-------------------------------------
Number:82
if: 18882225
case: 2625735
-------------------------------------
Number:83
if: 19041840
case: 2631060
-------------------------------------
Number:84
if: 19344630
case: 2625105
-------------------------------------
Number:85
if: 19434015
case: 2626845
-------------------------------------
Number:86
if: 19756770
case: 2625960
-------------------------------------
Number:87
if: 19816695
case: 2627760
-------------------------------------
Number:88
if: 20121810
case: 2628255
-------------------------------------
Number:89
if: 20227305
case: 2627610
-------------------------------------
Number:90
if: 20515200
case: 2623455
-------------------------------------
Number:91
if: 22120935
case: 2629860
-------------------------------------
Number:92
if: 20968785
case: 2625945
-------------------------------------
Number:93
if: 21100155
case: 2625705
-------------------------------------
Number:94
if: 21419655
case: 2624250
-------------------------------------
Number:95
if: 21858960
case: 2624640
-------------------------------------
Number:96
if: 21814755
case: 2623080
-------------------------------------
Number:97
if: 21769530
case: 2624640
-------------------------------------
Number:98
if: 22225110
case: 2625105
-------------------------------------
Number:99
if: 22131165
case: 2625195
-------------------------------------
Average ticks
if: 12387129
case: 2634786
Как видите — вариант с if практически сразу потерял свои позиции и под конец разница с switch на порядок.
А вот результаты сборки с оптимизацией мгновенно ставят все на свои места
gate ~ # g++ switch-if.cpp -O2 -o si; ./si
Number:0
if: 105
case: 90
-------------------------------------
Number:1
if: 105
case: 105
-------------------------------------
Number:2
if: 120
case: 105
-------------------------------------
Number:3
if: 120
case: 105
-------------------------------------
Number:4
if: 105
case: 105
-------------------------------------
Number:5
if: 120
case: 105
-------------------------------------
Number:6
if: 105
case: 105
-------------------------------------
Number:7
if: 105
case: 105
-------------------------------------
Number:8
if: 105
case: 105
-------------------------------------
Number:9
if: 105
case: 105
-------------------------------------
Number:10
if: 105
case: 105
-------------------------------------
Number:11
if: 105
case: 90
-------------------------------------
Number:12
if: 120
case: 105
-------------------------------------
Number:13
if: 105
case: 120
-------------------------------------
Number:14
if: 105
case: 105
-------------------------------------
Number:15
if: 105
case: 105
-------------------------------------
Number:16
if: 120
case: 105
-------------------------------------
Number:17
if: 105
case: 105
-------------------------------------
Number:18
if: 105
case: 105
-------------------------------------
Number:19
if: 105
case: 120
-------------------------------------
Number:20
if: 105
case: 90
-------------------------------------
Number:21
if: 105
case: 120
-------------------------------------
Number:22
if: 90
case: 120
-------------------------------------
Number:23
if: 105
case: 105
-------------------------------------
Number:24
if: 105
case: 105
-------------------------------------
Number:25
if: 120
case: 105
-------------------------------------
Number:26
if: 105
case: 120
-------------------------------------
Number:27
if: 90
case: 105
-------------------------------------
Number:28
if: 105
case: 105
-------------------------------------
Number:29
if: 120
case: 120
-------------------------------------
Number:30
if: 105
case: 120
-------------------------------------
Number:31
if: 105
case: 120
-------------------------------------
Number:32
if: 105
case: 120
-------------------------------------
Number:33
if: 105
case: 105
-------------------------------------
Number:34
if: 105
case: 105
-------------------------------------
Number:35
if: 105
case: 105
-------------------------------------
Number:36
if: 105
case: 105
-------------------------------------
Number:37
if: 105
case: 105
-------------------------------------
Number:38
if: 105
case: 105
-------------------------------------
Number:39
if: 105
case: 105
-------------------------------------
Number:40
if: 105
case: 105
-------------------------------------
Number:41
if: 105
case: 105
-------------------------------------
Number:42
if: 105
case: 105
-------------------------------------
Number:43
if: 105
case: 105
-------------------------------------
Number:44
if: 105
case: 105
-------------------------------------
Number:45
if: 105
case: 90
-------------------------------------
Number:46
if: 105
case: 105
-------------------------------------
Number:47
if: 120
case: 90
-------------------------------------
Number:48
if: 105
case: 105
-------------------------------------
Number:49
if: 105
case: 105
-------------------------------------
Number:50
if: 105
case: 105
-------------------------------------
Number:51
if: 105
case: 120
-------------------------------------
Number:52
if: 120
case: 105
-------------------------------------
Number:53
if: 105
case: 105
-------------------------------------
Number:54
if: 105
case: 105
-------------------------------------
Number:55
if: 120
case: 105
-------------------------------------
Number:56
if: 105
case: 105
-------------------------------------
Number:57
if: 105
case: 105
-------------------------------------
Number:58
if: 120
case: 105
-------------------------------------
Number:59
if: 105
case: 105
-------------------------------------
Number:60
if: 105
case: 105
-------------------------------------
Number:61
if: 105
case: 105
-------------------------------------
Number:62
if: 120
case: 105
-------------------------------------
Number:63
if: 105
case: 120
-------------------------------------
Number:64
if: 120
case: 105
-------------------------------------
Number:65
if: 105
case: 105
-------------------------------------
Number:66
if: 105
case: 105
-------------------------------------
Number:67
if: 105
case: 120
-------------------------------------
Number:68
if: 105
case: 105
-------------------------------------
Number:69
if: 120
case: 90
-------------------------------------
Number:70
if: 105
case: 105
-------------------------------------
Number:71
if: 105
case: 105
-------------------------------------
Number:72
if: 105
case: 105
-------------------------------------
Number:73
if: 105
case: 120
-------------------------------------
Number:74
if: 120
case: 105
-------------------------------------
Number:75
if: 105
case: 120
-------------------------------------
Number:76
if: 90
case: 105
-------------------------------------
Number:77
if: 105
case: 105
-------------------------------------
Number:78
if: 105
case: 105
-------------------------------------
Number:79
if: 105
case: 105
-------------------------------------
Number:80
if: 120
case: 105
-------------------------------------
Number:81
if: 105
case: 105
-------------------------------------
Number:82
if: 120
case: 105
-------------------------------------
Number:83
if: 105
case: 105
-------------------------------------
Number:84
if: 120
case: 90
-------------------------------------
Number:85
if: 105
case: 105
-------------------------------------
Number:86
if: 105
case: 120
-------------------------------------
Number:87
if: 120
case: 90
-------------------------------------
Number:88
if: 105
case: 105
-------------------------------------
Number:89
if: 120
case: 105
-------------------------------------
Number:90
if: 105
case: 120
-------------------------------------
Number:91
if: 120
case: 105
-------------------------------------
Number:92
if: 120
case: 90
-------------------------------------
Number:93
if: 105
case: 120
-------------------------------------
Number:94
if: 105
case: 105
-------------------------------------
Number:95
if: 105
case: 105
-------------------------------------
Number:96
if: 105
case: 105
-------------------------------------
Number:97
if: 105
case: 105
-------------------------------------
Number:98
if: 105
case: 105
-------------------------------------
Number:99
if: 105
case: 120
-------------------------------------
Average ticks
if: 107
case: 106
