C perché il tempo di esecuzione cambia in base al tipo di dati?

Question

Perché il tempo di esecuzione cambia utilizzando diversi tipi di dati in C. qui è il mio primo programma in cui ho utilizzato il tipo di dati long long int & ottenuto 4,61 tempo di esecuzione su codechef per un no. dei casi di test.

#include<stdio.h> 
#include<stdlib.h> 
#include<math.h> 
long long int prm[] = {2,3,5,7,11,13,17,19,23,29,31,37,41,43,47,53,59,61,67,71,73,79,83,89,97,101,103,107,109,113,127,131,137,139,149,151,157,163,167,173,179,181,191,193,197,199,211,223,227,229,233,239,241,251,257,263,269,271,277,281,283,293,307,311,313,317,331,337,347,349,353,359,367,373,379,383,389,397,401,409,419,421,431,433,439,443,449,457,461,463,467,479,487,491,499,503,509,521,523,541,547,557,563,569,571,577,587,593,599,601,607,613,617,619,631,641,643,647,653,659,661,673,677,683,691,701,709,719,727,733,739,743,751,757,761,769,773,787,797,809,811,821,823,827,829,839,853,857,859,863,877,881,883,887,907,911,919,929,937,941,947,953,967,971,977,983,991,997,1009,1013,1019,1021,1031,1033,1039,1049,1051,1061,1063,1069,1087,1091,1093,1097,1103,1109,1117,1123,1129,1151,1153,1163,1171,1181,1187,1193,1201,1213,1217,1223,1229,1231,1237,1249,1259,1277,1279,1283,1289,1291,1297,1301,1303,1307,1319,1321,1327,1361,1367,1373,1381,1399,1409,1423,1427,1429,1433,1439,1447,1451,1453,1459,1471,1481,1483,1487,1489,1493,1499,1511,1523,1531,1543,1549,1553,1559,1567,1571,1579,1583,1597,1601,1607,1609,1613,1619,1621,1627,1637,1657,1663,1667,1669,1693,1697,1699,1709,1721,1723,1733,1741,1747,1753,1759,1777,1783,1787,1789,1801,1811,1823,1831,1847,1861,1867,1871,1873,1877,1879,1889,1901,1907,1913,1931,1933,1949,1951,1973,1979,1987,1993,1997,1999,2003,2011,2017,2027,2029,2039,2053,2063,2069,2081,2083,2087,2089,2099,2111,2113,2129,2131,2137,2141,2143,2153,2161,2179,2203,2207,2213,2221,2237,2239,2243,2251,2267,2269,2273,2281,2287,2293,2297,2309,2311,2333,2339,2341,2347,2351,2357,2371,2377,2381,2383,2389,2393,2399,2411,2417,2423,2437,2441,2447,2459,2467,2473,2477,2503,2521,2531,2539,2543,2549,2551,2557,2579,2591,2593,2609,2617,2621,2633,2647,2657,2659,2663,2671,2677,2683,2687,2689,2693,2699,2707,2711,2713,2719,2729,2731,2741,2749,2753,2767,2777,2789,2791,2797,2801,2803,2819,2833,2837,2843,2851,2857,2861,2879,2887,2897,2903,2909,2917,2927,2939,2953,2957,2963,2969,2971,2999,3001,3011,3019,3023,3037,3041,3049,3061,3067,3079,3083,3089,3109,3119,3121,3137,3163,3167,3169,3181,3187,3191,3203,3209,3217,3221,3229,3251,3253,3257,3259,3271,3299,3301,3307,3313,3319,3323,3329,3331,3343,3347,3359,3361,3371,3373,3389,3391,3407,3413,3433,3449,3457,3461,3463,3467,3469,3491,3499,3511,3517,3527,3529,3533,3539,3541,3547,3557,3559,3571,3581,3583,3593,3607,3613,3617,3623,3631,3637,3643,3659,3671,3673,3677,3691,3697,3701,3709,3719,3727,3733,3739,3761,3767,3769,3779,3793,3797,3803,3821,3823,3833,3847,3851,3853,3863,3877,3881,3889,3907,3911,3917,3919,3923,3929,3931,3943,3947,3967,3989,4001,4003,4007,4013,4019,4021,4027,4049,4051,4057,4073,4079,4091,4093,4099,4111,4127,4129,4133,4139,4153,4157,4159,4177,4201,4211,4217,4219,4229,4231,4241,4243,4253,4259,4261,4271,4273,4283,4289,4297,4327,4337,4339,4349,4357,4363,4373,4391,4397,4409,4421,4423,4441,4447,4451,4457,4463,4481,4483,4493,4507,4513,4517,4519,4523,4547,4549,4561,4567,4583,4591,4597,4603,4621,4637,4639,4643,4649,4651,4657,4663,4673,4679,4691,4703,4721,4723,4729,4733,4751,4759,4783,4787,4789,4793,4799,4801,4813,4817,4831,4861,4871,4877,4889,4903,4909,4919,4931,4933,4937,4943,4951,4957,4967,4969,4973,4987,4993,4999,5003,5009,5011,5021,5023,5039,5051,5059,5077,5081,5087,5099,5101,5107,5113,5119,5147,5153,5167,5171,5179,5189,5197,5209,5227,5231,5233,5237,5261,5273,5279,5281,5297,5303,5309,5323,5333,5347,5351,5381,5387,5393,5399,5407,5413,5417,5419,5431,5437,5441,5443,5449,5471,5477,5479,5483,5501,5503,5507,5519,5521,5527,5531,5557,5563,5569,5573,5581,5591,5623,5639,5641,5647,5651,5653,5657,5659,5669,5683,5689,5693,5701,5711,5717,5737,5741,5743,5749,5779,5783,5791,5801,5807,5813,5821,5827,5839,5843,5849,5851,5857,5861,5867,5869,5879,5881,5897,5903,5923,5927,5939,5953,5981,5987,6007,6011,6029,6037,6043,6047,6053,6067,6073,6079,6089,6091,6101,6113,6121,6131,6133,6143,6151,6163,6173,6197,6199,6203,6211,6217,6221,6229,6247,6257,6263,6269,6271,6277,6287,6299,6301,6311,6317,6323,6329,6337,6343,6353,6359,6361,6367,6373,6379,6389,6397,6421,6427,6449,6451,6469,6473,6481,6491,6521,6529,6547,6551,6553,6563,6569,6571,6577,6581,6599,6607,6619,6637,6653,6659,6661,6673,6679,6689,6691,6701,6703,6709,6719,6733,6737,6761,6763,6779,6781,6791,6793,6803,6823,6827,6829,6833,6841,6857,6863,6869,6871,6883,6899,6907,6911,6917,6947,6949,6959,6961,6967,6971,6977,6983,6991,6997,7001,7013,7019,7027,7039,7043,7057,7069,7079,7103,7109,7121,7127,7129,7151,7159,7177,7187,7193,7207,7211,7213,7219,7229,7237,7243,7247,7253,7283,7297,7307,7309,7321,7331,7333,7349,7351,7369,7393,7411,7417,7433,7451,7457,7459,7477,7481,7487,7489,7499,7507,7517,7523,7529,7537,7541,7547,7549,7559,7561,7573,7577,7583,7589,7591,7603,7607,7621,7639,7643,7649,7669,7673,7681,7687,7691,7699,7703,7717,7723,7727,7741,7753,7757,7759,7789,7793,7817,7823,7829,7841,7853,7867,7873,7877,7879,7883,7901,7907,7919,7927,7933,7937,7949,7951,7963,7993,8009,8011,8017,8039,8053,8059,8069,8081,8087,8089,8093,8101,8111,8117,8123,8147,8161,8167,8171,8179,8191,8209,8219,8221,8231,8233,8237,8243,8263,8269,8273,8287,8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

int main() 
{ 
    int test, flag, sq; 
    long long int first, second, first_l, secd_l; 
    scanf("%d",&test); 
    while(test--) 
    { 
    scanf("%lld",&first); 
    scanf("%lld",&second); 
    if(first == 1) 
     first++; 
    if(first == 2 && second != 1) 
     printf("%d\n",2); 
    first += (first % 2 == 0) ? 1 : 0 ; 
    for(first_l = first; first_l <= second; first_l += 2) 
    { 
     flag = 0; 
     sq = sqrt(first_l); 
     for(secd_l = 0; sq >= prm[secd_l]; secd_l++) 
     { 
     if(first_l % prm[secd_l] == 0) 
     { 
      flag = 1; 
      break; 
     } 
     } 
     if(flag == 0) 
     { 
     printf("%lld\n",first_l); 
     } 
    } 
    } 
    return 0; 
} 

Ecco un altro programma molto simile alla prima ma utilizzando unsigned tipo di dati int anziché lungo tipo di dati long int. Il tempo di esecuzione che ho ottenuto per questo è di 2,78 secondi su codechef per gli stessi casi di test.

#include<stdio.h> 
#include<stdlib.h> 
#include<math.h> 
int prm[] ={same as before(max character limit in question)}; 

int main() 
{ 
    int test, flag, sq; 
    unsigned int first, second, first_l, secd_l; 
    scanf("%d",&test); 
    while(test--) 
    { 
    scanf("%u",&first); 
    scanf("%u",&second); 
    if(first == 1) 
     first++; 
    if(first == 2 && second != 1) 
     printf("%d\n",2); 
    first += (first % 2 == 0) ? 1 : 0 ; 
    for(first_l = first; first_l <= second; first_l += 2) 
    { 
     flag = 0; 
     sq = sqrt(first_l); 
     for(secd_l = 0; sq >= prm[secd_l]; secd_l++) 
     { 
     if(first_l % prm[secd_l] == 0) 
     { 
      flag = 1; 
      break; 
     } 
     } 
     if(flag == 0) 
     { 
     printf("%u\n",first_l); 
     } 
    } 
    } 
    return 0; 
} 

In un altro codice di programma ho semplicemente usato int invece di long long int/unsigned int su stessi casi di test & ho ottenuto 2,57 sec tempo di esecuzione.

Quindi, mi piacerebbe sapere perché avere int long long o unsigned int fa una così grande differenza per il mio tempo di esecuzione. Tutte le soluzioni sono testate su processori PIII 733 MHZ uguali su codechef.

Il formato di input è: - La prima riga contiene t, il numero di casi di test (inferiore o uguale a 10). Seguito da t linee che contengono due numeri m e n (1 < = m < = n < = 1000000000, n-m < = 100000) separati da uno spazio. Il programma genera numeri primi tra due numeri n & m.

Answer 1

ci sono molti fattori che determinano il tempo di esecuzione.

• Se si è su una macchina a 32 bit; eseguire operazioni matematiche con un tipo lungo 32 bit richiederà meno tempo rispetto alle operazioni su un tipo di 64 bit. (Uint è probabilmente a 32 bit, long long int è probabilmente 64bits)
• sua è più performante di aggiungere le variabili dello stesso tipo (perché allora non è necessario alcun tipo di casting)
• a seconda di come si scrive il codice, il compilatore potrebbe ottimizzare il codice in modo diverso (ma dal momento che si è cambiato solo il tipo di variabile che non interessa qui)
• ...

queste sono solo alcune idee; ottimizzazione del codice è un vasto argomento :)

Answer 2

sizeof(int) <= sizeof(long) 

Quindi, al fine di accedere a più byte nel tempo il tempo impiegato sarà più. Sebbene la dimensione dei tipi di dati sia completamente dipendente dalla piattaforma. Il punto qui è al fine di accedere a 64 bit più cicli di CPU è necessario rispetto all'accesso a 32 bit su una macchina a 32 bit.

Answer 3

Come sottolinea Chris Maes, nelle macchine a 32 bit le operazioni a 32 bit richiedono meno tempo rispetto a 64 bit. Il motivo è che hai bisogno di due registri a 32 bit (di solito EDX e EAX su x86, ALU calcola con numeri interi) per lavorare con loro o solo per visualizzarli - questo richiede più isottrazioni e più cicli di CPU, quindi è più lento lavorare a lungo (che è probabilmente a 64 bit) rispetto a int (probabilmente a 32 bit). Probabilmente, perché dipende dalla piattaforma, come sottolinea Gopi.

Su macchine a 64 bit, ci sono registri a 64 bit (come RAX) monouso per unità ALU, quindi in teoria potrebbe essere più veloce lì. Tuttavia, anche su macchine a 64 bit è possibile eseguire una logica di calcolo a 32 bit.

Answer 4

Su un computer a 32 bit, ovviamente, la matematica a 64 bit dovrebbe essere più lenta di quella a 32 bit. Pensa a quanto è più lento moltiplicare due numeri a 2 cifre nella tua testa che moltiplicare due numeri a 1 cifra. [Si noti che la maggior parte dei PC circa 2 decadi fa funzionare sistemi operativi a 32 e 64 bit, quindi int è un tipo a 32 bit il cui intervallo è [-2147483648, 2147483647]. Codechef, naturalmente, doesn't use DOS anymore così int dovrebbe essere anche 32 bit.

È necessario più memoria e più registri per memorizzare una variabile a 64 bit. Su un'architettura con un numero di registro basso come x86, meno registri a sinistra possono incorrere in più accessi di memoria per spillare i valori per le altre variabili. Dato che la larghezza di banda della memoria è limitata, meno variabili possono essere lette/scritte contemporaneamente e meno variabili si adattano anche alla cache, quindi sarà due volte più lento e se è necessario accedere a molta memoria, molto più quando manca una cache appare

È inoltre necessario altre istruzioni per elaborare valori superiori alla dimensione del registro. Per aggiungere/sottrarre 2 valori a 64 bit su sistemi x86 avete bisogno di 2 istruzioni

add edx, ebx // add low part 
adc eax, ecx // add high part with carry 

Ogni istruzione superiore aggiunge 2 registri a 32 bit, in modo da poter vedere che è necessario solo 1 istruzione di aggiungere due a 32 bit int s

Per operazioni più complesse come la moltiplicazione e la divisione, la differenza di prestazioni sarebbe molto più grande. Si può vedere che gcc genera molto di istruzioni per moltiplicare un numero a 32 bit con uno a 64 bit here. Il programma ha un'operazione di modulo in modo che il compilatore possa dover emettere una divisione, che è la più complessa in 4 operazioni di base. Pertanto, le prestazioni sono anche peggiori.

Si può leggere di più su questo here