Tests the speed of unrolling a 1024 element loop using pointer arithmetic vs. a nested for loop and outputs the result.

unroller.cpp

#include <iostream>
#include <cmath>
#include <ctime>
using namespace std;

int array[256][4] = {  
{ 1, 2, 3, 4 },
{ 5, 6, 7, 8 },
{ 9, 10, 11, 12 },
{ 13, 14, 15, 16 },
{ 17, 18, 19, 20 },
{ 21, 22, 23, 24 },
{ 25, 26, 27, 28 },
{ 29, 30, 31, 32 },
{ 33, 34, 35, 36 },
{ 37, 38, 39, 40 },
{ 41, 42, 43, 44 },
{ 45, 46, 47, 48 },
{ 49, 50, 51, 52 },
{ 53, 54, 55, 56 },
{ 57, 58, 59, 60 },
{ 61, 62, 63, 64 },
{ 65, 66, 67, 68 },
{ 69, 70, 71, 72 },
{ 73, 74, 75, 76 },
{ 77, 78, 79, 80 },
{ 81, 82, 83, 84 },
{ 85, 86, 87, 88 },
{ 89, 90, 91, 92 },
{ 93, 94, 95, 96 },
{ 97, 98, 99, 100 },
{ 101, 102, 103, 104 },
{ 105, 106, 107, 108 },
{ 109, 110, 111, 112 },
{ 113, 114, 115, 116 },
{ 117, 118, 119, 120 },
{ 121, 122, 123, 124 },
{ 125, 126, 127, 128 },
{ 129, 130, 131, 132 },
{ 133, 134, 135, 136 },
{ 137, 138, 139, 140 },
{ 141, 142, 143, 144 },
{ 145, 146, 147, 148 },
{ 149, 150, 151, 152 },
{ 153, 154, 155, 156 },
{ 157, 158, 159, 160 },
{ 161, 162, 163, 164 },
{ 165, 166, 167, 168 },
{ 169, 170, 171, 172 },
{ 173, 174, 175, 176 },
{ 177, 178, 179, 180 },
{ 181, 182, 183, 184 },
{ 185, 186, 187, 188 },
{ 189, 190, 191, 192 },
{ 193, 194, 195, 196 },
{ 197, 198, 199, 200 },
{ 201, 202, 203, 204 },
{ 205, 206, 207, 208 },
{ 209, 210, 211, 212 },
{ 213, 214, 215, 216 },
{ 217, 218, 219, 220 },
{ 221, 222, 223, 224 },
{ 225, 226, 227, 228 },
{ 229, 230, 231, 232 },
{ 233, 234, 235, 236 },
{ 237, 238, 239, 240 },
{ 241, 242, 243, 244 },
{ 245, 246, 247, 248 },
{ 249, 250, 251, 252 },
{ 253, 254, 255, 256 },
{ 257, 258, 259, 260 },
{ 261, 262, 263, 264 },
{ 265, 266, 267, 268 },
{ 269, 270, 271, 272 },
{ 273, 274, 275, 276 },
{ 277, 278, 279, 280 },
{ 281, 282, 283, 284 },
{ 285, 286, 287, 288 },
{ 289, 290, 291, 292 },
{ 293, 294, 295, 296 },
{ 297, 298, 299, 300 },
{ 301, 302, 303, 304 },
{ 305, 306, 307, 308 },
{ 309, 310, 311, 312 },
{ 313, 314, 315, 316 },
{ 317, 318, 319, 320 },
{ 321, 322, 323, 324 },
{ 325, 326, 327, 328 },
{ 329, 330, 331, 332 },
{ 333, 334, 335, 336 },
{ 337, 338, 339, 340 },
{ 341, 342, 343, 344 },
{ 345, 346, 347, 348 },
{ 349, 350, 351, 352 },
{ 353, 354, 355, 356 },
{ 357, 358, 359, 360 },
{ 361, 362, 363, 364 },
{ 365, 366, 367, 368 },
{ 369, 370, 371, 372 },
{ 373, 374, 375, 376 },
{ 377, 378, 379, 380 },
{ 381, 382, 383, 384 },
{ 385, 386, 387, 388 },
{ 389, 390, 391, 392 },
{ 393, 394, 395, 396 },
{ 397, 398, 399, 400 },
{ 401, 402, 403, 404 },
{ 405, 406, 407, 408 },
{ 409, 410, 411, 412 },
{ 413, 414, 415, 416 },
{ 417, 418, 419, 420 },
{ 421, 422, 423, 424 },
{ 425, 426, 427, 428 },
{ 429, 430, 431, 432 },
{ 433, 434, 435, 436 },
{ 437, 438, 439, 440 },
{ 441, 442, 443, 444 },
{ 445, 446, 447, 448 },
{ 449, 450, 451, 452 },
{ 453, 454, 455, 456 },
{ 457, 458, 459, 460 },
{ 461, 462, 463, 464 },
{ 465, 466, 467, 468 },
{ 469, 470, 471, 472 },
{ 473, 474, 475, 476 },
{ 477, 478, 479, 480 },
{ 481, 482, 483, 484 },
{ 485, 486, 487, 488 },
{ 489, 490, 491, 492 },
{ 493, 494, 495, 496 },
{ 497, 498, 499, 500 },
{ 501, 502, 503, 504 },
{ 505, 506, 507, 508 },
{ 509, 510, 511, 512 },
{ 513, 514, 515, 516 },
{ 517, 518, 519, 520 },
{ 521, 522, 523, 524 },
{ 525, 526, 527, 528 },
{ 529, 530, 531, 532 },
{ 533, 534, 535, 536 },
{ 537, 538, 539, 540 },
{ 541, 542, 543, 544 },
{ 545, 546, 547, 548 },
{ 549, 550, 551, 552 },
{ 553, 554, 555, 556 },
{ 557, 558, 559, 560 },
{ 561, 562, 563, 564 },
{ 565, 566, 567, 568 },
{ 569, 570, 571, 572 },
{ 573, 574, 575, 576 },
{ 577, 578, 579, 580 },
{ 581, 582, 583, 584 },
{ 585, 586, 587, 588 },
{ 589, 590, 591, 592 },
{ 593, 594, 595, 596 },
{ 597, 598, 599, 600 },
{ 601, 602, 603, 604 },
{ 605, 606, 607, 608 },
{ 609, 610, 611, 612 },
{ 613, 614, 615, 616 },
{ 617, 618, 619, 620 },
{ 621, 622, 623, 624 },
{ 625, 626, 627, 628 },
{ 629, 630, 631, 632 },
{ 633, 634, 635, 636 },
{ 637, 638, 639, 640 },
{ 641, 642, 643, 644 },
{ 645, 646, 647, 648 },
{ 649, 650, 651, 652 },
{ 653, 654, 655, 656 },
{ 657, 658, 659, 660 },
{ 661, 662, 663, 664 },
{ 665, 666, 667, 668 },
{ 669, 670, 671, 672 },
{ 673, 674, 675, 676 },
{ 677, 678, 679, 680 },
{ 681, 682, 683, 684 },
{ 685, 686, 687, 688 },
{ 689, 690, 691, 692 },
{ 693, 694, 695, 696 },
{ 697, 698, 699, 700 },
{ 701, 702, 703, 704 },
{ 705, 706, 707, 708 },
{ 709, 710, 711, 712 },
{ 713, 714, 715, 716 },
{ 717, 718, 719, 720 },
{ 721, 722, 723, 724 },
{ 725, 726, 727, 728 },
{ 729, 730, 731, 732 },
{ 733, 734, 735, 736 },
{ 737, 738, 739, 740 },
{ 741, 742, 743, 744 },
{ 745, 746, 747, 748 },
{ 749, 750, 751, 752 },
{ 753, 754, 755, 756 },
{ 757, 758, 759, 760 },
{ 761, 762, 763, 764 },
{ 765, 766, 767, 768 },
{ 769, 770, 771, 772 },
{ 773, 774, 775, 776 },
{ 777, 778, 779, 780 },
{ 781, 782, 783, 784 },
{ 785, 786, 787, 788 },
{ 789, 790, 791, 792 },
{ 793, 794, 795, 796 },
{ 797, 798, 799, 800 },
{ 801, 802, 803, 804 },
{ 805, 806, 807, 808 },
{ 809, 810, 811, 812 },
{ 813, 814, 815, 816 },
{ 817, 818, 819, 820 },
{ 821, 822, 823, 824 },
{ 825, 826, 827, 828 },
{ 829, 830, 831, 832 },
{ 833, 834, 835, 836 },
{ 837, 838, 839, 840 },
{ 841, 842, 843, 844 },
{ 845, 846, 847, 848 },
{ 849, 850, 851, 852 },
{ 853, 854, 855, 856 },
{ 857, 858, 859, 860 },
{ 861, 862, 863, 864 },
{ 865, 866, 867, 868 },
{ 869, 870, 871, 872 },
{ 873, 874, 875, 876 },
{ 877, 878, 879, 880 },
{ 881, 882, 883, 884 },
{ 885, 886, 887, 888 },
{ 889, 890, 891, 892 },
{ 893, 894, 895, 896 },
{ 897, 898, 899, 900 },
{ 901, 902, 903, 904 },
{ 905, 906, 907, 908 },
{ 909, 910, 911, 912 },
{ 913, 914, 915, 916 },
{ 917, 918, 919, 920 },
{ 921, 922, 923, 924 },
{ 925, 926, 927, 928 },
{ 929, 930, 931, 932 },
{ 933, 934, 935, 936 },
{ 937, 938, 939, 940 },
{ 941, 942, 943, 944 },
{ 945, 946, 947, 948 },
{ 949, 950, 951, 952 },
{ 953, 954, 955, 956 },
{ 957, 958, 959, 960 },
{ 961, 962, 963, 964 },
{ 965, 966, 967, 968 },
{ 969, 970, 971, 972 },
{ 973, 974, 975, 976 },
{ 977, 978, 979, 980 },
{ 981, 982, 983, 984 },
{ 985, 986, 987, 988 },
{ 989, 990, 991, 992 },
{ 993, 994, 995, 996 },
{ 997, 998, 999, 1000 },
{ 1001, 1002, 1003, 1004 },
{ 1005, 1006, 1007, 1008 },
{ 1009, 1010, 1011, 1012 },
{ 1013, 1014, 1015, 1016 },
{ 1017, 1018, 1019, 1020 },
{ 1021, 1022, 1023, 1024 }
};

int main() {
	/* Number of elements.  In a real world scenario, you would calculate
	this ahead of time, but we have it shortened to a const so that the test 
	only includes information on exactly how fast each approach is. */
	const int elements = 1024;
	
	/* TEST 1: NESTED LOOP (worst case O(n^2)) */
	cout << "Begin nested loop test: " << endl;
	clock_t start1 = clock();
	
	for (int i = 0; i < 256; i++) {
		for (int j = 0; j < 4; j++) {
			cout << array[i][j] << endl;
		}
	}
	
	clock_t end1 = clock();
	double time1 = (double) (end1-start1) / CLOCKS_PER_SEC;
	cout << "Nested loop time: " << time1 << endl;
	
	/* TEST 2: UNROLLED LOOP (worst case O(n)) ) */
	cout << "Begin unrolled loop test:" << endl;
	clock_t start2 = clock();
	
	/* How large should each incrementation be? */
	int incr = sizeof(int);
	
	/* Pointer to perform arithmetic on. */
	int *ptr = (int*)array;
	
	/* Array elements are still in sequential order in memory */
	for(int offset = 0; offset < elements; offset++) {
		cout << (*ptr) << endl;
		ptr++;
	}
	
	clock_t end2 = clock();
	double time2 = (double) (end2-start2) / CLOCKS_PER_SEC;
	cout << "Unrolled loop time: " << time2 << endl;
	
	/* Output results */
	if (time1 < time2) {
		cout << "Nested loop wins by " << (time2-time1) << "s" << endl;
	} else if (time2 < time1) {
		cout << "Unrolled loop wins by " << (time1-time2) << "s" << endl;
	} else {
	  cout << "Both unrolled and nested loops took approximately ";
	  cout << "the same amount of time." << endl;
	}
	
	return 0;
}