Fd 979: Fill & Download for Free

As some other answers said, it is mostly syntactic sugar. But in the case of unrolling a loop, by placing a -funroll-all-loops or by placing an explicit __attribute__((optimize("unroll-loops"))) in front of a function the for loop is easier to optimize, because the index variable it for the compiler visual.In modern compilers this should not be of any difference, but we are going to take a look into the details and see ourselves:#include <stdio.h> //__attribute__((optimize("unroll-loops"))) void forloop(int n){  int a=0;  for(int i=1; i<=n; i++){  a+=i;  printf("i=%d\ta=%d\n", i, a);  } } //__attribute__((optimize("unroll-loops"))) void whileloop(int n){  int a=0, i=1;  while(i<=n){  a+=i;  printf("i=%d\ta=%d\n", i, a);  ++i;  } } int main(int argc, char **argv){  printf("for-loop\n");  forloop(10);  printf("while-loop\n");  whileloop(10);  return 0; } We are going to compile with> cc -O3 -o forwhile forwhile.c Then take a look at the compilat (using: objdump -d forwhile).00000000000007a0 <forloop>:  7a0: 85 ff test edi,edi  7a2: 7e 43 jle 7e7 <forloop+0x47>  7a4: 41 55 push r13  7a6: 41 54 push r12  7a8: 4c 8d 2d 15 01 00 00 lea r13,[rip+0x115] # 8c4 <_IO_stdin_used+0x4>  7af: 55 push rbp  7b0: 53 push rbx  7b1: 41 89 fc mov r12d,edi  7b4: bb 01 00 00 00 mov ebx,0x1  7b9: 31 ed xor ebp,ebp  7bb: 48 83 ec 08 sub rsp,0x8  7bf: 90 nop  7c0: 01 dd add ebp,ebx  7c2: 89 da mov edx,ebx  7c4: 31 c0 xor eax,eax  7c6: 89 e9 mov ecx,ebp  7c8: 4c 89 ee mov rsi,r13  7cb: bf 01 00 00 00 mov edi,0x1  7d0: 83 c3 01 add ebx,0x1  7d3: e8 f0 fd ff ff call 5c8 <.plt.got+0x8>  7d8: 41 39 dc cmp r12d,ebx  7db: 7d e3 jge 7c0 <forloop+0x20>  7dd: 48 83 c4 08 add rsp,0x8  7e1: 5b pop rbx  7e2: 5d pop rbp  7e3: 41 5c pop r12  7e5: 41 5d pop r13  7e7: f3 c3 repz ret   7e9: 0f 1f 80 00 00 00 00 nop DWORD PTR [rax+0x0]  00000000000007f0 <whileloop>:  7f0: 85 ff test edi,edi  7f2: 7e 43 jle 837 <whileloop+0x47>  7f4: 41 55 push r13  7f6: 41 54 push r12  7f8: 4c 8d 2d c5 00 00 00 lea r13,[rip+0xc5] # 8c4 <_IO_stdin_used+0x4>  7ff: 55 push rbp  800: 53 push rbx  801: 41 89 fc mov r12d,edi  804: bb 01 00 00 00 mov ebx,0x1  809: 31 ed xor ebp,ebp  80b: 48 83 ec 08 sub rsp,0x8  80f: 90 nop  810: 01 dd add ebp,ebx  812: 89 da mov edx,ebx  814: 31 c0 xor eax,eax  816: 89 e9 mov ecx,ebp  818: 4c 89 ee mov rsi,r13  81b: bf 01 00 00 00 mov edi,0x1  820: 83 c3 01 add ebx,0x1  823: e8 a0 fd ff ff call 5c8 <.plt.got+0x8>  828: 41 39 dc cmp r12d,ebx  82b: 7d e3 jge 810 <whileloop+0x20>  82d: 48 83 c4 08 add rsp,0x8  831: 5b pop rbx  832: 5d pop rbp  833: 41 5c pop r12  835: 41 5d pop r13  837: f3 c3 repz ret   839: 0f 1f 80 00 00 00 00 nop DWORD PTR [rax+0x0] As we can see, byte for byte the same. Now let’s see what happens if we unroll-all-loops (or uncomment the attribute line in front of the functions):> cc -funroll-all-loops -O3 -o forwhile forwhile.c  The compilat will grow a bit. So keep with me with this, please.00000000000007a0 <forloop>:  7a0: 85 ff test edi,edi  7a2: 0f 8e 1d 02 00 00 jle 9c5 <forloop+0x225>  7a8: 41 56 push r14  7aa: 41 55 push r13  7ac: 44 8d 77 ff lea r14d,[rdi-0x1]  7b0: 41 54 push r12  7b2: 55 push rbp  7b3: 48 8d 2d ca 04 00 00 lea rbp,[rip+0x4ca] # c84 <_IO_stdin_used+0x4>  7ba: 53 push rbx  7bb: 41 89 fc mov r12d,edi  7be: 31 c0 xor eax,eax  7c0: b9 01 00 00 00 mov ecx,0x1  7c5: ba 01 00 00 00 mov edx,0x1  7ca: 48 89 ee mov rsi,rbp  7cd: bf 01 00 00 00 mov edi,0x1  7d2: 41 83 e6 07 and r14d,0x7  7d6: bb 01 00 00 00 mov ebx,0x1  7db: e8 e8 fd ff ff call 5c8 <.plt.got+0x8>  7e0: 41 83 fc 02 cmp r12d,0x2  7e4: 41 bd 02 00 00 00 mov r13d,0x2  7ea: 0f 8c cd 01 00 00 jl 9bd <forloop+0x21d>  7f0: 45 85 f6 test r14d,r14d  7f3: 0f 84 ff 00 00 00 je 8f8 <forloop+0x158>  7f9: 41 83 fe 01 cmp r14d,0x1  7fd: 0f 84 d1 00 00 00 je 8d4 <forloop+0x134>  803: 41 83 fe 02 cmp r14d,0x2  807: 0f 84 ac 00 00 00 je 8b9 <forloop+0x119>  80d: 41 83 fe 03 cmp r14d,0x3  811: 0f 84 87 00 00 00 je 89e <forloop+0xfe>  817: 41 83 fe 04 cmp r14d,0x4  81b: 74 66 je 883 <forloop+0xe3>  81d: 41 83 fe 05 cmp r14d,0x5  821: 74 45 je 868 <forloop+0xc8>  823: 41 83 fe 06 cmp r14d,0x6  827: 74 24 je 84d <forloop+0xad>  829: b9 03 00 00 00 mov ecx,0x3  82e: ba 02 00 00 00 mov edx,0x2  833: 48 89 ee mov rsi,rbp  836: bf 01 00 00 00 mov edi,0x1  83b: 31 c0 xor eax,eax  83d: bb 03 00 00 00 mov ebx,0x3  842: 41 bd 03 00 00 00 mov r13d,0x3  848: e8 7b fd ff ff call 5c8 <.plt.got+0x8>  84d: 44 01 eb add ebx,r13d  850: 44 89 ea mov edx,r13d  853: 48 89 ee mov rsi,rbp  856: 89 d9 mov ecx,ebx  858: bf 01 00 00 00 mov edi,0x1  85d: 31 c0 xor eax,eax  85f: 41 83 c5 01 add r13d,0x1  863: e8 60 fd ff ff call 5c8 <.plt.got+0x8>  868: 44 01 eb add ebx,r13d  86b: 44 89 ea mov edx,r13d  86e: 48 89 ee mov rsi,rbp  871: 89 d9 mov ecx,ebx  873: bf 01 00 00 00 mov edi,0x1  878: 31 c0 xor eax,eax  87a: 41 83 c5 01 add r13d,0x1  87e: e8 45 fd ff ff call 5c8 <.plt.got+0x8>  883: 44 01 eb add ebx,r13d  886: 44 89 ea mov edx,r13d  889: 48 89 ee mov rsi,rbp  88c: 89 d9 mov ecx,ebx  88e: bf 01 00 00 00 mov edi,0x1  893: 31 c0 xor eax,eax  895: 41 83 c5 01 add r13d,0x1  899: e8 2a fd ff ff call 5c8 <.plt.got+0x8>  89e: 44 01 eb add ebx,r13d  8a1: 44 89 ea mov edx,r13d  8a4: 48 89 ee mov rsi,rbp  8a7: 89 d9 mov ecx,ebx  8a9: bf 01 00 00 00 mov edi,0x1  8ae: 31 c0 xor eax,eax  8b0: 41 83 c5 01 add r13d,0x1  8b4: e8 0f fd ff ff call 5c8 <.plt.got+0x8>  8b9: 44 01 eb add ebx,r13d  8bc: 44 89 ea mov edx,r13d  8bf: 48 89 ee mov rsi,rbp  8c2: 89 d9 mov ecx,ebx  8c4: bf 01 00 00 00 mov edi,0x1  8c9: 31 c0 xor eax,eax  8cb: 41 83 c5 01 add r13d,0x1  8cf: e8 f4 fc ff ff call 5c8 <.plt.got+0x8>  8d4: 44 01 eb add ebx,r13d  8d7: 44 89 ea mov edx,r13d  8da: 31 c0 xor eax,eax  8dc: 89 d9 mov ecx,ebx  8de: 48 89 ee mov rsi,rbp  8e1: bf 01 00 00 00 mov edi,0x1  8e6: 41 83 c5 01 add r13d,0x1  8ea: e8 d9 fc ff ff call 5c8 <.plt.got+0x8>  8ef: 45 39 ec cmp r12d,r13d  8f2: 0f 8c c5 00 00 00 jl 9bd <forloop+0x21d>  8f8: 44 01 eb add ebx,r13d  8fb: 44 89 ea mov edx,r13d  8fe: 48 89 ee mov rsi,rbp  901: 89 d9 mov ecx,ebx  903: bf 01 00 00 00 mov edi,0x1  908: 31 c0 xor eax,eax  90a: e8 b9 fc ff ff call 5c8 <.plt.got+0x8>  90f: 41 8d 55 01 lea edx,[r13+0x1]  913: 48 89 ee mov rsi,rbp  916: bf 01 00 00 00 mov edi,0x1  91b: 31 c0 xor eax,eax  91d: 01 d3 add ebx,edx  91f: 89 d9 mov ecx,ebx  921: e8 a2 fc ff ff call 5c8 <.plt.got+0x8>  926: 41 8d 55 02 lea edx,[r13+0x2]  92a: 48 89 ee mov rsi,rbp  92d: bf 01 00 00 00 mov edi,0x1  932: 31 c0 xor eax,eax  934: 01 d3 add ebx,edx  936: 89 d9 mov ecx,ebx  938: e8 8b fc ff ff call 5c8 <.plt.got+0x8>  93d: 41 8d 55 03 lea edx,[r13+0x3]  941: 48 89 ee mov rsi,rbp  944: bf 01 00 00 00 mov edi,0x1  949: 31 c0 xor eax,eax  94b: 01 d3 add ebx,edx  94d: 89 d9 mov ecx,ebx  94f: e8 74 fc ff ff call 5c8 <.plt.got+0x8>  954: 41 8d 55 04 lea edx,[r13+0x4]  958: 48 89 ee mov rsi,rbp  95b: bf 01 00 00 00 mov edi,0x1  960: 31 c0 xor eax,eax  962: 01 d3 add ebx,edx  964: 89 d9 mov ecx,ebx  966: e8 5d fc ff ff call 5c8 <.plt.got+0x8>  96b: 41 8d 55 05 lea edx,[r13+0x5]  96f: 48 89 ee mov rsi,rbp  972: bf 01 00 00 00 mov edi,0x1  977: 31 c0 xor eax,eax  979: 01 d3 add ebx,edx  97b: 89 d9 mov ecx,ebx  97d: e8 46 fc ff ff call 5c8 <.plt.got+0x8>  982: 41 8d 55 06 lea edx,[r13+0x6]  986: 48 89 ee mov rsi,rbp  989: bf 01 00 00 00 mov edi,0x1  98e: 31 c0 xor eax,eax  990: 01 d3 add ebx,edx  992: 89 d9 mov ecx,ebx  994: e8 2f fc ff ff call 5c8 <.plt.got+0x8>  999: 41 8d 55 07 lea edx,[r13+0x7]  99d: 31 c0 xor eax,eax  99f: 48 89 ee mov rsi,rbp  9a2: bf 01 00 00 00 mov edi,0x1  9a7: 41 83 c5 08 add r13d,0x8  9ab: 01 d3 add ebx,edx  9ad: 89 d9 mov ecx,ebx  9af: e8 14 fc ff ff call 5c8 <.plt.got+0x8>  9b4: 45 39 ec cmp r12d,r13d  9b7: 0f 8d 3b ff ff ff jge 8f8 <forloop+0x158>  9bd: 5b pop rbx  9be: 5d pop rbp  9bf: 41 5c pop r12  9c1: 41 5d pop r13  9c3: 41 5e pop r14  9c5: f3 c3 repz ret   9c7: 66 0f 1f 84 00 00 00 nop WORD PTR [rax+rax*1+0x0]  9ce: 00 00   00000000000009d0 <whileloop>:  9d0: 85 ff test edi,edi  9d2: 0f 8e 1d 02 00 00 jle bf5 <whileloop+0x225>  9d8: 41 56 push r14  9da: 41 55 push r13  9dc: 44 8d 77 ff lea r14d,[rdi-0x1]  9e0: 41 54 push r12  9e2: 55 push rbp  9e3: 48 8d 2d 9a 02 00 00 lea rbp,[rip+0x29a] # c84 <_IO_stdin_used+0x4>  9ea: 53 push rbx  9eb: 41 89 fc mov r12d,edi  9ee: 31 c0 xor eax,eax  9f0: b9 01 00 00 00 mov ecx,0x1  9f5: ba 01 00 00 00 mov edx,0x1  9fa: 48 89 ee mov rsi,rbp  9fd: bf 01 00 00 00 mov edi,0x1  a02: 41 83 e6 07 and r14d,0x7  a06: bb 01 00 00 00 mov ebx,0x1  a0b: e8 b8 fb ff ff call 5c8 <.plt.got+0x8>  a10: 41 83 fc 02 cmp r12d,0x2  a14: 41 bd 02 00 00 00 mov r13d,0x2  a1a: 0f 8c cd 01 00 00 jl bed <whileloop+0x21d>  a20: 45 85 f6 test r14d,r14d  a23: 0f 84 ff 00 00 00 je b28 <whileloop+0x158>  a29: 41 83 fe 01 cmp r14d,0x1  a2d: 0f 84 d1 00 00 00 je b04 <whileloop+0x134>  a33: 41 83 fe 02 cmp r14d,0x2  a37: 0f 84 ac 00 00 00 je ae9 <whileloop+0x119>  a3d: 41 83 fe 03 cmp r14d,0x3  a41: 0f 84 87 00 00 00 je ace <whileloop+0xfe>  a47: 41 83 fe 04 cmp r14d,0x4  a4b: 74 66 je ab3 <whileloop+0xe3>  a4d: 41 83 fe 05 cmp r14d,0x5  a51: 74 45 je a98 <whileloop+0xc8>  a53: 41 83 fe 06 cmp r14d,0x6  a57: 74 24 je a7d <whileloop+0xad>  a59: b9 03 00 00 00 mov ecx,0x3  a5e: ba 02 00 00 00 mov edx,0x2  a63: 48 89 ee mov rsi,rbp  a66: bf 01 00 00 00 mov edi,0x1  a6b: 31 c0 xor eax,eax  a6d: bb 03 00 00 00 mov ebx,0x3  a72: 41 bd 03 00 00 00 mov r13d,0x3  a78: e8 4b fb ff ff call 5c8 <.plt.got+0x8>  a7d: 44 01 eb add ebx,r13d  a80: 44 89 ea mov edx,r13d  a83: 48 89 ee mov rsi,rbp  a86: 89 d9 mov ecx,ebx  a88: bf 01 00 00 00 mov edi,0x1  a8d: 31 c0 xor eax,eax  a8f: 41 83 c5 01 add r13d,0x1  a93: e8 30 fb ff ff call 5c8 <.plt.got+0x8>  a98: 44 01 eb add ebx,r13d  a9b: 44 89 ea mov edx,r13d  a9e: 48 89 ee mov rsi,rbp  aa1: 89 d9 mov ecx,ebx  aa3: bf 01 00 00 00 mov edi,0x1  aa8: 31 c0 xor eax,eax  aaa: 41 83 c5 01 add r13d,0x1  aae: e8 15 fb ff ff call 5c8 <.plt.got+0x8>  ab3: 44 01 eb add ebx,r13d  ab6: 44 89 ea mov edx,r13d  ab9: 48 89 ee mov rsi,rbp  abc: 89 d9 mov ecx,ebx  abe: bf 01 00 00 00 mov edi,0x1  ac3: 31 c0 xor eax,eax  ac5: 41 83 c5 01 add r13d,0x1  ac9: e8 fa fa ff ff call 5c8 <.plt.got+0x8>  ace: 44 01 eb add ebx,r13d  ad1: 44 89 ea mov edx,r13d  ad4: 48 89 ee mov rsi,rbp  ad7: 89 d9 mov ecx,ebx  ad9: bf 01 00 00 00 mov edi,0x1  ade: 31 c0 xor eax,eax  ae0: 41 83 c5 01 add r13d,0x1  ae4: e8 df fa ff ff call 5c8 <.plt.got+0x8>  ae9: 44 01 eb add ebx,r13d  aec: 44 89 ea mov edx,r13d  aef: 48 89 ee mov rsi,rbp  af2: 89 d9 mov ecx,ebx  af4: bf 01 00 00 00 mov edi,0x1  af9: 31 c0 xor eax,eax  afb: 41 83 c5 01 add r13d,0x1  aff: e8 c4 fa ff ff call 5c8 <.plt.got+0x8>  b04: 44 01 eb add ebx,r13d  b07: 44 89 ea mov edx,r13d  b0a: 31 c0 xor eax,eax  b0c: 89 d9 mov ecx,ebx  b0e: 48 89 ee mov rsi,rbp  b11: bf 01 00 00 00 mov edi,0x1  b16: 41 83 c5 01 add r13d,0x1  b1a: e8 a9 fa ff ff call 5c8 <.plt.got+0x8>  b1f: 45 39 ec cmp r12d,r13d  b22: 0f 8c c5 00 00 00 jl bed <whileloop+0x21d>  b28: 44 01 eb add ebx,r13d  b2b: 44 89 ea mov edx,r13d  b2e: 48 89 ee mov rsi,rbp  b31: 89 d9 mov ecx,ebx  b33: bf 01 00 00 00 mov edi,0x1  b38: 31 c0 xor eax,eax  b3a: e8 89 fa ff ff call 5c8 <.plt.got+0x8>  b3f: 41 8d 55 01 lea edx,[r13+0x1]  b43: 48 89 ee mov rsi,rbp  b46: bf 01 00 00 00 mov edi,0x1  b4b: 31 c0 xor eax,eax  b4d: 01 d3 add ebx,edx  b4f: 89 d9 mov ecx,ebx  b51: e8 72 fa ff ff call 5c8 <.plt.got+0x8>  b56: 41 8d 55 02 lea edx,[r13+0x2]  b5a: 48 89 ee mov rsi,rbp  b5d: bf 01 00 00 00 mov edi,0x1  b62: 31 c0 xor eax,eax  b64: 01 d3 add ebx,edx  b66: 89 d9 mov ecx,ebx  b68: e8 5b fa ff ff call 5c8 <.plt.got+0x8>  b6d: 41 8d 55 03 lea edx,[r13+0x3]  b71: 48 89 ee mov rsi,rbp  b74: bf 01 00 00 00 mov edi,0x1  b79: 31 c0 xor eax,eax  b7b: 01 d3 add ebx,edx  b7d: 89 d9 mov ecx,ebx  b7f: e8 44 fa ff ff call 5c8 <.plt.got+0x8>  b84: 41 8d 55 04 lea edx,[r13+0x4]  b88: 48 89 ee mov rsi,rbp  b8b: bf 01 00 00 00 mov edi,0x1  b90: 31 c0 xor eax,eax  b92: 01 d3 add ebx,edx  b94: 89 d9 mov ecx,ebx  b96: e8 2d fa ff ff call 5c8 <.plt.got+0x8>  b9b: 41 8d 55 05 lea edx,[r13+0x5]  b9f: 48 89 ee mov rsi,rbp  ba2: bf 01 00 00 00 mov edi,0x1  ba7: 31 c0 xor eax,eax  ba9: 01 d3 add ebx,edx  bab: 89 d9 mov ecx,ebx  bad: e8 16 fa ff ff call 5c8 <.plt.got+0x8>  bb2: 41 8d 55 06 lea edx,[r13+0x6]  bb6: 48 89 ee mov rsi,rbp  bb9: bf 01 00 00 00 mov edi,0x1  bbe: 31 c0 xor eax,eax  bc0: 01 d3 add ebx,edx  bc2: 89 d9 mov ecx,ebx  bc4: e8 ff f9 ff ff call 5c8 <.plt.got+0x8>  bc9: 41 8d 55 07 lea edx,[r13+0x7]  bcd: 31 c0 xor eax,eax  bcf: 48 89 ee mov rsi,rbp  bd2: bf 01 00 00 00 mov edi,0x1  bd7: 41 83 c5 08 add r13d,0x8  bdb: 01 d3 add ebx,edx  bdd: 89 d9 mov ecx,ebx  bdf: e8 e4 f9 ff ff call 5c8 <.plt.got+0x8>  be4: 45 39 ec cmp r12d,r13d  be7: 0f 8d 3b ff ff ff jge b28 <whileloop+0x158>  bed: 5b pop rbx  bee: 5d pop rbp  bef: 41 5c pop r12  bf1: 41 5d pop r13  bf3: 41 5e pop r14  bf5: f3 c3 repz ret   bf7: 66 0f 1f 84 00 00 00 nop WORD PTR [rax+rax*1+0x0]  bfe: 00 00  In the beginning of our two loops there is no difference, command to command:And also in the end, will every single step of machine code, every single byte, the same:The for loop once, before the times of modern compilers and optimizers was an invention of FORTRAN and other ancient languages, that indeed were able to optimize this construction and do the things that modern compilers do en passant.So, what once was essential information for our compiler is now reduced to some syntactic sugar. This is, too, the reason why modern languages like Lua only very recently (5.0) have introduced the for loop as a syntactic construction.You need a while() and a repeat-until (do-while();) structure. And additionally to that you need a break;, a continue; and a goto, to create all kinds of loops that are possible. You can not replace the goto by the way. Yeah, yeah, don’t start with recursion. The Google code policy forbids you to even use recursion, because people don’t understand them, don’t use them correctly, have been brainwashed to use it with bad example like faculty() and fibonacci() which are both situations were you definitely and absolutely not must use recursion.And because our teachers were doing such a bad job, sadly the recursion is dead in major companies, while I personally like recursions but when I see how they are used normally, I would totally ban them from my codespace, too, if I were a big company. They are used by lazy people, not by geniuses. Anyway. Goto does most of the jobs a tail recursion does better means you can see and understand what the algorithm is about and you don’t have to unbreak some tail recursion in your head. It’s stupid to create such a headcramp, just because from ideological reasons you are proud to remove every goto from your code, even those that are absolutely needed and necessary and that do a good job.Tail recursion is cool, but it is a tool like any of the others: it has a point where you should use them and where you should not. There are no bad tools, just bad applications of tools. So, do yourself a favor, get rid of ideology when you are programming.It is needless in modern times, but it gives something to talk about when teachers want to school you about the construction of loops.Here, by the way, you can see the Duff’s device, that is used to unroll our loops, sadly all examples for Duff’s device on Wikipedia are broken, at the moment at least.So much about the quality of Wikipedia. The working version of that device is looking like this, where you can see how this device can be made slower or faster by a simple change of code.And this is all what we have to know about for() and while() loops, I guess. You can also build a loop like that with gotos, by the way but I think they won’t get optimized. But I didn’t test that (yet).For() is syntactic sugar. Real programmers work with dynamic jump indices that is handled in the same addition opcode as the loop counter. The original Duff’s device was working with a calculated jump. Our compiler’s isn’t that smart.Hope I could help.#include <stdio.h> #include <stdlib.h> #include <string.h> #include <limits.h> #include <x86intrin.h> //timer #include <inttypes.h>  #define STARTTIMER(runs) {\  for(int rrr=runs; rrr; rrr--){\  min=UINT_MAX;\  t=__rdtsc()+72;\ } #define STOPTIMER(x,r) {\  t=__rdtsc()-t;\  min=(min>t)?t:min;\  }\  printf("%-10s cycles \t%e, avg/data: \t%lu\n", #x, (double)min, min/(r));\ } inline void duff(register long *to, register long *from, register size_t count){  register size_t n = (count + 7) / 8;  switch (count % 8) {  case 0: do { *to++ = *from++;  case 7: *to++ = *from++;  case 6: *to++ = *from++;  case 5: *to++ = *from++;  case 4: *to++ = *from++;  case 3: *to++ = *from++;  case 2: *to++ = *from++;  case 1: *to++ = *from++;  } while (--n > 0);  }  return; } inline void duff1(register long *to, register long *from, register size_t count){  register size_t n = (count + 7)>>3, i=0;  switch (count & 7) {  case 0: do { to[i] = from[i]; ++i;  case 7: to[i] = from[i]; ++i;  case 6: to[i] = from[i]; ++i;  case 5: to[i] = from[i]; ++i;  case 4: to[i] = from[i]; ++i;  case 3: to[i] = from[i]; ++i;  case 2: to[i] = from[i]; ++i;  case 1: to[i] = from[i]; ++i;  } while (--n > 0);  }  return; } int main(int argc, char **argv){  register uint64_t t; // timer cycle count  uint64_t min;   size_t SIZE;  SIZE=argc>1?atoll(argv[1]):50L;  long *a=(long *)malloc(SIZE*sizeof(long)),  *b=(long *)malloc(SIZE*sizeof(long));  if(!(a && b)) exit(EXIT_FAILURE);   for(size_t i=0; i<SIZE; i++) a[i]=i;  #define REP 100   STARTTIMER(REP)  duff(&b[0], &a[0], SIZE);  STOPTIMER(duff, REP)   STARTTIMER(REP)  memcpy(&b[0], &a[0], SIZE);  STOPTIMER(memcpy, REP)   STARTTIMER(REP)  duff1(&b[0], &a[0], SIZE);  STOPTIMER(duff1, REP)  if(SIZE<10000) for(size_t i=0; i<SIZE; i++) printf("%ld ", b[i]);   free(b); free(a);  return 0; } Outcome, while it is interesting that unroll-all-loops makes my Duff slower, because the compilers version is indeed slower, because it’s a template, a macro. My code is like it should be done in C. In Assembly that even gets a little bit shorter. Memcpy on the other hand is getting as slow as 400 cycles sometimes. It only is of advantage with very very large data fields. It is badly done.You could make a memcpy with the help of YMM registers, but they didn’t. Guess our system can be pushed at lot if somebody finally either finance me to do that or does that himself. I mean… factor 35… THIRTYFIVE. *gulp*In the end, the answer to your question is: you are using the for() loop for syntactic sugar reasons, means if you have an index variable or increment or something that is needed for your loop, you put that in there. Because it is easier to read for a human.There is no other reason for doing that.> cc -O3 -s -o duff duff.c > ./duff  duff cycles 5.380000e+02, avg/data: 5  memcpy cycles 9.181000e+03, avg/data: 91  duff1 cycles 2.400000e+02, avg/data: 2  0 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 % 

A number of states encode your name and date of birth in your license number. These include Maryland, Michigan, and Minnesota (prior to December 13, 2004 only). These states use the same system of encoding, or very similar ones. Given someone's driver's license number from one of these states, you can take good guesses at someone's name and exactly determine day of birth (but not year). With someone's name and date of birth you can guess some or all of their driver's license number.A typical license would look like this:S530-429-085-151 LLLL-FFF-MMM-BBB The above is for John Bennett Smith, born on February 27th.LLLL - Last Name, Soundex CodedLLLL is the Soundex coded last name. Soundex attempts to code similar sounding names to the same four character code. See my Soundex page for details on how to encode it.FFF - First Name, CodedFFF is the encoded first name. The name is looked up on the following table. If the exact name isn't on the table, look up the longest prefix that is on the table.a 027 aa 028 ab 029 ac 030 ad 031 ae 032 af 033 ag 034 ah 035 ai 036 aj 037 ak 038 al 039 ala 040 alb 041 alc 042 ald 043 ale 044 alf 045 alg 046 alh 047 ali 048 alj 049 alk 050 all 051 alm 052 aln 053 alo 054 alp 055 alq 056 alr 057 als 058 alt 059 alu 060 alv 061 alw 062 alx 063 aly 064 alz 065 am 066 an 067 ao 068 ap 069 aq 070 ar 071 as 072 at 073 au 074 av 075 aw 076 ax 077 ay 078 az 079  b 080 ba 081 bb 082 bc 083 bd 084 be 085 bf 086 bg 087 bh 088 bi 089 bj 090 bk 091 bl 092 bm 093 bn 094 bo 095 bp 096 bq 097 br 098 bs 099 bt 100 bu 101 bv 102 bw 103 bx 104 by 105 bz 106  c 107 ca 108 cb 109 cc 110 cd 111 ce 112 cf 113 cg 114 ch 115 ci 116 cj 117 ck 118 cl 119 cm 120 cn 121 co 122 cp 123 cq 124 cr 125 cs 126 ct 127 cu 128 cv 129 cw 130 cx 131 cy 132 cz 133  d 134 da 135 db 136 dc 137 dd 138 de 139 df 140 dg 141 dh 142 di 143 dj 144 dk 145 dl 146 dm 147 dn 148 do 149 dp 150 dq 151 dr 152 ds 153 dt 154 du 155 dv 156 dw 157 dx 158 dy 159 dz 160  e 161 ea 162 eb 163 ec 164 ed 165 eda 166 edb 167 edc 168 edd 169 ede 170 edf 171 edg 172 edh 173 edi 174 edj 175 edk 176 edl 177 edm 178 edn 179 edo 180 edp 181 edq 182 edr 183 eds 184 edt 185 edu 186 edv 187 edw 188 edward 189 edx 190 edy 191 edz 192 ee 193 ef 194 eg 195 eh 196 ei 197 ej 198 ek 199 el 200 ela 201 elb 202 elc 203 eld 204 ele 205 elf 206 elg 207 elh 208 eli 209 elizabeth 210 elj 211 elk 212 ell 213 ellen 214 elm 215 eln 216 elo 217 elp 218 elq 219 elr 220 els 221 elt 222 elu 223 elv 224 elw 225 elx 226 ely 227 elz 228 em 229 en 230 eo 231 ep 232 eq 233 er 234 es 235 et 236 eu 237 ev 238 ew 239 ex 240 ey 241 ez 242  f 243 fa 244 fb 245 fc 246 fd 247 fe 248 ff 249 fg 250 fh 251 fi 252 fj 253 fk 254 fl 255 fm 256 fn 257 fo 258 fp 259 fq 260 fr 261 fs 262 ft 263 fu 264 fv 265 fw 266 fx 267 fy 268 fz 269  g 270 ga 271 gb 272 gc 273 gd 274 ge 275 gf 276 gg 277 gh 278 gi 279 gj 280 gk 281 gl 282 gm 283 gn 284 go 285 gp 286 gq 287 gr 288 gs 289 gt 290 gu 291 gv 292 gw 293 gx 294 gy 295 gz 296  h 297 ha 298 hb 299 hc 300 hd 301 he 302 henry 303 hf 304 hg 305 hh 306 hi 307 hj 308 hk 309 hl 310 hm 311 hn 312 ho 313 hp 314 hq 315 hr 316 hs 317 ht 318 hu 319 hv 320 hw 321 hx 322 hy 323 hz 324  i 325 ia 326 ib 327 ic 328 id 329 ie 330 if 331 ig 332 ih 333 ii 334 ij 335 ik 336 il 337 im 338 in 339 io 340 ip 341 iq 342 ir 343 is 344 it 345 iu 346 iv 347 iw 348 ix 349 iy 350 iz 351  j 352 ja 353 jaa 354 jab 355 jac 356 jad 357 jae 358 jaf 359 jag 360 jah 361 jai 362 jaj 363 jak 364 jal 365 jam 366 james 367 jan 368 jao 369 jap 370 jaq 371 jar 372 jas 373 jat 374 jau 375 jav 376 jaw 377 jax 378 jay 379 jaz 380 jb 381 jc 382 jd 383 je 384 jea 385 jeb 386 jec 387 jed 388 jee 389 jef 390 jeg 391 jeh 392 jei 393 jej 394 jek 395 jel 396 jem 397 jen 398 jeo 399 jep 400 jeq 401 jer 402 jes 403 jet 404 jeu 405 jev 406 jew 407 jex 408 jey 409 jez 410 jf 411 jg 412 jh 413 ji 414 jj 415 jk 416 jl 417 jm 418 jn 419 jo 420 joa 421 job 422 joc 423 jod 424 joe 425 jof 426 jog 427 joh 428 john 429 joi 430 joj 431 jok 432 jol 433 jom 434 jon 435 joo 436 jop 437 joq 438 jor 439 jos 440 joseph 441 jot 442 jou 443 jov 444 jow 445 jox 446 joy 447 joz 448 jp 449 jq 450 jr 451 js 452 jt 453 ju 454 jv 455 jw 456 jx 457 jy 458 jz 459  k 460 ka 461 kb 462 kc 463 kd 464 ke 465 kf 466 kg 467 kh 468 ki 469 kj 470 kk 471 kl 472 km 473 kn 474 ko 475 kp 476 kq 477 kr 478 ks 479 kt 480 ku 481 kv 482 kw 483 kx 484 ky 485 kz 486  l 487 la 488 lb 489 lc 490 ld 491 le 492 lea 493 leb 494 lec 495 led 496 lee 497 lef 498 leg 499 leh 500 lei 501 lej 502 lek 503 lel 504 lem 505 len 506 leo 507 lep 508 leq 509 ler 510 les 511 let 512 leu 513 lev 514 lew 515 lex 516 ley 517 lez 518 lf 519 lg 520 lh 521 li 522 lj 523 lk 524 ll 525 lm 526 ln 527 lo 528 loa 529 lob 530 loc 531 lod 532 loe 533 lof 534 log 535 loh 536 loi 537 loj 538 lok 539 lol 540 lom 541 lon 542 loo 543 lop 544 loq 545 lor 546 los 547 lot 548 lou 549 lov 550 low 551 lox 552 loy 553 loz 554 lp 555 lq 556 lr 557 ls 558 lt 559 lu 560 lv 561 lw 562 lx 563 ly 564 lz 565  m 566 ma 567 maa 568 mab 569 mac 570 mad 571 mae 572 maf 573 mag 574 mah 575 mai 576 maj 577 mak 578 mal 579 mam 580 man 581 mao 582 map 583 maq 584 mar 585 margaret 586 mary 587 mas 588 mat 589 mau 590 mav 591 maw 592 max 593 may 594 maz 595 mb 596 mc 597 md 598 me 599 mf 600 mg 601 mh 602 mi 603 mj 604 mk 605 ml 606 mm 607 mn 608 mo 609 mp 610 mq 611 mr 612 ms 613 mt 614 mu 615 mv 616 mw 617 mx 618 my 619 mz 620  n 621 na 622 nb 623 nc 624 nd 625 ne 626 nf 627 ng 628 nh 629 ni 630 nj 631 nk 632 nl 633 nm 634 nn 635 no 636 np 637 nq 638 nr 639 ns 640 nt 641 nu 642 nv 643 nw 644 nx 645 ny 646 nz 647  o 648 oa 649 ob 650 oc 651 od 652 oe 653 of 654 og 655 oh 656 oi 657 oj 658 ok 659 ol 660 om 661 on 662 oo 663 op 664 oq 665 or 666 os 667 ot 668 ou 669 ov 670 ow 671 ox 672 oy 673 oz 674  p 675 pa 676 pb 677 pc 678 pd 679 pe 680 pf 681 pg 682 ph 683 pi 684 pj 685 pk 686 pl 687 pm 688 pn 689 po 690 pp 691 pq 692 pr 693 ps 694 pt 695 pu 696 pv 697 pw 698 px 699 py 700 pz 701  q 702 qa 703 qb 704 qc 705 qd 706 qe 707 qf 708 qg 709 qh 710 qi 711 qj 712 qk 713 ql 714 qm 715 qn 716 qo 717 qp 718 qq 719 qr 720 qs 721 qt 722 qu 723 qv 724 qw 725 qx 726 qy 727 qz 728  r 729 ra 730 rb 731 rc 732 rd 733 re 734 rf 735 rg 736 rh 737 ri 738 rj 739 rk 740 rl 741 rm 742 rn 743 ro 744 robert 745 rp 746 rq 747 rr 748 rs 749 rt 750 ru 751 rv 752 rw 753 rx 754 ry 755 rz 756  s 757 sa 758 sb 759 sc 760 sd 761 se 762 sf 763 sg 764 sh 765 si 766 sj 767 sk 768 sl 769 sm 770 sn 771 so 772 sp 773 sq 774 sr 775 ss 776 st 777 su 778 sv 779 sw 780 sx 781 sy 782 sz 783  t 784 ta 785 tb 786 tc 787 td 788 te 789 tf 790 tg 791 th 792 ti 793 tj 794 tk 795 tl 796 tm 797 tn 798 to 799 tp 800 tq 801 tr 802 ts 803 tt 804 tu 805 tv 806 tw 807 tx 808 ty 809 tz 810  u 811 ua 812 ub 813 uc 814 ud 815 ue 816 uf 817 ug 818 uh 819 ui 820 uj 821 uk 822 ul 823 um 824 un 825 uo 826 up 827 uq 828 ur 829 us 830 ut 831 uu 832 uv 833 uw 834 ux 835 uy 836 uz 837  v 838 va 839 vb 840 vc 841 vd 842 ve 843 vf 844 vg 845 vh 846 vi 847 vj 848 vk 849 vl 850 vm 851 vn 852 vo 853 vp 854 vq 855 vr 856 vs 857 vt 858 vu 859 vv 860 vw 861 vx 862 vy 863 vz 864  w 865 wa 866 wb 867 wc 868 wd 869 we 870 wf 871 wg 872 wh 873 wi 874 wia 875 wib 876 wic 877 wid 878 wie 879 wif 880 wig 881 wih 882 wii 883 wij 884 wik 885 wil 886 william 887 wim 888 win 889 wio 890 wip 891 wiq 892 wir 893 wis 894 wit 895 wiu 896 wiv 897 wiw 898 wix 899 wiy 900 wiz 901 wj 902 wk 903 wl 904 wm 905 wn 906 wo 907 wp 908 wq 909 wr 910 ws 911 wt 912 wu 913 wv 914 ww 915 wx 916 wy 917 wz 918  x 919 xa 920 xb 921 xc 922 xd 923 xe 924 xf 925 xg 926 xh 927 xi 928 xj 929 xk 930 xl 931 xm 932 xn 933 xo 934 xp 935 xq 936 xr 937 xs 938 xt 939 xu 940 xv 941 xw 942 xx 943 xy 944 xz 945  y 946 ya 947 yb 948 yc 949 yd 950 ye 951 yf 952 yg 953 yh 954 yi 955 yj 956 yk 957 yl 958 ym 959 yn 960 yo 961 yp 962 yq 963 yr 964 ys 965 yt 966 yu 967 yv 968 yw 969 yx 970 yy 971 yz 972  z 973 za 974 zb 975 zc 976 zd 977 ze 978 zf 979 zg 980 zh 981 zi 982 zj 983 zk 984 zl 985 zm 986 zn 987 zo 988 zp 989 zq 990 zr 991 zs 992 zt 993 zu 994 zv 995 zw 996 zx 997 zy 998 zz 999 MMM - Middle Name, CodedThis is the middle name, coded using the above table.As a special case, if there is no middle name and the first name is fully coded (say, "John" or "Mary"), this is 000. If the first name is not fully coded, encode the first unused character from the first name on this table and use it as MMM:a 001 b 002 c 003 d 004 e 005 f 006 g 007 h 008 i 009 j 010 k 011 l 012 m 013 n 014 o 015 p 016 q 017 r 018 s 019 t 020 u 021 v 022 w 023 x 024 y 025 z 026 BBB - Birth day and month, CodedLook up the birst day of month and the birth month on this table to find the find three characters.In the event of two or more people having identical driver's licence numbers, this final group of digits will be used to differeniate them. Simple add one to the final group of digits until you find an unused entry. If you reach a number allocated to a different date, instead subtract one until you find an unused entry. I don't know what is down if while moving down you hit a number allocated to another date or when you generate a number over 999 or below 001.January (001)01 => 002 02 => 007 03 => 010 04 => 012 05 => 017 06 => 020 07 => 022 08 => 025 09 => 027 10 => 030 11 => 032 12 => 035 13 => 037 14 => 040 15 => 042 16 => 045 17 => 047 18 => 050 19 => 052 20 => 055 21 => 057 22 => 060 23 => 062 24 => 065 25 => 067 26 => 070 27 => 072 28 => 075 29 => 077 30 => 080 31 => 082 February (085)01 => 086 02 => 088 03 => 091 04 => 093 05 => 096 06 => 098 07 => 101 08 => 103 09 => 106 10 => 108 11 => 111 12 => 113 13 => 116 14 => 118 15 => 121 16 => 123 17 => 126 18 => 128 19 => 131 20 => 133 21 => 136 22 => 138 23 => 141 24 => 143 25 => 146 26 => 148 27 => 151 28 => 153 29 => 156 March (158)01 => 159 02 => 162 03 => 164 04 => 167 05 => 169 06 => 172 07 => 174 08 => 177  09 => 182 10 => 184 11 => 187 12 => 189 13 => 192 14 => 194 15 => 197 16 => 199 17 => 202 18 => 204 19 => 207  20 => 227 21 => 229 22 => 232 23 => 234 24 => 237 25 => 239 26 => 242 27 => 244 28 => 247 29 => 249 30 => 252 31 => 254 April (257)01 => 258 02 => 260 03 => 263 04 => 265 05 => 268 06 => 270 07 => 273 08 => 275 09 => 278 10 => 280 11 => 283 12 => 285 13 => 288 14 => 290 15 => 293 16 => 295 17 => 298 18 => 300 19 => 303 20 => 305 21 => 308 22 => 310 23 => 313 24 => 315 25 => 318 26 => 320 27 => 323 28 => 325 29 => 328 30 => 330 May (333)01 => 334 02 => 336 03 => 339 04 => 341 05 => 344 06 => 346 07 => 349 08 => 351 09 => 354 10 => 356 11 => 359 12 => 361 13 => 364 14 => 366 15 => 369 16 => 371 17 => 374 18 => 376 19 => 379 20 => 381 21 => 384 22 => 386 23 => 389 24 => 391 25 => 394 26 => 396 27 => 399 28 => 401 29 => 404 30 => 406 31 => 409 June (411)01 => 412 02 => 415 03 => 417 04 => 420 05 => 422 06 => 425 07 => 427 08 => 430 09 => 432 10 => 435 11 => 437 12 => 440 13 => 442 14 => 445 15 => 447 16 => 450 17 => 452 18 => 467 19 => 470 20 => 472 21 => 475 22 => 477 23 => 480 24 => 482 25 => 497 26 => 500 27 => 502 28 => 505 29 => 507  30 => 517 July (520)01 => 521 02 => 523 03 => 526 04 => 528 05 => 534 06 => 537 07 => 539 08 => 542 09 => 544 10 => 547 11 => 549 12 => 552 13 => 554 14 => 557 15 => 559 16 => 562 17 => 564 18 => 567 19 => 569 20 => 572 21 => 574 22 => 577 23 => 579 24 => 582 25 => 584 26 => 587 27 => 589 28 => 592 29 => 594 30 => 597 31 => 599 August (602) 01 => 603 02 => 605 03 => 608 04 => 610 05 => 613 06 => 615 07 => 618 08 => 620 09 => 623 10 => 625 11 => 628 12 => 630 13 => 633 14 => 635 15 => 638 16 => 640 17 => 643 18 => 645 19 => 648 20 => 650 21 => 653 22 => 655 23 => 658 24 => 660 25 => 663 26 => 665 27 => 668 28 => 670 29 => 673 30 => 675 31 => 678 September (680)01 => 681 02 => 684 03 => 686 04 => 689 05 => 691 06 => 694 07 => 696 08 => 699 09 => 701 10 => 704 11 => 706 12 => 709 13 => 711 14 => 714 15 => 716 16 => 719 17 => 721 18 => 724 19 => 726 20 => 729 21 => 731 22 => 734 23 => 736 24 => 739 25 => 741 26 => 744 27 => 746 28 => 749 29 => 751 30 => 754 October (756)01 => 757 02 => 760 03 => 762 04 => 765 05 => 767 06 => 770 07 => 772 08 => 775 09 => 777 10 => 780 11 => 782 12 => 785 13 => 787 14 => 790 15 => 792 16 => 797 17 => 800 18 => 802 19 => 807 20 => 810 21 => 812 22 => 815 23 => 817 24 => 820 25 => 822 26 => 825 27 => 827 28 => 830 29 => 832 30 => 835 31 => 837 November (840)01 => 841 02 => 843 03 => 846 04 => 848 05 => 851 06 => 853 07 => 856 08 => 858 09 => 861 10 => 863 11 => 866 12 => 868 13 => 871 14 => 873 15 => 876 16 => 878 17 => 881 18 => 883 19 => 886 20 => 888 21 => 891 22 => 893 23 => 896 24 => 898 25 => 901 26 => 903 27 => 906 28 => 908 29 => 911 30 => 913 December (916)01 => 917 02 => 919 03 => 922 04 => 924 05 => 927 06 => 929 07 => 932 08 => 934 09 => 937 10 => 939 11 => 942 12 => 944 13 => 947 14 => 949 15 => 952 16 => 954 17 => 957 18 => 959 19 => 962 20 => 964 21 => 967 22 => 969 23 => 972 24 => 974 25 => 977 26 => 983 27 => 985 28 => 990 29 => 993 30 => 995 31 => 998 Thanks to Joseph Gallian for providing me with the information on which this is based.