cpp / expert
Snippet
Optimizing Sequential Access via Loop Interleaving
Duff's Device is a classic expert-level technique for loop unrolling. It combines a switch statement with a do-while loop to reduce the overhead of jump instructions and condition checking. While modern compilers often unroll simple loops, this manual pattern remains relevant in specific low-level optimization scenarios and state machine implementations.
snippet.cpp
cpp
1
2
3
4
5
6
7
8
9
10
11
12
13
14
void fast_copy(int* __restrict dest, const int* __restrict src, size_t count) {size_t n = (count + 7) / 8;switch (count % 8) {case 0: do { *dest++ = *src++;case 7: *dest++ = *src++;case 6: *dest++ = *src++;case 5: *dest++ = *src++;case 4: *dest++ = *src++;case 3: *dest++ = *src++;case 2: *dest++ = *src++;case 1: *dest++ = *src++;} while (--n > 0);}}
Breakdown
1
switch (count % 8)
Handles the 'remainder' elements before entering the main unrolled block.
2
case 0: do { ... } while (--n > 0);
Creates a fall-through mechanism that jumps directly into the loop body.