Fixed The C Loop So you don't have to. #285
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loops/c/code.c
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| int main (int argc, char** argv) { | ||
| int u = atoi(argv[1]); // Get an input number from the command line | ||
| srand(time(NULL)); // FIX random seed | ||
| int r = rand() % 10000; // Get a random integer 0 <= r < 10k | ||
| int32_t a[10000] = {0}; // Array of 10k elements initialized to 0 | ||
| for (int i = 0; i < 10000; i++) { // 10k outer loop iterations | ||
| for (int j = 0; j < 100000; j++) { // 100k inner loop iterations, per outer loop iteration | ||
| a[i] = a[i] + j%u; // Simple sum | ||
| a[i] = a[i] + REM(j,u) // Simple sum |
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COOL, but why the compiler does not do
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I don't know to be honest, the only time I saw a compiler optimizes modules is if you tried doing module power of 2, in that case it replaces it with "and" instruction, so for example if
a = 35
b = 32
a % b = a & (b - 1)
which in this case is 3
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Thanks for contributing! 🙏
Can you explain for someone not familiar with Casey? What are the trade-offs? (I am assuming there are trade-offs since the compiler doesn't do this optimization.) And please also provide benchmarks showing what effects the change has to motivate the use of a macro.
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@PEZ @He-Pin full explanation can be found in this video (https://www.youtube.com/watch?v=RrHGX1wwSYM), thing is the original code can't be vectorized, it uses idiv instruction (for example on x86 machines) which is very slow, compared to this version also uses division however this version the compiler can vectorize it, by using SIMD (Single Instruction Multiple Data), instead of working on one piece of data one instruction can manipulate multiple data, why the compiler doesn't do that already is out of my scope to be honest. |
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Without enabling a more advanced instruction set like Avx2 or Avx512 this change might actually slow things down. Additionally the same change could be made for many other languages as well. |
Added a casey inspired macro.
Thanks,