New submission from Nikita Kniazev: I have failed to find previous discussion when `while (n--)` was changed to `n++; while (--n)`. (commits 306d6b1ea6bf2582b9284be2fd27275abbade3e1, 165eee214bc388eb588db33385ca49ddbb305565)
It is clear for me that n++; while (--n) and for (; n; --n) are interchangable statements, but here is the prof of it http://coliru.stacked-crooked.com/a/a6fc4108b223e7b2. According to asm out https://godbolt.org/g/heHM33 the `for` loop is even shorter (takes less instructions). While I believe that location of `cmp`/`jmp` instruction makes no sense and performance is the same, but I have made a benchmark. ``` Run on (4 X 3310 MHz CPU s) 02/27/17 22:10:55 Benchmark Time CPU Iterations ------------------------------------------------------------ BM_while_loop/2 13 ns 13 ns 56089384 BM_while_loop/4 17 ns 16 ns 40792279 BM_while_loop/8 24 ns 24 ns 29914338 BM_while_loop/16 40 ns 40 ns 20396140 BM_while_loop/32 84 ns 80 ns 8974301 BM_while_loop/64 146 ns 146 ns 4487151 BM_while_loop/128 270 ns 269 ns 2492862 BM_while_loop/128 267 ns 266 ns 2639500 BM_while_loop/512 1022 ns 1022 ns 641022 BM_while_loop/4096 8203 ns 8344 ns 89743 BM_while_loop/32768 66971 ns 66750 ns 11218 BM_while_loop/262144 545833 ns 546003 ns 1000 BM_while_loop/2097152 4376095 ns 4387528 ns 160 BM_while_loop/8388608 17654654 ns 17883041 ns 41 BM_for_loop/2 13 ns 13 ns 56089384 BM_for_loop/4 15 ns 15 ns 49857230 BM_for_loop/8 21 ns 21 ns 32051077 BM_for_loop/16 37 ns 37 ns 19509351 BM_for_loop/32 81 ns 80 ns 8974301 BM_for_loop/64 144 ns 128 ns 4985723 BM_for_loop/128 265 ns 263 ns 3205108 BM_for_loop/128 265 ns 266 ns 2639500 BM_for_loop/512 1036 ns 1022 ns 641022 BM_for_loop/4096 8314 ns 8344 ns 89743 BM_for_loop/32768 67345 ns 66750 ns 11218 BM_for_loop/262144 541310 ns 546004 ns 1000 BM_for_loop/2097152 4354986 ns 4387528 ns 160 BM_for_loop/8388608 17592428 ns 17122061 ns 41 ``` ```cpp #include <benchmark/benchmark.h> #define MAKE_ROTL_BENCHMARK(name) \ static void BM_##name(benchmark::State& state) { \ while (state.KeepRunning()) { \ int n = name(state.range(0)); \ } \ } \ /**/ int while_loop(int n) { int sum = 0; n++; while (--n) { sum += 1; } return sum; } int for_loop(int n) { int sum = 0; for(; n; --n) { sum += 1; } return sum; } MAKE_ROTL_BENCHMARK(while_loop) MAKE_ROTL_BENCHMARK(for_loop) BENCHMARK(BM_while_loop)->RangeMultiplier(2)->Range(2, 8<<4); BENCHMARK(BM_while_loop)->Range(8<<4, 8<<20); BENCHMARK(BM_for_loop)->RangeMultiplier(2)->Range(2, 8<<4); BENCHMARK(BM_for_loop)->Range(8<<4, 8<<20); BENCHMARK_MAIN() ``` ---------- components: Interpreter Core messages: 288815 nosy: Kojoley priority: normal pull_requests: 329 severity: normal status: open title: _collectionsmodule.c: Replace `n++; while (--n)` with `for (; n; --n)` type: enhancement versions: Python 3.6, Python 3.7 _______________________________________ Python tracker <rep...@bugs.python.org> <http://bugs.python.org/issue29698> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
