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| Subject | X[]==Y[] is 7X slower than it should be -- why? |
| From | Don <nospam@nospam.com.au> |
| Date | Fri, 20 Jun 2008 17:02:25 +0200 |
| Newsgroups | digitalmars.D |
I've been doing some timing tests, and found that the built-in array comparison is amazingly slow. Given: uint [2000] X, Y; bool b = (X[] == Y[]); on my Pentium M, this takes ~30000 cycles in the case where both arrays are equal. It's not hard to write an optimal asm routine; this takes 4000 cycles (7.5 times faster). But, it's also possible to perform the comparison in simple D code, something like: int compareArrays(uint [] x, uint y[]) { for (int i=0; i<x.length; ++i) { if (x[i] != y[i]) return x[i]-y[i]; } return 0; } and this takes 6000 cycles when compiled with -release -O, 5X better than the built-in version. So, why is the built-in compare so slow? Using the CPU performance counters: X[]==Y[] --> 10000 branches, 35000 memory loads, 30000 cycles. for loop --> 4000 branches, 6000 memory loads, 6000 cycles. asm --> 4000 branches, 4000 memory loads, 4000 cycles. What's it doing? Is it doing a byte-by-byte comparison? (it shouldn't do that even for strings). Is it doing bounds checking for every array element?? (even with -release -O) There's significant optimisation potential here. | |
X[]==Y[] is 7X slower than it should be -- why? (Current message)
