What is the effect of ordering if…else if statements by probability?
Specifically, if I have a series of if...else if statements, and I somehow know beforehand the relative probability that each statement will evalua
-
Just my 5 cents. It seems the effect of ordering if statements should depend on:
Probability of each if statement.
Number of iterations, so the branch predictor could kick in.
Likely/unlikely compiler hints, i.e. code layout.
To explore those factors, I benchmarked the following functions:
ordered_ifs()
for (i = 0; i < data_sz * 1024; i++) { if (data[i] < check_point) // highly likely s += 3; else if (data[i] > check_point) // samewhat likely s += 2; else if (data[i] == check_point) // very unlikely s += 1; }reversed_ifs()
for (i = 0; i < data_sz * 1024; i++) { if (data[i] == check_point) // very unlikely s += 1; else if (data[i] > check_point) // samewhat likely s += 2; else if (data[i] < check_point) // highly likely s += 3; }ordered_ifs_with_hints()
for (i = 0; i < data_sz * 1024; i++) { if (likely(data[i] < check_point)) // highly likely s += 3; else if (data[i] > check_point) // samewhat likely s += 2; else if (unlikely(data[i] == check_point)) // very unlikely s += 1; }reversed_ifs_with_hints()
for (i = 0; i < data_sz * 1024; i++) { if (unlikely(data[i] == check_point)) // very unlikely s += 1; else if (data[i] > check_point) // samewhat likely s += 2; else if (likely(data[i] < check_point)) // highly likely s += 3; }data
The data array contains random numbers between 0 and 100:
const int RANGE_MAX = 100; uint8_t data[DATA_MAX * 1024]; static void data_init(int data_sz) { int i; srand(0); for (i = 0; i < data_sz * 1024; i++) data[i] = rand() % RANGE_MAX; }The Results
The following results are for Intel i5@3,2 GHz and G++ 6.3.0. The first argument is the check_point (i.e. probability in %% for the highly likely if statement), the second argument is data_sz (i.e. number of iterations).
--------------------------------------------------------------------- Benchmark Time CPU Iterations --------------------------------------------------------------------- ordered_ifs/50/4 4660 ns 4658 ns 150948 ordered_ifs/50/8 25636 ns 25635 ns 27852 ordered_ifs/75/4 4326 ns 4325 ns 162613 ordered_ifs/75/8 18242 ns 18242 ns 37931 ordered_ifs/100/4 1673 ns 1673 ns 417073 ordered_ifs/100/8 3381 ns 3381 ns 207612 reversed_ifs/50/4 5342 ns 5341 ns 126800 reversed_ifs/50/8 26050 ns 26050 ns 26894 reversed_ifs/75/4 3616 ns 3616 ns 193130 reversed_ifs/75/8 15697 ns 15696 ns 44618 reversed_ifs/100/4 3738 ns 3738 ns 188087 reversed_ifs/100/8 7476 ns 7476 ns 93752 ordered_ifs_with_hints/50/4 5551 ns 5551 ns 125160 ordered_ifs_with_hints/50/8 23191 ns 23190 ns 30028 ordered_ifs_with_hints/75/4 3165 ns 3165 ns 218492 ordered_ifs_with_hints/75/8 13785 ns 13785 ns 50574 ordered_ifs_with_hints/100/4 1575 ns 1575 ns 437687 ordered_ifs_with_hints/100/8 3130 ns 3130 ns 221205 reversed_ifs_with_hints/50/4 6573 ns 6572 ns 105629 reversed_ifs_with_hints/50/8 27351 ns 27351 ns 25568 reversed_ifs_with_hints/75/4 3537 ns 3537 ns 197470 reversed_ifs_with_hints/75/8 16130 ns 16130 ns 43279 reversed_ifs_with_hints/100/4 3737 ns 3737 ns 187583 reversed_ifs_with_hints/100/8 7446 ns 7446 ns 93782Analysis
1. The Ordering Does Matter
For 4K iterations and (almost) 100% probability of highly liked statement the difference is huge 223%:
--------------------------------------------------------------------- Benchmark Time CPU Iterations --------------------------------------------------------------------- ordered_ifs/100/4 1673 ns 1673 ns 417073 reversed_ifs/100/4 3738 ns 3738 ns 188087For 4K iterations and 50% probability of highly liked statement the difference is about 14%:
--------------------------------------------------------------------- Benchmark Time CPU Iterations --------------------------------------------------------------------- ordered_ifs/50/4 4660 ns 4658 ns 150948 reversed_ifs/50/4 5342 ns 5341 ns 1268002. Number of Iterations Does Matter
The difference between 4K and 8K iterations for (almost) 100% probability of highly liked statement is about two times (as expected):
--------------------------------------------------------------------- Benchmark Time CPU Iterations --------------------------------------------------------------------- ordered_ifs/100/4 1673 ns 1673 ns 417073 ordered_ifs/100/8 3381 ns 3381 ns 207612But the difference between 4K and 8K iterations for 50% probability of highly liked statement is 5,5 times:
--------------------------------------------------------------------- Benchmark Time CPU Iterations --------------------------------------------------------------------- ordered_ifs/50/4 4660 ns 4658 ns 150948 ordered_ifs/50/8 25636 ns 25635 ns 27852Why is so? Because of branch predictor misses. Here is the branch misses for each mentioned above case:
ordered_ifs/100/4 0.01% of branch-misses ordered_ifs/100/8 0.01% of branch-misses ordered_ifs/50/4 3.18% of branch-misses ordered_ifs/50/8 15.22% of branch-missesSo on my i5 the branch predictor fails spectacularly for not-so-likely branches and large data sets.
3. Hints Help a Bit
For 4K iterations the results are somewhat worse for 50% probability and somewhat better for close to 100% probability:
--------------------------------------------------------------------- Benchmark Time CPU Iterations --------------------------------------------------------------------- ordered_ifs/50/4 4660 ns 4658 ns 150948 ordered_ifs/100/4 1673 ns 1673 ns 417073 ordered_ifs_with_hints/50/4 5551 ns 5551 ns 125160 ordered_ifs_with_hints/100/4 1575 ns 1575 ns 437687But for 8K iterations the results are always a bit better:
--------------------------------------------------------------------- Benchmark Time CPU Iterations --------------------------------------------------------------------- ordered_ifs/50/8 25636 ns 25635 ns 27852 ordered_ifs/100/8 3381 ns 3381 ns 207612 ordered_ifs_with_hints/50/8 23191 ns 23190 ns 30028 ordered_ifs_with_hints/100/8 3130 ns 3130 ns 221205So, the hints also help, but just a tiny bit.
Overall conclusion is: always benchmark the code, because the results may surprise.
Hope that helps.
加载中...
- 热议问题