c - learning sample of likely() and unlikely() compiler hints -

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how can demonstrate students usability of likely , unlikely compiler hints (__builtin_expect)?

can write sample code, several times faster these hints comparing code without hints.

here 1 use, inefficient implementation of fibonacci numbers:

#include <stdio.h> #include <inttypes.h> #include <time.h> #include <assert.h>  #define likely(x) __builtin_expect((x),1) #define unlikely(x) __builtin_expect((x),0)  uint64_t fib(uint64_t n) {     if (opt(n == 0 || n == 1)) {         return n;     } else {         return fib(n - 2) + fib(n - 1);     } }  int main(int argc, char **argv) {     int i, max = 45;     clock_t tm;      if (argc == 2) {         max = atoi(argv[1]);         assert(max > 0);     } else {         assert(argc == 1);     }      tm = -clock();     (i = 0; <= max; ++i)         printf("fib(%d) = %" priu64 "\n", i, fib(i));     tm += clock();      printf("time elapsed: %.3fs\n", (double)tm / clocks_per_sec);     return 0; }

to demonstrate, using gcc:

~% gcc -o2 -dopt= -o test-nrm test.c ~% ./test-nrm ... fib(45) = 1134903170 time elapsed: 34.290s  ~% gcc -o2 -dopt=unlikely -o test-opt test.c ~% ./test-opt ... fib(45) = 1134903170 time elapsed: 33.530s

a few hundred milliseconds less. gain due programmer-aided branch prediction.

but now, programmer should doing instead:

~% gcc -o2 -dopt= -fprofile-generate -o test.prof test.c ~% ./test.prof  ... fib(45) = 1134903170 time elapsed: 77.530s  /this run slowed down profile generation.  ~% gcc -o2 -dopt= -fprofile-use -o test.good test.c ~% ./test.good fib(45) = 1134903170 time elapsed: 17.760s

with compiler-aided runtime profiling, managed reduce original 34.290s 17.760s. better programmer-aided branch prediction!


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