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

Question

How can I demonstrate for students the usability of likely and unlikely compiler hints (__builtin_expect)?
Can you write an sample code, which will be several times faster with these hints comparing the code without hints.

Answer 1

Here is the one I use, a really inefficient implementation of the 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();
for (i = 0; i <= 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. This gain is due to the programmer-aided branch prediction.
But now, for what the programmer should really be doing instead:
~% gcc -O2 -Dopt= -fprofile-generate -o test.prof test.c
~% ./test.prof
...
fib(45) = 1134903170
Time elapsed: 77.530s /this run is slowed down by 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, we managed to reduce from the original 34.290s to 17.760s. Much better than with programmer-aided branch prediction!

Answer 2

From this blog post. I think likely and unlikely are mostly obsolete. Very cheap CPUs (ARM Cortex A20 in the example) have branch predictors and there is no penalty regardless of jump is taken / jump is not taken. When you introduce likely/unlikely the results will be either the same or worse (because compiler has generated more instructions).