可能的（），并不太可能（）编译器提示的学习样本

问题描述

我怎么能证明学生的可用性可能和不可能编译器提示（ __builtin_expect ）？

How can I demonstrate for students the usability of likely and unlikely compiler hints (__builtin_expect)?

您可以写一个样本code，这将是更快的与这些提示不提示比较code几次。

Can you write an sample code, which will be several times faster with these hints comparing the code without hints.

推荐答案

下面是我使用的一个，一个真正的低效执行Fibonacci数的：

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;
}

为了演示，使用GCC：

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

几百毫秒以下。这个增益是由于程序员辅助分支prediction

A few hundred milliseconds less. This gain is due to the programmer-aided branch prediction.

但现在，什么程序员真正应该做的，而不是：

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

使用编译器辅助运行分析，我们成功地从原来的34.290s，以17.760s减少。多比程序员辅助分支prediction更好！

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!

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