gcc分支预测

Question

这是我的演示程序:

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

int cmp(const void *d1, const void *d2)
{
    int a, b;

    a = *(int const *) d1;
    b = *(int const *) d2;

    if (a > b)
        return 1;
    else if (a == b)
        return 0;

    return -1;
}

int main()
{
    int seed = time(NULL);
    srandom(seed);

    int i, n, max = 32768, a[max];

    for (n=0; n < max; n++) {

        int r = random() % 256;
        a[n] = r;

    }

    qsort(a, max, sizeof(int), cmp);

    clock_t beg = clock();

    long long int sum = 0;

    for (i=0; i < 20000; i++) 
    {
        for (n=0; n < max; n++) {
            if (a[n] >= 128)
                sum += a[n];
        }
    }

    clock_t end = clock();

    double sec = (end - beg) / CLOCKS_PER_SEC;

    printf("sec: %f\n", sec);
    printf("sum: %lld\n", sum);

    return 0;
}



unsorted
sec: 5.000000
sum: 63043880000

sorted
sec: 1.000000
sum: 62925420000

这是程序的两个版本的汇编差异,一个有qsort一个,一个没有:

--- unsorted.s  
+++ sorted.s    
@@ -58,7 +58,7 @@
    shrl    $4, %eax
    sall    $4, %eax
    subl    %eax, %esp
-   leal    4(%esp), %eax
+   leal    16(%esp), %eax
    addl    $15, %eax
    shrl    $4, %eax
    sall    $4, %eax
@@ -83,6 +83,13 @@
    movl    -16(%ebp), %eax
    cmpl    -24(%ebp), %eax
    jl  .L7
+   movl    -24(%ebp), %eax
+   movl    $cmp, 12(%esp)
+   movl    $4, 8(%esp)
+   movl    %eax, 4(%esp)
+   movl    -32(%ebp), %eax
+   movl    %eax, (%esp)
+   call    qsort
    movl    $0, -48(%ebp)
    movl    $0, -44(%ebp)
    movl    $0, -12(%ebp)

据我所知,汇编输出,由于传递值,排序版本只有更多的代码qsort,但我没有看到任何分支优化/预测/任何事情.也许我看错了方向？

Answer 1

分支预测不是您在汇编代码级别会看到的; 它由CPU本身完成.