在 C 中 a += b 比 a = a + b 更有效吗? [英] Is a += b more efficient than a = a + b in C?

问题描述

我知道以下一些语言:

a += b

比:

a = a + b

因为它不需要创建临时变量.在 C 中是这种情况吗?使用 += 是否更有效(因此还有 -= *= 等)

because it removes the need for creating a temporary variable. Is this the case in C? Is it more efficient to use += (and, therefore also -= *= etc)

推荐答案

这里有一个明确的答案...

So here's a definitive answer...

$ cat junk1.c
#include <stdio.h>

int main()
{
    long a, s = 0;
    for (a = 0; a < 1000000000; a++)
    {
        s = s + a * a;
    }
    printf("Final sum: %ld
", s);
}

michael@isolde:~/junk$ cat junk2.c
#include <stdio.h>

int main()
{
    long a, s = 0;
    for (a = 0; a < 1000000000; a++)
    {
        s += a * a;
    }
    printf("Final sum: %ld
", s);
}

michael@isolde:~/junk$ for a in *.c ; do gcc -O3 -o ${a%.c} $a ; done
michael@isolde:~/junk$ time ./junk1
Final sum: 3338615082255021824

real    0m2.188s
user    0m2.120s
sys 0m0.000s
michael@isolde:~/junk$ time ./junk2
Final sum: 3338615082255021824

real    0m2.179s
user    0m2.120s
sys 0m0.000s

...用于在我的 操作系统上运行的我的 计算机和我的 编译器.您的结果可能会也可能不会有所不同.然而，在我的系统上，时间是相同的:用户时间 2.120 秒.

...for my computer and my compiler running on my operating system. Your results may or may not vary. On my system, however, the time is identical: user time 2.120s.

现在只是为了向您展示现代编译器是多么令人印象深刻，您会注意到我在赋值中使用了表达式 a * a.这是因为这个小问题:

Now just to show you how impressive modern compilers can be, you'll note that I used the expression a * a in the assignment. This is because of this little problem:

$ cat junk.c
#include <stdio.h>

int main()
{
    long a, s = 0;
    for (a = 0; a < 1000000000; a++)
    {
        s = s + a;
    }
    printf("Final sum: %ld
", s);
}

michael@isolde:~/junk$ gcc -O3 -S junk.c
michael@isolde:~/junk$ cat junk.s 
    .file   "junk.c"
    .section    .rodata.str1.1,"aMS",@progbits,1
.LC0:
    .string "Final sum: %ld
"
    .text
    .p2align 4,,15
.globl main
    .type   main, @function
main:
.LFB22:
    .cfi_startproc
    movabsq $499999999500000000, %rdx
    movl    $.LC0, %esi
    movl    $1, %edi
    xorl    %eax, %eax
    jmp __printf_chk
    .cfi_endproc
.LFE22:
    .size   main, .-main
    .ident  "GCC: (Ubuntu 4.4.3-4ubuntu5) 4.4.3"
    .section    .note.GNU-stack,"",@progbits

编译器找出我的循环并将其展开到计算累积总和的程度，然后将其嵌入为一个常量，然后继续打印出来，完全跳过任何类型的循环构造.面对如此聪明的优化器，您真的认为您会在区分 s = s + a 和 s += a 时找到任何有意义的优势吗?！

The compiler figured out my loop and unrolled it to the point of calculating the cumulative sum and just embedded that as a constant which it proceeded to print out, skipping any kind of looping construct entirely. In the face of optimizers that clever do you really think you're going to find any meaningful edge in distinguishing between s = s + a and s += a?!

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