为A + = B比在C = A + B更有效？ [英] Is a += b more efficient than a = a + b in C?

问题描述

我知道在一些语言如下：

I know in some languages the following:

a += b

的效率比：

a = a + b

，因为它消除了对创建临时变量的需要。这是在C的情况下？它是更有效地使用+ =（因此也 - = * = 等）

推荐答案

所以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\n", 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\n", 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

...对于我计算机和我编译器上运行的我操作系统。结果可以或可以不变化。在我系统，但是，时间是相同的：用户时间2.120s

...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.

现在只是给你看pressive现代编译器如何IM就可以了，你会注意到，我使用了前pression 在A * A 分配。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\n", 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\n"
    .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

编译器想通了，我的循环，它展开来计算累积和的点，只是嵌入，作为一个常数，它进行打印出来，跳过任何一种循环完全建构的。在优化的脸聪明，你真的认为你会发现任何有意义的边缘 S =秒之间区别+ A 和 S + = A ？！

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