嵌套if语句和"&&"运算符

Question

if(a() && b() && c() && d())
   doSomething();


if(a())
   if(b()) 
      if(c())
         if(d())
            doSomething();

这两者之间是否存在"任何"性能差异？

例如,在a()变为0的情况下,它是否会在第一个if语句中继续运行b(),c()和d()？或者它会与第二个嵌套的if语句一样工作吗？

Answer 1

它们完全相同.

要自己测试一下,运行gcc -S test.c(假设这是你放置源代码的地方)并观察内容test.s.

以下是嵌套if方法如何使用默认选项(使用注释注释)在gcc 4.8.1中编译:

main:
.LFB0:
    .cfi_startproc
    pushq   %rbp
    .cfi_def_cfa_offset 16
    .cfi_offset 6, -16
    movq    %rsp, %rbp
    .cfi_def_cfa_register 6
    movl    $0, %eax
    call    A                        # try to call A
    testl   %eax, %eax               # look at its return value
    je  .L3                          # short-circuit if it returned 0
    movl    $0, %eax                 # ...repeat for B, et al.
    call    B
    testl   %eax, %eax
    je  .L3
    movl    $0, %eax
    call    C
    testl   %eax, %eax
    je  .L3
    movl    $0, %eax
    call    D
    testl   %eax, %eax
    je  .L3
    movl    $0, %eax
    call    doSomething
.L3:
    popq    %rbp
    .cfi_def_cfa 7, 8
    ret
    .cfi_endproc

以下是该&&方法的编译方式:

main:
.LFB0:
    .cfi_startproc
    pushq   %rbp
    .cfi_def_cfa_offset 16
    .cfi_offset 6, -16
    movq    %rsp, %rbp
    .cfi_def_cfa_register 6
    movl    $0, %eax
    call    A                           # try to call A
    testl   %eax, %eax                  # look at its return value
    je  .L3                             # short-circuit if it returned 0
    movl    $0, %eax                    # ...repeat for B, et al.
    call    B
    testl   %eax, %eax
    je  .L3
    movl    $0, %eax
    call    C
    testl   %eax, %eax
    je  .L3
    movl    $0, %eax
    call    D
    testl   %eax, %eax
    je  .L3
    movl    $0, %eax
    call    doSomething
.L3:
    popq    %rbp
    .cfi_def_cfa 7, 8
    ret
    .cfi_endproc