更新时间:2023-02-17 10:26:41
它们完全相同。
要自己测试一下,运行 gcc -S test.c
(假设这是您放置源代码的地方)并观察内容 test.s
。
To test this yourself, run gcc -S test.c
(presuming that this is where you've put your source) and observe the contents of test.s
.
以下是嵌套的方法 - 如果
方法在gcc 4.8.1中使用默认选项编译(带注释注释):
Here's how the nested-if
approach compiles in gcc 4.8.1 with default options (annotated with comments):
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&&
方法的编制方式:
Here's how the &&
approach compiles:
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