Linux C语言内联汇编-条件跳转

void jump(){
    int i = 10, sum = 0;

    __asm__("movl $0, %0;"
            "a:;"
            "cmpl $0, %2;"
            "jle b;"
            "addl %2, %0;"
            "subl $1, %1;"
            "jmp a;"
            "b:;"
            :"=r"(sum), "=r"(i)
            :"1"(i)
    );

    cout << sum;
}

当sum的约束条件是"r"时，内联汇编中应该初始化一下，movl $0, %0; 

相当于初始化sum的中转寄存器;

比如下面，编译器选择%edx来暂存sum，如果不清零操作，其中的值是不确定的；

再看看生成的.s文件

	subq	$16, %rsp
	movl	$10, -4(%rbp)
	movl	$0, -8(%rbp)
	movl	-4(%rbp), %eax
#APP
# 52 "main.cpp" 1
	movl $0, %edx;a:;cmpl $0, %eax;jle b;addl %eax, %edx;subl $1, %eax;jmp a;b:;
# 0 "" 2
#NO_APP
	movl	%edx, -8(%rbp)
	movl	%eax, -4(%rbp)
	movl	-8(%rbp), %eax
	movl	%eax, %esi
	leaq	_ZSt4cout(%rip), %rdi
	call	[email protected]
	nop