发生了什么事对你来说，当C程序被编译成汇编语言，你的toupper函数会弄成这个样子，也许是：

What happened for you is that when the C program was compiled into assembly language, your toUpper function ended up like this, perhaps:

_toUpper:
LFB4:
        pushq   %rbp
LCFI3:
        movq    %rsp, %rbp
LCFI4:
        movb    %dil, -4(%rbp)
        cmpb    $96, -4(%rbp)
        jle     L8
        cmpb    $122, -4(%rbp)
        jg      L8
        movzbl  -4(%rbp), %eax
        subl    $32, %eax
        movb    %al, -4(%rbp)
L8:
        leave
        ret

32的减法是在％eax寄存器进行。而在x86调用约定，就是在返回值预计将注册！所以...你真的很幸运。

The subtraction of 32 was carried out in the %eax register. And in the x86 calling convention, that is the register in which the return value is expected to be! So... you got lucky.

但请注意警告。他们是有原因的！