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Possible Duplicate:
Not sure why we add the registers %rdx and %rax when the assembly code has been using %eax and %edx

all. So, here's my minimal program:

int main(int argc, char * argv[]){
    volatile int a;
    volatile int b;
    volatile int c;
    a = 9;
    b = 15;
    c = a+b;
    return c;
}

I now run gcc -S on this, and here's the meat of the generated assembly:

.LCFI1:
    movl    %edi, -20(%rbp)    //unknown setup
    movq    %rsi, -32(%rbp)    //unknown setup
    movl    $9, -4(%rbp)       //a=9, so -4(%rbp) is the register for a.
    movl    $15, -8(%rbp)      //b=15, so -8(%rbp) is b's register.
    movl    -4(%rbp), %edx     //move a into the register for addition.
    movl    -8(%rbp), %eax     //move b into the register for additon.
    leal    (%rdx,%rax), %eax  //Add a and b, store in %eax. (PROBLEM LINE)
    movl    %eax, -12(%rbp)    //Store %eax in c's register.
    movl    -12(%rbp), %eax    //get ready to return c. 
    leave                      //we're done here.
    ret

Okay, so you see the line I indicate as the problem line. Here's my question: What on earth are %rdx and %rax? The only registers I've loaded things into are %edx and %eax.

Since the program works, %rdx and %rax must be some sort of alias to %edx and %eax, accordingly. Can someone possibly explain the nuances of x86 register nomenclature? I'm completely in the dark on this.

(Of note, if I change the problem line to addl %edx, %eax, the result is identical, but if I change it to addl %rdx, %rax, I get "Error: Incorrect register '%rax' used with l suffix")

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如果您不确定寄存器及其大小，请查看此处。这样想

union{
  struct{
    uint32_t eax;
    uint32_t padd;
  };
  uint64_t rax;
};

eax并且rax共享同一个寄存器，eax是该寄存器的低部分。

这就是为什么addl不能使用以 r 为前缀的寄存器的原因，它们比addl预期的要长。试试addq吧。

于 2012-12-18T03:59:29.273 回答