汇编64位:如何返回双精度值? [英] Assembly 64bit: How to return a double value?

问题描述

如何在Assembler中返回64位值? 我试过了:

How do I return in Assembler a 64bit value ? I tried this:

C程序:

#include <stdio.h>

double result=0;
double a = 10;
extern double func(double a);

 int main() {       
    result = func(a);
    printf("result: %f\n", result);     
    return 0;
   }

组装:

      section .bss
      x: resq 1

      section .text 

      global func

      func:

      movq qword[x],xmm0
      fld qword [x]
      fld qword [x]
      fadd
      movq xmm0,qword[x]

      ret

应返回20.0，但始终为10.0 我怎么了

It should return 20.0 but instead it is always 10.0 What did I wrong?

推荐答案

@ petch @ Michael Petch指出，使用以下代码可以使整个函数更高效:

@Michael Petch noted that the whole function could be much more efficient with the following code:

addsd xmm0, xmm0   ; Add input parameter to itself
ret                ; Done!  (return values go in xmm0)

x86-64在XMM寄存器(而不是内存或x87堆栈)中传递/返回double.(适用于x86-64 System V ABI/调用约定和Windows x64.请参阅 x86标签wiki )

发布的代码没有评论.评论它会对OP有所帮助，所以...

The code posted didn't have comments. Commenting it would have helped the OP, so...

;; Buggy original version with comments
movq qword[x],xmm0  ; Store current value in memory  [Why?]
fld qword [x]       ; Load current value from memory [Why??]
fld qword [x]       ; Load current value from memory again
fadd                ; Add top two stack items

movq xmm0,qword[x]  ; reload original value from memory, unmodified

@ElderBug指出，OP在执行最后的movq之前忘记将fadd的结果存储到内存中，因此该函数只是返回其输入，例如
double foo(double x) { return x; }，但在x87堆栈上留下了垃圾.

@ElderBug noted that the OP forgot to store the result of the fadd into memory before doing the final movq, so this function simply returns its input, like
double foo(double x) { return x; } but leaving garbage on the x87 stack.

@Michael Petch继续指出，原始代码在浮点堆栈上留下了大量碎片"-没有尝试使用各种pop版本的指令(fstp，或faddp而不是fadd).这样就为下一个浮点函数留出了更少的空间-直到最终导致浮点堆栈溢出，从而导致意外的NaN！

@Michael Petch went on to note that the original code left a large amount of 'debris' on the floating point stack - there was no attempt to clean it up with various pop versions of the instructions (fstp, or faddp instead of fadd). This leaves less room for the next floating point function - until finally a floating-point stack overflow is caused, resulting in an unexpected NaN!

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