程序集 64 位无效有效地址

Question

我正在创建一个可以从 64 位 asm 中的 C 代码调用的函数这是 C 等效函数：

 /*
 * x and y are two arrays of floats, this function calculates the
 * distance beetween those two objects
 */
float dist(float *x, float *y, int length)
{
    float c = 0;
    int i;
    for (i = 0; i < length; ++i)
    {
        c += (x[i] - y[i]) * (x[i] - y[i]);

    }

    return  sqrt(c);
}

这是汇编代码：

section .text       

global distanza64

distanza64:



         push    rbp             ; save base pointer
         mov     rbp, rsp         
         pushaq                  ; save general registers


         ; C function
         ; float dist(float *x, float *y, int length)
         ; in xmm0 there is *x, in xmm1 float *y,  in rdi there is length 
loop:
         cmp rdi, 0              ; cycle counter
         je end_loop
         movss xmm2, [xmm0]      ; x[i]
         subss xmm2, [xmm1]      ; x[i] = x[i] - y[i] i.e (a-b)
         mulss xmm2, xmm2        ; x[i] = x[i] * x[i] i.e (a-b)*(a-b)
         addss xmm3, xmm2        ; c += x[i] i.e c = (a-b)*(a-b)
         addsd xmm0, 8           ; vgo to next address 8*8 = 64-bit
         addsd xmm1, 8           ; same as above
         dec   rdi               ; length--
end_loop:
         sqrtss xmm3, xmm3       ; c = sqrt(c)
         movss  xmm0, xmm3       ; in xmm0 there is the final value

         popaq                   
         mov     rsp, rbp        
         pop     rbp             
        ret

我使用nasm编译：nasm -f elf64 distanza.asm
问题是当我尝试使用 xmm0 和 xmm1:

处的地址获取 x[i] 和 y[i] 的值时

 movss xmm2, [xmm0]      
 subss xmm2, [xmm1]

无法编译：有效地址无效。如何使用存储在 xmm0 中的地址来获取内存中的值？我必须使用 xmm0，因为它是存储参数 float *x 的寄存器。

Answer 1

我相信一个地址只能使用通用寄存器，比如rax，加上rip。您需要在 "dereferencing" 之前将值移出 1。然而，我必须警告，我已经有一段时间没有组装了。

编辑

以下是英特尔手册中的相关部分：

3.7.5.1 Specifying an Offset in 64-Bit Mode

The offset part of a memory address in 64-bit mode can be specified directly as a static value or through an address computation made up of one or more of the following components:

Displacement — An 8-bit, 16-bit, or 32-bit value.

Base — The value in a 64-bit general-purpose register.

Index — The value in a 64-bit general-purpose register.

Scale factor — A value of 2, 4, or 8 that is multiplied by the index value.

The base and index value can be specified in one of sixteen available general-purpose registers in most cases. See Chapter 2, “Instruction Format,” in the Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 2A.

The following unique combination of address components is also available.

RIP + Displacement ⎯ In 64-bit mode, RIP-relative addressing uses a signed 32-bit displacement to calculate the effective address of the next instruction by sign-extend the 32-bit value and add to the 64-bit value in RIP.

这与@Jester 提到的调用约定问题不同。

Answer 2

float* 是一个指针，它肯定不在 xmm0.

中

; float dist(float *x, float *y, int length)

; in xmm0 there is *x, in xmm1 float *y, in rdi there is length

实际上，rdi是*x，rsi是*y，rdx是length。阅读 abi documentation or the overview at wikipedia.

另外 pusha/popa 在 64 位模式下不存在。

程序集 64 位无效有效地址

Assembly 64-bit invalid effective address

c

64-bit

assembly

x86-64

calling-convention