Question

[编辑] 使用gdb调试器运行代码后，movapd确实不是问题所在（感谢所有指出这一点的人）。逐行进行，故障在于xmm7和xmm0寄存器之间的第二次比较。流控制如下所示：

检查半径较大的一切都很好；

comisd xmm1, xmm3进行相应比较
（p $xmm1.v2_double $1 = {10, 0} p $xmm3.v2_double $2 = {2, 0}）

认为xmm1更大，并跳转到first标签

如果comisd以某种方式推断出xmm7大于xmm1

，那么事情就出错了

（{p $xmm7.v2_double $3 = {1.4142135623730951, 1}

p $xmm1.v2_double $4 = {10, 0}）

（另外，我非常确定gdb中print命令大括号中的值是相反的顺序。我的意思是在xmm1中10实际上在较低的四字中，而在较高的四字中为零。，所以我无法理解1.41如何大于10）

这是汇编代码

#extern int circles(int n, double* cr);

.intel_syntax   noprefix

.data

three:      .int 3

.text

.global circles

circles:

        enter   0, 0


        mov     rax, 1
        cpuid
        test    rdx, 0x2000000
        jz      notSupported

        mov     rbx, rsp
        and     rsp, 0xfffffffffffffff0
        sub     rsp, 512
        fxsave  [rsp]



        xor     r10, r10    #pair counter
        xor     r8, r8      #outter loop counter
        xor     r9, r9      #inner loop counter
        mov     rax, rdi
        mul     dword ptr three
        sub     rax, 3
        #rax value 3n-3 since our step is length of 3

        pivotCircle:

        cmp     r8, rax
        je      done

        #this is the line where it goes wrong 
        movupd  xmm0, [rsi + 8*r8]          #first circle center
        movsd   xmm1, [rsi + 8*r8 + 16]     #radius first circle

        mov     r9, r8

        nextCircle:
        add     r9, 3

        cmp     r9, rax
        jg      nextPivot

        movupd  xmm2, [rsi + 8*r9]          #second circle center
        movsd   xmm3, [rsi + 8*r9 + 16]     #second circle radius

        #calculating distance between the centers

        movapd  xmm7, xmm0
        subpd   xmm7, xmm2
        mulpd   xmm7, xmm7
        movapd  xmm2, xmm7
        shufpd  xmm2, xmm2, 0b11
        addsd   xmm7, xmm2
        sqrtsd  xmm7, xmm7                  # |c1 - c2| in xmm7



        #checking which radius is bigger

        comisd  xmm1, xmm3      

        jge     first
        jmp     second

        first:                  #first one greater

        comisd  xmm7, xmm1
        jge     nextCircle
        movsd   xmm6, xmm1
        subsd   xmm6, xmm7
        comisd  xmm3, xmm6
        jg      nextCircle

        jmp     found

        second:                 #second one greater

        comisd  xmm7, xmm3
        jge     nextCircle
        movsd   xmm6, xmm3
        subsd   xmm6, xmm7
        comisd  xmm1, xmm6
        jg      nextCircle

        found:

        inc     r10
        jmp     nextCircle

        nextPivot:
        add     r8, 3
        jmp pivotCircle

        done:

        fxrstor [rsp]
        mov     rsp, rbx

        mov     rax, r10
        leave
        ret


        notSupported:

        mov     rdi, 1
        call    exit

这是main.c文件。数组中的元素以三个（a，b，半径）为一组。

#include    <stdio.h>
#include    <stdlib.h>
#include    <assert.h>


extern int circles(int n, double* cr);


int main(int argc, char const *argv[])
{
    int n;

    double* cr;

    scanf("%d", &n);
    assert(n > 0);

    cr = malloc(n * sizeof(double) * 3);
    assert(cr != NULL);

    for (int i = 0; i < n; i++)
    {
        scanf("%lf%lf%lf", cr+i*3, cr+i*3+1, cr+i*3+2);
        assert(*(cr+3*i+2) > 0);
    }

    printf("%d\n", circles(n, cr));


    return 0;
}

COMISD比较不正确

0 个答案: