Question

我使用的是Windows 7 64位，使用VS2013 Express for Desktop。

默认编译（使用SSE）

D:\Work>cl printf-test.c
Microsoft (R) C/C++ Optimizing Compiler Version 18.00.21005.1 for x86

D:\Work>printf-test.exe
FPU control word 0x0000037f
0. FPU status 0x00000000
1. 0x43380000 0x00000000
1. FPU status 0x00000000
2. 0x43380000 0x00000001
2. FPU status 0x00000000
3. 0x43380000 0x00000001          //@@1
3. FPU status 0x00000000

仅使用x87进行编译。

D:\Work>cl /arch:IA32 printf-test.c
Microsoft (R) C/C++ Optimizing Compiler Version 18.00.21005.1 for x86

D:\Work>printf-test.exe
FPU control word 0x0000037f
0. FPU status 0x00000000
1. 0x43380000 0x00000000
1. FPU status 0x00000020
2. 0x43380000 0x00000001   
2. FPU status 0x00000220
3. 0x43380000 0x00000000         //@@1 this is the difference. why?
3. FPU status 0x00000020

测试代码如下

#include <stdio.h>

void print_double_hex( double d , int idx)
{
    union UD
    {
        double d;
        int i[2];
    } c;
    c.d = d + 6755399441055744.0;
    printf("%d. 0x%08x 0x%08x\n", idx, c.i[1], c.i[0]);
}

void PrintControlWord()
{
    int a;
    int c;
    __asm {
        fstcw [esp - 4]
        mov eax, [esp - 4]
        and eax, 0xFFFF
        mov c, eax
    }
    printf("FPU control word 0x%08x\n", c);
}

void PrintStatus(int idx)
{
    int a;
    int c;
    __asm {
        fstsw [esp - 4]
        mov eax, [esp - 4]
        and eax, 0xFFFF
        mov c, eax
    }
    printf("%d. FPU status 0x%08x\n", idx, c);
}

int main()
{
    double a = .5;
    double b = 0.501;
    double c = 0.5001;
    int fpu = 0;

    __asm {                 // @@2 Remove it, see result below
        finit
    }

    PrintControlWord();
    PrintStatus(0); 

    print_double_hex(a, 1);
    PrintStatus(1); 

    print_double_hex(b, 2);
    PrintStatus(2);

    print_double_hex(c, 3);
    PrintStatus(3);

    return 0;
}

请注意，如果我将其删除，我会手动初始化FPU。结果会改变！

删除finit后，编译为默认

D:\Work>cl printf-test.c
Microsoft (R) C/C++ Optimizing Compiler Version 18.00.21005.1 for x86

D:\Work>printf-test.exe
FPU control word 0x0000027f      //@@2 FPU control word changed, why?
0. FPU status 0x00000000
1. 0x43380000 0x00000000
1. FPU status 0x00000000
2. 0x43380000 0x00000001
2. FPU status 0x00000000
3. 0x43380000 0x00000001
3. FPU status 0x00000000

仅使用x87进行编译

D:\Work>cl /arch:IA32 printf-test.c
Microsoft (R) C/C++ Optimizing Compiler Version 18.00.21005.1 for x86

D:\Work>printf-test.exe
FPU control word 0x0000027f
0. FPU status 0x00000000
1. 0x43380000 0x00000000
1. FPU status 0x00000020
2. 0x43380000 0x00000001
2. FPU status 0x00000020
3. 0x43380000 0x00000001         //@@3, why?
3. FPU status 0x00000020

所以我的问题是：

@@1，为什么会有不同？这是一个错误吗？
对于@@3，为什么价值会发生变化？

使用不同的cl.exe编译器选项时的结果不同

0 个答案: