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我在 Windows 7 64bit 上，使用 VS2013 Express for Desktop。

编译为默认值（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 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，如果我删除它，我会手动初始化 FPU。结果会改变！

删除后finit，默认编译

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 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. 对于@@1，为什么会有不同？它是一个错误吗？

  2. 对于@@3，为什么值改变了？

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0 回答 0