在MSVC中将扩展的精度浮点数(80位)转换为双精度(64位)

问题描述

在MSVC win32/win64中从扩展精度浮点数(80位值，在某些编译器中也称为long double)到双精度(64位)转换的最可移植，最正确"的方法是什么? /p>

MSVC当前(截至2010年)假定long double是double的同义词.

我可能可以在内联汇编中编写fld/fstp汇编程序对，但是内联汇编不适用于MSVC中的win64代码.我是否需要将此汇编代码移动到单独的.asm文件?真的是这样，没有好的解决方法吗?

解决方案

这只是在x86代码中完成的...

    .686P
    .XMM

_TEXT   SEGMENT

EXTRN   __fltused:DWORD

PUBLIC  _cvt80to64
PUBLIC  _cvt64to80

_cvt80to64 PROC

    mov eax, dword ptr [esp+4]
    fld TBYTE PTR [eax]

    ret 0
_cvt80to64 ENDP


_cvt64to80 PROC
    mov eax, DWORD PTR [esp+12]
    fld QWORD PTR [esp+4]
    fstp    TBYTE PTR [eax]
    ret 0
_cvt64to80 ENDP

ENDIF

_TEXT   ENDS
    END

What is the most portable and "right" way to do conversion from extended precision float (80-bit value, also known as long double in some compilers) to double (64-bit) in MSVC win32/win64?

MSVC currently (as of 2010) assumes that long double is double synonym.

I could probably write fld/fstp assembler pair in inline asm, but inline asm is not available for win64 code in MSVC. Do I need to move this assembler code to separate .asm file? Is that really so there are no good solution?

解决方案

Just did this in x86 code...

    .686P
    .XMM

_TEXT   SEGMENT

EXTRN   __fltused:DWORD

PUBLIC  _cvt80to64
PUBLIC  _cvt64to80

_cvt80to64 PROC

    mov eax, dword ptr [esp+4]
    fld TBYTE PTR [eax]

    ret 0
_cvt80to64 ENDP


_cvt64to80 PROC
    mov eax, DWORD PTR [esp+12]
    fld QWORD PTR [esp+4]
    fstp    TBYTE PTR [eax]
    ret 0
_cvt64to80 ENDP

ENDIF

_TEXT   ENDS
    END

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