MSVC win32：将扩展精度浮点数（80位）转换为双精度（64位）

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问题描述

在MSVC win32 / win64中，扩展精度浮点（80位值，在某些编译器中也称为long double）转换为double（64位）的最便携和正确方式是什么？ MSVC目前（截至2010年）假设long double是double的同义词。

我可能可以在内联asm编写fld / fstp汇编程序对，但内联asm不适用于MSVC中的win64代码。我是否需要将此汇编程序代码移到单独的.asm文件中？这真的是没有好的解决方案吗？

解决方案

$ b

  .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 
 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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