visual c++ - Convert extended precision float (80-bit) to double (64-bit) in MSVC

Question

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?

Answer 1

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