[boost] xtime_get unsafe on Windows for IA64

From: Ben Hutchings (ben.hutchings_at_[hidden])
Date: 2004-08-25 11:41:54

The implementation of the boost::xtime_get() function in
libs/thread/src/xtime.cpp includes the following code for Windows:

#if defined(BOOST_HAS_FTIME)
        FILETIME ft;
        ...
        const boost::uint64_t TIMESPEC_TO_FILETIME_OFFSET =
            ((boost::uint64_t)27111902UL << 32) +
            (boost::uint64_t)3577643008UL;
        xtp->sec = (int)((*(__int64*)&ft - TIMESPEC_TO_FILETIME_OFFSET)
                         / 10000000);
        xtp->nsec = (int)((*(__int64*)&ft - TIMESPEC_TO_FILETIME_OFFSET
                           - ((__int64)xtp->sec * (__int64)10000000))
                          * 100);
        return clock_type;
#elif ...

(This is reformatted slightly so that Outlook doesn't screw it up.)

The pointer casts are not safe because although the representations
of FILETIME and __int64 are compatible, FILETIME is only 32-bit-
aligned whereas __int64 must be 64-bit-aligned on IA64 (in some
processor modes).

One can instead use union { FILETIME ft; __int64 i64; } and read out
i64, though I'm not sure this code should be unconditionally using
__int64 anyway.  There is a longer explanation at
<http://weblogs.asp.net/oldnewthing/archive/2004/08/25/220195.aspx>.

Ben.