From: Ben Hutchings (ben.hutchings_at_[hidden])
Date: 2004-08-27 10:45:39
Michael Glassford <glassfordm_at_[hidden]> wrote:
> Ben Hutchings wrote:
> > The implementation of the boost::xtime_get() function in
> > libs/thread/src/xtime.cpp includes the following code for Windows:
<snip>
> > 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>.
>
> How about this instead:
>
> const boost::uint64_t TIMESPEC_TO_FILETIME_OFFSET =
> (static_cast<boost::uint64_t>(27111902UL) << 32)
> + 3577643008UL;
I think this should use the UINT64_C macro instead of messing about
with casts.
> const boost::uint64_t ft64 =
> (static_cast<boost::uint64_t>(ft.dwHighDateTime) << 32)
> + ft.dwLowDateTime;
>
> xtp->sec = static_cast<int>(
> (ft64 - TIMESPEC_TO_FILETIME_OFFSET) / 10000000
> );
The cast should be to int_fast64_t rather than to int.
> xtp->nsec = static_cast<int>((
> ft64 - TIMESPEC_TO_FILETIME_OFFSET
> - (static_cast<boost::uint64_t>(xtp->sec) * 10000000)
> ) * 100
> );
>
> ?
The cast should be to int_fast32_t. Also the use of multiplication
and subtraction to calculate the remainder seems to be a premature
optimisation that obscures what's really going on. A smart compiler
can combine the division and remainder operations, and I have
confirmed that Visual C++ 7.1 does so when generating optimised i386
code (even for 64-bit division where it uses a subroutine rather than
a single instruction). So I would suggest:
static const boost::uint64_t TIMESPEC_TO_FILETIME_OFFSET =
UINT64_C(116444736000000000);
const boost::uint64_t ft64 =
(static_cast<boost::uint64_t>(ft.dwHighDateTime) << 32)
+ ft.dwLowDateTime;
// or use the union trick which MS appears to have blessed
xtp->sec = static_cast<int_fast64_t>(
(ft64 - TIMESPEC_TO_FILETIME_OFFSET) / 10000000
);
xtp->nsec = static_cast<int_fast32_t>(
((ft64 - TIMESPEC_TO_FILETIME_OFFSET) % 10000000) * 100
);
Ben.