[Libevent-users] optimize evutil_gettimeofday()'s Windows implementation

[Dave Hart <davehart@xxxxxxxxx>]

The code for evutil_gettimeofday() on Windows is:

	/* Conversion logic taken from Tor, which in turn took it
	 * from Perl.  GetSystemTimeAsFileTime returns its value as
	 * an unaligned (!) 64-bit value containing the number of
	 * 100-nanosecond intervals since 1 January 1601 UTC. */
#define EPOCH_BIAS U64_LITERAL(116444736000000000)
#define UNITS_PER_SEC U64_LITERAL(10000000)
#define USEC_PER_SEC U64_LITERAL(1000000)
#define UNITS_PER_USEC U64_LITERAL(10)
	union {
		FILETIME ft_ft;
		ev_uint64_t ft_64;
	} ft;

	if (tv == NULL)
		return -1;

	GetSystemTimeAsFileTime(&ft.ft_ft);

	if (EVUTIL_UNLIKELY(ft.ft_64 < EPOCH_BIAS)) {
		/* Time before the unix epoch. */
		return -1;
	}
	ft.ft_64 -= EPOCH_BIAS;
	tv->tv_sec = (long) (ft.ft_64 / UNITS_PER_SEC);
	tv->tv_usec = (long) ((ft.ft_64 / UNITS_PER_USEC) % USEC_PER_SEC);
	return 0;

Note it ends with three 64-bit divisions.  Given the routine can be
(though shouldn't be) a hot path, a micro-optimization seems worth
considering:

	ft.ft_64 -= EPOCH_BIAS;
	tv->tv_sec = (long) (ft.ft_64 / UNITS_PER_SEC);
	tv->tv_usec = (long) ((ft.ft_64 - (UNITS_PER_SEC * tv->tv_sec)) /
UNITS_PER_USEC);

That substitutes a multiplication followed by subtraction in place of
the modulo 1 million.  While I haven't tested it, I believe mul + sub
will be faster than modulo (division).  I'm not actually using
libevent on Windows at this point.

Cheers,
Dave Hart
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