Function ConvertFloat80ToDouble

IDEHelper/WinDebugger.cpp:408–474

conversion logic based on table at http://en.wikipedia.org/wiki/Extended_precision CDH TODO put this somewhere more general

// conversion logic based on table at http://en.wikipedia.org/wiki/Extended_precision
//CDH TODO put this somewhere more general
static double ConvertFloat80ToDouble(const byte fp80[10])
{
	uint16 e = *((uint16*)&fp80[8]);
	uint64 m = *((uint64*)&fp80[0]);
	uint64 bit63 = (uint64)1 << 63;
	uint64 bit62 = (uint64)1 << 62;

	bool isNegative = (e & 0x8000) != 0;
	double s = isNegative ? -1.0 : 1.0;
	e &= 0x7fff;

	if (!e)
	{
		// the high bit and mantissa content will determine whether it's an actual zero, or a denormal or
		// pseudo-denormal number with an effective exponent of -16382.  But since that exponent is so far
		// below anything we can handle in double-precision (even accounting for denormal bit shifts), we're
		// effectively still dealing with zero.
		return s * 0.0;
	}
	else if (e == 0x7fff)
	{
		if (m & bit63)
		{
			if (m & bit62)
			{
				return std::numeric_limits<double>::quiet_NaN();
			}
			else
			{
				if (m == bit63)
					return s * std::numeric_limits<double>::infinity();
				else
					return std::numeric_limits<double>::signaling_NaN();
			}
		}
		else
		{
			return std::numeric_limits<double>::quiet_NaN();
		}
	}
	else
	{
		if (!(m & bit63))
			return std::numeric_limits<double>::quiet_NaN(); // unnormal (we don't handle these since 80387 and later treat them as invalid operands anyway)

		// else is a normalized value
	}

	int useExponent = (int)e - 16383;
	if (useExponent < -1022)
		return s * 0.0; // we could technically support e from -1023 to -1074 as denormals, but don't bother with that for now.
	else if (useExponent > 1023)
		return s * HUGE_VAL;

	useExponent += 1023;

	BF_ASSERT((useExponent > 0) && (useExponent < 0x7ff)); // assume we've filtered for valid exponent range
	BF_ASSERT(m & bit63); // assume we've filtered out values that aren't normalized by now