internal static value128 compute_product_approximation(int bitPrecision, long q, ulong w)
{
int index = 2 * (int)(q - Constants.smallest_power_of_five);
// For small values of q, e.g., q in [0,27], the answer is always exact because
// The line value128 firstproduct = full_multiplication(w, power_of_five_128[index]);
// gives the exact answer.
value128 firstproduct = FullMultiplication(w, Constants.power_of_five_128[index]);
//static_assert((bit_precision >= 0) && (bit_precision <= 64), " precision should be in (0,64]");
ulong precision_mask = (bitPrecision < 64) ? ((ulong)(0xFFFFFFFFFFFFFFFF) >> bitPrecision) : (ulong)(0xFFFFFFFFFFFFFFFF);
if ((firstproduct.high & precision_mask) == precision_mask)
{ // could further guard with (lower + w < lower)
// regarding the second product, we only need secondproduct.high, but our expectation is that the compiler will optimize this extra work away if needed.
value128 secondproduct = FullMultiplication(w, Constants.power_of_five_128[index + 1]);
firstproduct.low += secondproduct.high;
if (secondproduct.high > firstproduct.low)
{
firstproduct.high++;
}
}
return(firstproduct);
}
/// <summary>
///
/// </summary>
/// <param name="q"></param>
/// <param name="w"></param>
///
/// <returns></returns>
internal static AdjustedMantissa ComputeFloat(long q, ulong w)
{
var answer = new AdjustedMantissa();
if ((w == 0) || (q < FloatBinaryConstants.smallest_power_of_ten))
{
answer.power2 = 0;
answer.mantissa = 0;
// result should be zero
return(answer);
}
if (q > FloatBinaryConstants.largest_power_of_ten)
{
// we want to get infinity:
answer.power2 = FloatBinaryConstants.infinite_power;
answer.mantissa = 0;
return(answer);
}
// At this point in time q is in [smallest_power_of_five, largest_power_of_five].
// We want the most significant bit of i to be 1. Shift if needed.
int lz = BitOperations.LeadingZeroCount(w);
w <<= lz;
// The required precision is mantissa_explicit_bits() + 3 because
// 1. We need the implicit bit
// 2. We need an extra bit for rounding purposes
// 3. We might lose a bit due to the "upperbit" routine (result too small, requiring a shift)
value128 product = Utils.compute_product_approximation(FloatBinaryConstants.mantissa_explicit_bits + 3, q, w);
if (product.low == 0xFFFFFFFFFFFFFFFF)
{ // could guard it further
// In some very rare cases, this could happen, in which case we might need a more accurate
// computation that what we can provide cheaply. This is very, very unlikely.
//
bool inside_safe_exponent = (q >= -27) && (q <= 55); // always good because 5**q <2**128 when q>=0,
// and otherwise, for q<0, we have 5**-q<2**64 and the 128-bit reciprocal allows for exact computation.
if (!inside_safe_exponent)
{
answer.power2 = -1; // This (a negative value) indicates an error condition.
return(answer);
}
}
// The "compute_product_approximation" function can be slightly slower than a branchless approach:
// value128 product = compute_product(q, w);
// but in practice, we can win big with the compute_product_approximation if its additional branch
// is easily predicted. Which is best is data specific.
int upperbit = (int)(product.high >> 63);
answer.mantissa = product.high >> (upperbit + 64 - FloatBinaryConstants.mantissa_explicit_bits - 3);
answer.power2 = (int)(Utils.power((int)(q)) + upperbit - lz - FloatBinaryConstants.minimum_exponent);
if (answer.power2 <= 0)
{ // we have a subnormal?
// Here have that answer.power2 <= 0 so -answer.power2 >= 0
if (-answer.power2 + 1 >= 64)
{ // if we have more than 64 bits below the minimum exponent, you have a zero for sure.
answer.power2 = 0;
answer.mantissa = 0;
// result should be zero
return(answer);
}
// next line is safe because -answer.power2 + 1 < 64
answer.mantissa >>= -answer.power2 + 1;
// Thankfully, we can't have both "round-to-even" and subnormals because
// "round-to-even" only occurs for powers close to 0.
answer.mantissa += (answer.mantissa & 1); // round up
answer.mantissa >>= 1;
// There is a weird scenario where we don't have a subnormal but just.
// Suppose we start with 2.2250738585072013e-308, we end up
// with 0x3fffffffffffff x 2^-1023-53 which is technically subnormal
// whereas 0x40000000000000 x 2^-1023-53 is normal. Now, we need to round
// up 0x3fffffffffffff x 2^-1023-53 and once we do, we are no longer
// subnormal, but we can only know this after rounding.
// So we only declare a subnormal if we are smaller than the threshold.
answer.power2 = (answer.mantissa < ((ulong)(1) << FloatBinaryConstants.mantissa_explicit_bits)) ? 0 : 1;
return(answer);
}
// usually, we round *up*, but if we fall right in between and and we have an
// even basis, we need to round down
// We are only concerned with the cases where 5**q fits in single 64-bit word.
if ((product.low <= 1) && (q >= FloatBinaryConstants.min_exponent_round_to_even) && (q <= FloatBinaryConstants.max_exponent_round_to_even) &&
((answer.mantissa & 3) == 1))
{ // we may fall between two floats!
// To be in-between two floats we need that in doing
// answer.mantissa = product.high >> (upperbit + 64 - mantissa_explicit_bits() - 3);
// ... we dropped out only zeroes. But if this happened, then we can go back!!!
if ((answer.mantissa << (upperbit + 64 - FloatBinaryConstants.mantissa_explicit_bits - 3)) == product.high)
{
answer.mantissa &= ~(ulong)(1); // flip it so that we do not round up
}
}
answer.mantissa += (answer.mantissa & 1); // round up
answer.mantissa >>= 1;
if (answer.mantissa >= ((ulong)(2) << FloatBinaryConstants.mantissa_explicit_bits))
{
answer.mantissa = ((ulong)(1) << FloatBinaryConstants.mantissa_explicit_bits);
answer.power2++; // undo previous addition
}
answer.mantissa &= ~((ulong)(1) << FloatBinaryConstants.mantissa_explicit_bits);
if (answer.power2 >= FloatBinaryConstants.infinite_power)
{ // infinity
answer.power2 = FloatBinaryConstants.infinite_power;
answer.mantissa = 0;
}
return(answer);
}