FPRSqrtEstimate: Improve documentation of RecipSqrtEstimate

This commit is contained in:
MerryMage 2018-07-23 11:25:14 +01:00
parent 0a64a66b26
commit bda86fd167

View file

@ -20,7 +20,9 @@
namespace Dynarmic::FP {
/// Input is a u1.8 fixed point number.
/// Input is a u0.9 fixed point number. Only values in [0.25, 1.0) are valid.
/// Output is a u0.8 fixed point number, with an implied 1 prefixed.
/// i.e.: The output is a value in [1.0, 2.0).
static u8 RecipSqrtEstimate(u64 a) {
using LUT = std::array<u8, 512>;
@ -28,15 +30,25 @@ static u8 RecipSqrtEstimate(u64 a) {
LUT result{};
for (u64 i = 128; i < result.size(); i++) {
u64 a = i;
// Convert to u.10 (with 8 significant bits), force to odd
if (a < 256) {
// [0.25, 0.5)
a = a * 2 + 1;
} else {
// [0.5, 1.0)
a = (a | 1) * 2;
}
// Calculate largest b which for which b < 1.0 / sqrt(a).
// Start from b = 1.0 (in u.9) since b cannot be smaller.
u64 b = 512;
// u.10 * u.9 * u.9 -> u.28
while (a * (b + 1) * (b + 1) < (1u << 28)) {
b++;
}
// Round to nearest u0.8 (with implied set integer bit).
result[i] = static_cast<u8>((b + 1) / 2);
}
return result;