FPRSqrtEstimate: Improve documentation of RecipSqrtEstimate

src/common/fp/op/FPRSqrtEstimate.cpp

@@ -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;