add comments

dbms/src/Functions/divide.cpp

@@ -67,14 +67,20 @@ struct TiDBDivideFloatingImpl<A, B, false>
         /// basically refer to https://stackoverflow.com/a/71634489
         if constexpr (std::is_integral_v<Result> || std::is_same_v<Result, Int256>)
         {
+            /// 1. do division first, get the quotient and mod, todo:(perf) find a unified `divmod` function to speed up this.
             Result quotient = x / d;
             Result mod = x % d;
+            /// 2. get the half of divisor, which is threshold to decide whether to round up or down.
+            /// note: don't directly use bit operation here, because it may cause unexpected result.
             Result half = (d / 2) + (d % 2);
 
+            /// 3. compare the abstract values of mod and half, if mod >= half, then round up.
             Result abs_m = mod < 0 ? -mod : mod;
             Result abs_h = half < 0 ? -half : half;
             if (abs_m >= abs_h)
             {
+                /// 4. now we need to round up, i.e., add 1 to the quotient's absolute value.
+                ///    if the signs of dividend and divisor are the same, then the quotient should be positive, otherwise negative.
                 if ((x < 0) == (d < 0)) // same_sign, i.e., quotient >= 0
                     quotient = quotient + 1;
                 else