Improve div_var_fast(), mostly by making comments better.

The integer overflow situation in div_var_fast() is a great deal more
complicated than the pre-existing comments would suggest.  Moreover, the
comments were also flat out incorrect as to the precise statement of the
maxdiv loop invariant.  Upon clarifying that, it becomes apparent that the
way in which we updated maxdiv after a carry propagation pass was overly
slow, complex, and conservative: we can just reset it to one, which is much
easier and also reduces the number of times carry propagation occurs.
Fix that and improve the relevant comments.

Since this is mostly a comment fix, with only a rather marginal performance
boost, no need for back-patch.

Tom Lane and Dean Rasheed

src/backend/utils/adt/numeric.c

@@ -6169,6 +6169,12 @@ div_var(NumericVar *var1, NumericVar *var2, NumericVar *result,
  *	exactly.  We compute DIV_GUARD_DIGITS extra digits, but there is
  *	no certainty that that's enough.  We use this only in the transcendental
  *	function calculation routines, where everything is approximate anyway.
+ *
+ *	Although we provide a "round" argument for consistency with div_var,
+ *	it is unwise to use this function with round=false.  In truncation mode
+ *	it is possible to get a result with no significant digits, for example
+ *	with rscale=0 we might compute 0.99999... and truncate that to 0 when
+ *	the correct answer is 1.
  */
 static void
 div_var_fast(NumericVar *var1, NumericVar *var2, NumericVar *result,
@@ -6251,8 +6257,9 @@ div_var_fast(NumericVar *var1, NumericVar *var2, NumericVar *result,
 
 	/*
 	 * We estimate each quotient digit using floating-point arithmetic, taking
-	 * the first four digits of the (current) dividend and divisor. This must
-	 * be float to avoid overflow.
+	 * the first four digits of the (current) dividend and divisor.  This must
+	 * be float to avoid overflow.  The quotient digits will generally be off
+	 * by no more than one from the exact answer.
 	 */
 	fdivisor = (double) var2digits[0];
 	for (i = 1; i < 4; i++)
@@ -6274,6 +6281,11 @@ div_var_fast(NumericVar *var1, NumericVar *var2, NumericVar *result,
 	 * To avoid overflow in maxdiv itself, it represents the max absolute
 	 * value divided by NBASE-1, ie, at the top of the loop it is known that
 	 * no div[] entry has an absolute value exceeding maxdiv * (NBASE-1).
+	 *
+	 * Actually, though, that holds good only for div[] entries after div[qi];
+	 * the adjustment done at the bottom of the loop may cause div[qi + 1] to
+	 * exceed the maxdiv limit, so that div[qi] in the next iteration is
+	 * beyond the limit.  This does not cause problems, as explained below.
 	 */
 	maxdiv = 1;
 
@@ -6325,10 +6337,10 @@ div_var_fast(NumericVar *var1, NumericVar *var2, NumericVar *result,
 
 				/*
 				 * All the div[] digits except possibly div[qi] are now in the
-				 * range 0..NBASE-1.
+				 * range 0..NBASE-1.  We do not need to consider div[qi] in
+				 * the maxdiv value anymore, so we can reset maxdiv to 1.
 				 */
-				maxdiv = Abs(newdig) / (NBASE - 1);
-				maxdiv = Max(maxdiv, 1);
+				maxdiv = 1;
 
 				/*
 				 * Recompute the quotient digit since new info may have
@@ -6348,7 +6360,16 @@ div_var_fast(NumericVar *var1, NumericVar *var2, NumericVar *result,
 				maxdiv += Abs(qdigit);
 			}
 
-			/* Subtract off the appropriate multiple of the divisor */
+			/*
+			 * Subtract off the appropriate multiple of the divisor.
+			 *
+			 * The digits beyond div[qi] cannot overflow, because we know they
+			 * will fall within the maxdiv limit.  As for div[qi] itself, note
+			 * that qdigit is approximately trunc(div[qi] / vardigits[0]),
+			 * which would make the new value simply div[qi] mod vardigits[0].
+			 * The lower-order terms in qdigit can change this result by not
+			 * more than about twice INT_MAX/NBASE, so overflow is impossible.
+			 */
 			if (qdigit != 0)
 			{
 				int			istop = Min(var2ndigits, div_ndigits - qi + 1);
@@ -6360,9 +6381,25 @@ div_var_fast(NumericVar *var1, NumericVar *var2, NumericVar *result,
 
 		/*
 		 * The dividend digit we are about to replace might still be nonzero.
-		 * Fold it into the next digit position.  We don't need to worry about
-		 * overflow here since this should nearly cancel with the subtraction
-		 * of the divisor.
+		 * Fold it into the next digit position.
+		 *
+		 * There is no risk of overflow here, although proving that requires
+		 * some care.  Much as with the argument for div[qi] not overflowing,
+		 * if we consider the first two terms in the numerator and denominator
+		 * of qdigit, we can see that the final value of div[qi + 1] will be
+		 * approximately a remainder mod (vardigits[0]*NBASE + vardigits[1]).
+		 * Accounting for the lower-order terms is a bit complicated but ends
+		 * up adding not much more than INT_MAX/NBASE to the possible range.
+		 * Thus, div[qi + 1] cannot overflow here, and in its role as div[qi]
+		 * in the next loop iteration, it can't be large enough to cause
+		 * overflow in the carry propagation step (if any), either.
+		 *
+		 * But having said that: div[qi] can be more than INT_MAX/NBASE, as
+		 * noted above, which means that the product div[qi] * NBASE *can*
+		 * overflow.  When that happens, adding it to div[qi + 1] will always
+		 * cause a cancelling overflow so that the end result is correct.  We
+		 * could avoid the intermediate overflow by doing the multiplication
+		 * and addition in int64 arithmetic, but so far there appears no need.
 		 */
 		div[qi + 1] += div[qi] * NBASE;
 
@@ -6381,9 +6418,12 @@ div_var_fast(NumericVar *var1, NumericVar *var2, NumericVar *result,
 	div[qi] = qdigit;
 
 	/*
-	 * Now we do a final carry propagation pass to normalize the result, which
-	 * we combine with storing the result digits into the output. Note that
-	 * this is still done at full precision w/guard digits.
+	 * Because the quotient digits might be off by one, some of them might be
+	 * -1 or NBASE at this point.  The represented value is correct in a
+	 * mathematical sense, but it doesn't look right.  We do a final carry
+	 * propagation pass to normalize the digits, which we combine with storing
+	 * the result digits into the output.  Note that this is still done at
+	 * full precision w/guard digits.
 	 */
 	alloc_var(result, div_ndigits + 1);
 	res_digits = result->digits;