IEEE754 Perf Improvement

We don't know whether BHL or BLH is the correct number
When nDigits is set(>=0), that means the consumer is not
interested with the entire value.
That means, we may not just roundup to the higher bound.
Doing so would break IEEE754
Imagine LowerBound is 4999... while the UpperBoud is 50001..
We need to know where actually the number is instead of picking
either Lower or UpperBoud
In this case, we should skip and fail
and let FDblToRgbPrecise do the math.

Perf Improvement [new]:
Exception for a number that has smaller amount of fractional part
than the number of digits are being asked.

lib/Common/Common/NumberUtilities_strtod.cpp

@@ -1565,7 +1565,7 @@ Get mantissa bytes (BCD).
 ***************************************************************************/
 _Success_(return)
 static BOOL FDblToRgbFast(double dbl, _Out_writes_to_(kcbMaxRgb, (*ppbLim - prgb)) byte *prgb,
-                          int *pwExp10, byte **ppbLim, const int highBound = 1)
+                          int *pwExp10, byte **ppbLim, const int nDigits = -1)
 {
     int ib;
     int iT;
@@ -1811,21 +1811,29 @@ static BOOL FDblToRgbFast(double dbl, _Out_writes_to_(kcbMaxRgb, (*ppbLim - prgb
         // the difference.
         prgb[ib++] = (bHL + bLH + 1) / 2;
     }
-    // We don't know whether BHL or BLH is the correct one
-    // When highBound is set, that means the consumer is not
-    // interested with the entire value. On the other hand, we may not
-    // just roundup to the higher bound. That breaks IEEE754
-    // Imagine LowerBound is 4999... while the UpperBoud is 50001..
-    // We need to know where actually the number is instead of picking
-    // either Lower or UpperBoud
-    // at this point, we should skip and fail
-    // let FDblToRgbPrecise do the math.
-    else if (highBound == 1 && (0 != luHL || !numHL.FZero() || 0 == (Js::NumberUtilities::LuLoDbl(dbl) & 1)))
+
+    else if (0 != luHL || !numHL.FZero() || 0 == (Js::NumberUtilities::LuLoDbl(dbl) & 1))
     {
         Assert(ib < kcbMaxRgb);
         if(!(ib < kcbMaxRgb))
             goto LFail;
 
+        // We don't know whether BHL or BLH is the correct number
+        // When nDigits is set(>=0), that means the consumer is not
+        // interested with the entire value.
+        // That means, we may not just roundup to the higher bound.
+        // Doing so would break IEEE754
+        // Imagine LowerBound is 4999... while the UpperBoud is 50001..
+        // We need to know where actually the number is instead of picking
+        // either Lower or UpperBoud
+        // In this case, we should skip and fail
+        // and let FDblToRgbPrecise do the math.
+        // Perf Check:
+        // Exception for a number that has smaller amount of fractional part
+        // than the number of digits are being asked.
+        if (nDigits > 0 && (ib - wExp10 >= nDigits))
+            goto LFail;
+
         // We can just use bHL because this guarantees that we're bigger than
         // LH and less than HL, so must convert to the double.
         prgb[ib++] = bHL;
@@ -2307,10 +2315,9 @@ int Js::NumberUtilities::FDblToStr(double dbl, Js::NumberUtilities::FormatType f
             Js::NumberUtilities::LuHiDbl(dbl) &= 0x7FFFFFFF;
         }
 
-        const int hBound = nDigits == -1 ? 1 : 0;
         // in case we restrict the number of digits, do not push for a higher bound
-        if (!FDblToRgbFast(dbl, rgb, &wExp10, &pbLim, hBound) &&
-            !FDblToRgbPrecise(dbl, rgb, &wExp10, &pbLim, hBound))
+        if (!FDblToRgbFast(dbl, rgb, &wExp10, &pbLim, nDigits) &&
+            !FDblToRgbPrecise(dbl, rgb, &wExp10, &pbLim, nDigits != -1 ? 0 : 1))
         {
             AssertMsg(FALSE, "Failure in FDblToRgbPrecise");
             return FALSE;

test/Number/toString.js

@@ -46,6 +46,14 @@ function runTest(numberToTestAsString)
          !(8.255.toFixed(2) + "" == "8.26") ) {
         throw Error("1.255.toFixed(2) != 1.25 or 8.255.toFixed(2) != 8.26 ??");
     }
+
+    if (-4.223372036854776e+12 + "" != -4.223372036854776e+12.toFixed(3)) {
+        // original number is;
+        // -4223372036854.77587890625
+        // We don't know the 8 after 775
+        // Our default approach is to pick upperBound
+        throw Error("-4.223372036854776e+12 -> -4223372036854.776");
+    }
     writeLine("");
 }