Use BigMul in BigMul (#100918)

* Use BigMul in BigMul

* Reverse usage of BigMul(uint, uint)

* Add aggressive inline to Math.BigMul(ulong, ulong)

Author: lilinus

src/libraries/System.Private.CoreLib/src/System/Math.cs

@@ -238,13 +238,24 @@ public static long BigMul(long a, long b, out long low)
             return (long)high - ((a >> 63) & b) - ((b >> 63) & a);
         }
 
+        /// <summary>Produces the full product of two unsigned 64-bit numbers.</summary>
+        /// <param name="a">The first number to multiply.</param>
+        /// <param name="b">The second number to multiply.</param>
+        /// <returns>The full product of the specified numbers.</returns>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        internal static UInt128 BigMul(ulong a, ulong b)
+        {
+            ulong high = BigMul(a, b, out ulong low);
+            return new UInt128(high, low);
+        }
+
         /// <summary>Produces the full product of two 64-bit numbers.</summary>
         /// <param name="a">The first number to multiply.</param>
         /// <param name="b">The second number to multiply.</param>
         /// <returns>The full product of the specified numbers.</returns>
         internal static Int128 BigMul(long a, long b)
         {
-            long high = Math.BigMul(a, b, out long low);
+            long high = BigMul(a, b, out long low);
             return new Int128((ulong)high, (ulong)low);
         }
 

src/libraries/System.Private.CoreLib/src/System/UInt128.cs

@@ -1396,18 +1396,18 @@ internal static UInt128 BigMul(UInt128 left, UInt128 right, out UInt128 lower)
             // Basically, it's an optimized version of FOIL method applied to
             // low and high qwords of each operand
 
-            UInt128 al = left._lower;
-            UInt128 ah = left._upper;
+            ulong al = left._lower;
+            ulong ah = left._upper;
 
-            UInt128 bl = right._lower;
-            UInt128 bh = right._upper;
+            ulong bl = right._lower;
+            ulong bh = right._upper;
 
-            UInt128 mull = al * bl;
-            UInt128 t = ah * bl + mull._upper;
-            UInt128 tl = al * bh + t._lower;
+            UInt128 mull = Math.BigMul(al, bl);
+            UInt128 t = Math.BigMul(ah, bl) + mull._upper;
+            UInt128 tl = Math.BigMul(al, bh) + t._lower;
 
             lower = new UInt128(tl._lower, mull._lower);
-            return ah * bh + t._upper + tl._upper;
+            return Math.BigMul(ah, bh) + t._upper + tl._upper;
         }
 
         //