BigMul with 64-bit operands

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

@@ -18,6 +18,7 @@
 using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
 using System.Runtime.CompilerServices;
+using System.Runtime.Intrinsics.X86;
 using System.Runtime.Versioning;
 
 namespace System
@@ -113,6 +114,52 @@ public static long BigMul(int a, int b)
             return ((long)a) * b;
         }
 
+        [CLSCompliant(false)]
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static unsafe ulong BigMul(ulong a, ulong b, out ulong low)
+        {
+            if (Bmi2.X64.IsSupported)
+            {
+                ulong tmp;
+                ulong high = Bmi2.X64.MultiplyNoFlags(a, b, &tmp);
+                low = tmp;
+                return high;
+            }
+
+            return SoftwareFallback(a, b, out low);
+
+            static ulong SoftwareFallback(ulong a, ulong b, out ulong low)
+            {
+                // It's adoption of algorithm for multiplication
+                // of 32-bit unsigned integers described
+                // in Hacker's Delight by Henry S. Warren, Jr. (ISBN 0-201-91465-4), Chapter 8
+                // Basically, it's an optimized version of FOIL method applied to
+                // low and high dwords of each operand
+                const ulong lowBitsMask = 0xFFFFFFFFU;
+
+                ulong al = a & lowBitsMask;
+                ulong ah = a >> 32;
+                ulong bl = b & lowBitsMask;
+                ulong bh = b >> 32;
+
+                ulong mull = al * bl;
+                ulong t = ah * bl + (mull >> 32);
+                ulong tl = t & lowBitsMask;
+
+                tl += al * bh;
+                low = tl << 32 | mull & lowBitsMask;
+
+                return ah * bh + (t >> 32) + (tl >> 32);
+            }
+        }
+
+        public static long BigMul(long a, long b, out long low)
+        {
+            ulong high = BigMul((ulong)a, (ulong)b, out ulong ulow);
+            low = (long)ulow;
+            return (long)high - ((a >> 63) & b) - ((b >> 63) & a);
+        }
+
         public static double BitDecrement(double x)
         {
             long bits = BitConverter.DoubleToInt64Bits(x);

src/libraries/System.Runtime.Extensions/tests/System/Math.cs

@@ -1749,6 +1749,36 @@ public static void BigMul()
             Assert.Equal(0L, Math.BigMul(0, 0));
         }
 
+        [Theory]
+        [InlineData(0U, 0U, "00000000000000000000000000000000")]
+        [InlineData(0U, 1U, "00000000000000000000000000000000")]
+        [InlineData(1U, 0U, "00000000000000000000000000000000")]
+        [InlineData(2U, 3U, "00000000000000000000000000000006")]
+        [InlineData(ulong.MaxValue, 2, "0000000000000001FFFFFFFFFFFFFFFE")]
+        [InlineData(ulong.MaxValue, 1, "0000000000000000FFFFFFFFFFFFFFFF")]
+        [InlineData(ulong.MaxValue, ulong.MaxValue, "FFFFFFFFFFFFFFFE0000000000000001")]
+        [InlineData(ulong.MaxValue, 3, "0000000000000002FFFFFFFFFFFFFFFD")]
+        public static void BigMul128_Unsigned(ulong a, ulong b, string result)
+        {
+            ulong high = Math.BigMul(a, b, out ulong low);
+            Assert.Equal(result, high.ToString("X16") + low.ToString("X16"));
+        }
+
+        [Theory]
+        [InlineData(0L, 0L, "00000000000000000000000000000000")]
+        [InlineData(0L, 1L, "00000000000000000000000000000000")]
+        [InlineData(1L, 0L, "00000000000000000000000000000000")]
+        [InlineData(2L, 3L, "00000000000000000000000000000006")]
+        [InlineData(3L, -2L, "FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFA")]
+        [InlineData(-1L, -1L, "00000000000000000000000000000001")]
+        [InlineData(-1L, long.MinValue, "00000000000000008000000000000000")]
+        [InlineData(1L, long.MinValue, "FFFFFFFFFFFFFFFF8000000000000000")]
+        public static void BigMul128_Signed(long a, long b, string result)
+        {
+            long high = Math.BigMul(a, b, out long low);
+            Assert.Equal(result, high.ToString("X16") + low.ToString("X16"));
+        }
+
         [Theory]
         [InlineData(1073741, 2147483647, 2000, 1647)]
         [InlineData(6, 13952, 2000, 1952)]

src/libraries/System.Runtime/ref/System.Runtime.cs

@@ -2548,6 +2548,9 @@ public static partial class Math
         public static double Atan2(double y, double x) { throw null; }
         public static double Atanh(double d) { throw null; }
         public static long BigMul(int a, int b) { throw null; }
+        [System.CLSCompliantAttribute(false)]
+        public static ulong BigMul(ulong a, ulong b, out ulong low) { throw null; }
+        public static long BigMul(long a, long b, out long low) { throw null; }
         public static double BitDecrement(double x) { throw null; }
         public static double BitIncrement(double x) { throw null; }
         public static double Cbrt(double d) { throw null; }