#if MONO

Author: Daniel-Svensson

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

@@ -171,7 +171,7 @@ public static long BigMul(int a, int b)
             return ((long)a) * b;
         }
 
-#if !(TARGET_ARM64 || (TARGET_AMD64 && CORECLR)) // BigMul 64*64 has high performance intrinsics on ARM64 and AMD64 (but not yet on MONO)
+#if !(TARGET_ARM64 || (TARGET_AMD64 && !MONO)) // BigMul 64*64 has high performance intrinsics on ARM64 and AMD64 (but not yet on MONO)
         /// <summary>
         /// Perform multiplication between 64 and 32 bit numbers, returning lower 64 bits in <paramref name="low"/>
         /// </summary>
@@ -205,8 +205,10 @@ public static ulong BigMul(ulong a, ulong b, out ulong low)
 #if MONO // Multiply is not yet implemented in MONO
             if (Bmi2.X64.IsSupported)
             {
-                low = a * b;
-                return Bmi2.X64.MultiplyNoFlags(a, b);
+                ulong tmp;
+                ulong high = Bmi2.X64.MultiplyNoFlags(a, b, &tmp);
+                low = tmp;
+                return high;
             }
 #else
             if (X86Base.X64.IsSupported)
@@ -254,7 +256,7 @@ static ulong SoftwareFallback(ulong a, ulong b, out ulong low)
         /// <returns>The high 64-bit of the product of the specified numbers.</returns>
         public static long BigMul(long a, long b, out long low)
         {
-#if CORECLR // Multiply is not yet implemented in MONO
+#if !MONO // Multiply is not yet implemented in MONO
             if (X86Base.X64.IsSupported)
             {
                 (low, long hi) = X86Base.X64.Multiply(a, b);

src/libraries/System.Private.CoreLib/src/System/Runtime/Intrinsics/X86/X86Base.cs

@@ -67,7 +67,7 @@ internal X64() { }
             [Experimental(Experimentals.X86BaseDivRemDiagId, UrlFormat = Experimentals.SharedUrlFormat)]
             public static (long Quotient, long Remainder) DivRem(ulong lower, long upper, long divisor) => DivRem(lower, upper, divisor);
 
-#if CORECLR
+#if !MONO
             /// <summary>
             ///   <para>unsigned _umul128(unsigned __int64 Multiplier, unsigned __int64  Multiplicand, unsigned __int64 * HighProduct)</para>
             ///   <para>  MUL reg/m64</para>
@@ -143,7 +143,7 @@ public static unsafe (int Eax, int Ebx, int Ecx, int Edx) CpuId(int functionId,
         [Experimental(Experimentals.X86BaseDivRemDiagId, UrlFormat = Experimentals.SharedUrlFormat)]
         public static (nint Quotient, nint Remainder) DivRem(nuint lower, nint upper, nint divisor) => DivRem(lower, upper, divisor);
 
-#if CORECLR
+#if !MONO
         /// <summary>
         ///   <para>  MUL reg/m32</para>
         /// </summary>