From 5f385e241e9cd44815e7120762bcdd35666d43ad Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Petryka?=
 <35800402+MichalPetryka@users.noreply.github.com>
Date: Wed, 7 Jun 2023 18:12:47 +0200
Subject: [PATCH] Simplify Random for ulong

Use Math.BigMul instead of UInt128 since both values are always ulongs.
---
 .../System.Private.CoreLib/src/System/Random.ImplBase.cs | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/src/libraries/System.Private.CoreLib/src/System/Random.ImplBase.cs b/src/libraries/System.Private.CoreLib/src/System/Random.ImplBase.cs
index 6bd1b8a1ac943b..c7857b156262df 100644
--- a/src/libraries/System.Private.CoreLib/src/System/Random.ImplBase.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Random.ImplBase.cs
@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System;
 using System.Runtime.CompilerServices;
 
 namespace System
@@ -57,8 +58,7 @@ internal static uint NextUInt32(uint maxValue, XoshiroImpl xoshiro)
             [MethodImpl(MethodImplOptions.AggressiveInlining)]
             internal static ulong NextUInt64(ulong maxValue, XoshiroImpl xoshiro)
             {
-                UInt128 randomProduct = (UInt128)maxValue * xoshiro.NextUInt64();
-                ulong lowPart = (ulong)randomProduct;
+                ulong randomProduct = Math.BigMul(maxValue, xoshiro.NextUInt64(), out ulong lowPart);
 
                 if (lowPart < maxValue)
                 {
@@ -66,12 +66,11 @@ internal static ulong NextUInt64(ulong maxValue, XoshiroImpl xoshiro)
 
                     while (lowPart < remainder)
                     {
-                        randomProduct = (UInt128)maxValue * xoshiro.NextUInt64();
-                        lowPart = (ulong)randomProduct;
+                        randomProduct = Math.BigMul(maxValue, xoshiro.NextUInt64(), out lowPart);
                     }
                 }
 
-                return (ulong)(randomProduct >> 64);
+                return randomProduct;
             }
         }
     }