JIT: Avoid boxing ArgumentNullException.ThrowIfNull arguments in Tier-0

src/coreclr/jit/importercalls.cpp

@@ -3256,6 +3256,9 @@ GenTree* Compiler::impIntrinsic(CORINFO_CLASS_HANDLE    clsHnd,
             // to avoid some unnecessary boxing
             case NI_System_Enum_HasFlag:
 
+            // This one is made intrinsic specifically to avoid boxing in Tier0
+            case NI_System_ArgumentNullException_ThrowIfNull:
+
             // Most atomics are compiled to single instructions
             case NI_System_Threading_Interlocked_And:
             case NI_System_Threading_Interlocked_Or:
@@ -3786,6 +3789,32 @@ GenTree* Compiler::impIntrinsic(CORINFO_CLASS_HANDLE    clsHnd,
                 break;
             }
 
+            case NI_System_ArgumentNullException_ThrowIfNull:
+            {
+                // void ThrowIfNull(object? argument, string? paramName = null)
+                // void ThrowIfNull(object? argument, ExceptionArgument paramName)
+                assert(sig->numArgs == 2);
+                assert(sig->retType == CORINFO_TYPE_VOID);
+
+                // if we see:
+                //
+                //   ArgumentNullException_ThrowIfNull(GT_BOX(...), ...)
+                //
+                // We should be able to remove the call (and the box). It is done via intrinsic only
+                // because Tier0 is not able to remove the box itself.
+                //
+                GenTree* arg = impStackTop(1).val;
+                if (arg->OperIs(GT_BOX) && !opts.compDbgCode && !opts.jitFlags->IsSet(JitFlags::JIT_FLAG_MIN_OPT))
+                {
+                    impSpillSideEffects(true, CHECK_SPILL_ALL DEBUGARG("spill side effects for ThrowIfNull"));
+                    gtTryRemoveBoxUpstreamEffects(arg, BR_REMOVE_AND_NARROW);
+                    impPopStack();
+                    impPopStack();
+                    retNode = gtNewNothingNode();
+                }
+                break;
+            }
+
             case NI_System_Enum_HasFlag:
             {
                 GenTree* thisOp  = impStackTop(1).val;
@@ -9825,6 +9854,13 @@ NamedIntrinsic Compiler::lookupNamedIntrinsic(CORINFO_METHOD_HANDLE method)
                             result = NI_System_Activator_DefaultConstructorOf;
                         }
                     }
+                    else if (strcmp(className, "ArgumentNullException") == 0)
+                    {
+                        if (strcmp(methodName, "ThrowIfNull") == 0)
+                        {
+                            result = NI_System_ArgumentNullException_ThrowIfNull;
+                        }
+                    }
                     else if (strcmp(className, "Array") == 0)
                     {
                         if (strcmp(methodName, "Clone") == 0)

src/coreclr/jit/namedintrinsiclist.h

@@ -13,6 +13,8 @@ enum NamedIntrinsic : unsigned short
 {
     NI_Illegal = 0,
 
+    NI_System_ArgumentNullException_ThrowIfNull,
+
     NI_System_Enum_HasFlag,
 
     NI_System_BitConverter_DoubleToInt64Bits,

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

@@ -51,6 +51,7 @@ protected ArgumentNullException(SerializationInfo info, StreamingContext context
         /// <summary>Throws an <see cref="ArgumentNullException"/> if <paramref name="argument"/> is null.</summary>
         /// <param name="argument">The reference type argument to validate as non-null.</param>
         /// <param name="paramName">The name of the parameter with which <paramref name="argument"/> corresponds.</param>
+        [Intrinsic] // Tier0 intrinsic to avoid redundant boxing in generics
         public static void ThrowIfNull([NotNull] object? argument, [CallerArgumentExpression(nameof(argument))] string? paramName = null)
         {
             if (argument is null)
@@ -59,6 +60,7 @@ public static void ThrowIfNull([NotNull] object? argument, [CallerArgumentExpres
             }
         }
 
+        [Intrinsic] // Tier0 intrinsic to avoid redundant boxing in generics
         internal static void ThrowIfNull([NotNull] object? argument, ExceptionArgument paramName)
         {
             if (argument is null)