Fold always false type checks (#99761)

If we have a program like:

```csharp
class Never { }

class Program
{
    static void Main()
    {
        Test(null);
    }

    [MethodImpl(MethodImplOptions.NoInlining)]
    static void Test(object o)
    {
        if (o is Never)
            Console.WriteLine("Hello!");
        if (o.GetType() == typeof(Never))
            Console.WriteLine("Hello!");
    }
}
```

We know these checks are never going to be true thanks to the whole program view. Fold these to `false`.

Author: MichalStrehovsky

src/coreclr/inc/corinfo.h

@@ -2983,7 +2983,7 @@ class ICorStaticInfo
             CORINFO_CLASS_HANDLE*       vcTypeRet       /* OUT */
             ) = 0;
 
-    // Obtains a list of exact classes for a given base type. Returns 0 if the number of
+    // Obtains a list of exact classes for a given base type. Returns -1 if the number of
     // the exact classes is greater than maxExactClasses or if more types might be loaded
     // in future.
     virtual int getExactClasses(

src/coreclr/inc/jiteeversionguid.h

@@ -43,11 +43,11 @@ typedef const GUID *LPCGUID;
 #define GUID_DEFINED
 #endif // !GUID_DEFINED
 
-constexpr GUID JITEEVersionIdentifier = { /* 6fd660c7-96be-4832-a84c-4200141f7d08 */
-    0x6fd660c7,
-    0x96be,
-    0x4832,
-    {0xa8, 0x4c, 0x42, 0x00, 0x14, 0x1f, 0x7d, 0x08}
+constexpr GUID JITEEVersionIdentifier = { /* bdf34b26-0725-4ad6-9935-40bfd2a4c4fc */
+    0xbdf34b26,
+    0x0725,
+    0x4ad6,
+    {0x99, 0x35, 0x40, 0xbf, 0xd2, 0xa4, 0xc4, 0xfc}
   };
 
 //////////////////////////////////////////////////////////////////////////////////////////////////////////

src/coreclr/jit/gentree.cpp

@@ -14117,6 +14117,18 @@ GenTree* Compiler::gtFoldTypeCompare(GenTree* tree)
         return tree;
     }
 
+    // Check if an object of this type can even exist
+    if (info.compCompHnd->getExactClasses(clsHnd, 0, nullptr) == 0)
+    {
+        JITDUMP("Runtime reported %p (%s) is never allocated\n", dspPtr(clsHnd), eeGetClassName(clsHnd));
+
+        const bool operatorIsEQ  = (oper == GT_EQ);
+        const int  compareResult = operatorIsEQ ? 0 : 1;
+        JITDUMP("Runtime reports comparison is known at jit time: %u\n", compareResult);
+        GenTree* result = gtNewIconNode(compareResult);
+        return result;
+    }
+
     // We're good to go.
     JITDUMP("Optimizing compare of obj.GetType()"
             " and type-from-handle to compare method table pointer\n");

src/coreclr/jit/importer.cpp

@@ -5380,14 +5380,39 @@ GenTree* Compiler::impOptimizeCastClassOrIsInst(GenTree* op1, CORINFO_RESOLVED_T
         return nullptr;
     }
 
+    CORINFO_CLASS_HANDLE toClass = pResolvedToken->hClass;
+    if (info.compCompHnd->getExactClasses(toClass, 0, nullptr) == 0)
+    {
+        JITDUMP("\nClass %p (%s) can never be allocated\n", dspPtr(toClass), eeGetClassName(toClass));
+
+        if (!isCastClass)
+        {
+            JITDUMP("Cast will fail, optimizing to return null\n");
+
+            // If the cast was fed by a box, we can remove that too.
+            if (op1->IsBoxedValue())
+            {
+                JITDUMP("Also removing upstream box\n");
+                gtTryRemoveBoxUpstreamEffects(op1);
+            }
+
+            if (gtTreeHasSideEffects(op1, GTF_SIDE_EFFECT))
+            {
+                impAppendTree(op1, CHECK_SPILL_ALL, impCurStmtDI);
+            }
+            return gtNewNull();
+        }
+
+        JITDUMP("Cast will always throw, but not optimizing yet\n");
+    }
+
     // See what we know about the type of the object being cast.
     bool                 isExact   = false;
     bool                 isNonNull = false;
     CORINFO_CLASS_HANDLE fromClass = gtGetClassHandle(op1, &isExact, &isNonNull);
 
     if (fromClass != nullptr)
     {
-        CORINFO_CLASS_HANDLE toClass = pResolvedToken->hClass;
         JITDUMP("\nConsidering optimization of %s from %s%p (%s) to %p (%s)\n", isCastClass ? "castclass" : "isinst",
                 isExact ? "exact " : "", dspPtr(fromClass), eeGetClassName(fromClass), dspPtr(toClass),
                 eeGetClassName(toClass));

src/coreclr/jit/importercalls.cpp

@@ -6596,7 +6596,7 @@ void Compiler::considerGuardedDevirtualization(GenTreeCall*            call,
         {
             JITDUMP("No exact classes implementing %s\n", eeGetClassName(baseClass))
         }
-        else if (numExactClasses > maxTypeChecks)
+        else if (numExactClasses < 0 || numExactClasses > maxTypeChecks)
         {
             JITDUMP("Too many exact classes implementing %s (%d > %d)\n", eeGetClassName(baseClass), numExactClasses,
                     maxTypeChecks)

src/coreclr/tools/Common/Compiler/DevirtualizationManager.cs

@@ -203,13 +203,17 @@ protected virtual MethodDesc ResolveVirtualMethod(MethodDesc declMethod, DefType
 
 #if !READYTORUN
         /// <summary>
-        /// Gets a value indicating whether it might be possible to obtain a constructed type data structure for the given type.
+        /// Gets a value indicating whether it might be possible to obtain a constructed type data structure for the given type
+        /// in this compilation (i.e. is it possible to reference a constructed MethodTable symbol for this).
         /// </summary>
-        /// <remarks>
-        /// This is a bit of a hack, but devirtualization manager has a global view of all allocated types
-        /// so it can answer this question.
-        /// </remarks>
-        public virtual bool CanConstructType(TypeDesc type) => true;
+        public virtual bool CanReferenceConstructedMethodTable(TypeDesc type) => true;
+
+        /// <summary>
+        /// Gets a value indicating whether a (potentially canonically-equlivalent) constructed MethodTable could
+        /// exist. This is similar to <see cref="CanReferenceConstructedMethodTable"/>, but will return true
+        /// for List&lt;__Canon&gt; if a constructed MethodTable for List&lt;object&gt; exists.
+        /// </summary>
+        public virtual bool CanReferenceConstructedTypeOrCanonicalFormOfType(TypeDesc type) => true;
 
         public virtual TypeDesc[] GetImplementingClasses(TypeDesc type) => null;
 #endif

src/coreclr/tools/Common/JitInterface/CorInfoImpl.cs

@@ -4284,7 +4284,7 @@ private HRESULT getPgoInstrumentationResults(CORINFO_METHOD_STRUCT_* ftnHnd, ref
 #pragma warning disable SA1001, SA1113, SA1115 // Commas should be spaced correctly
                     ComputeJitPgoInstrumentationSchema(ObjectToHandle, pgoResultsSchemas, out var nativeSchemas, _cachedMemoryStream
 #if !READYTORUN
-                        , _compilation.CanConstructType
+                        , _compilation.CanReferenceConstructedMethodTable
 #endif
                         );
 #pragma warning restore SA1001, SA1113, SA1115 // Commas should be spaced correctly

src/coreclr/tools/aot/ILCompiler.Compiler/Compiler/Compilation.cs

@@ -103,9 +103,14 @@ public bool CanInline(MethodDesc caller, MethodDesc callee)
             return _inliningPolicy.CanInline(caller, callee);
         }
 
-        public bool CanConstructType(TypeDesc type)
+        public bool CanReferenceConstructedMethodTable(TypeDesc type)
         {
-            return NodeFactory.DevirtualizationManager.CanConstructType(type);
+            return NodeFactory.DevirtualizationManager.CanReferenceConstructedMethodTable(type);
+        }
+
+        public bool CanReferenceConstructedTypeOrCanonicalFormOfType(TypeDesc type)
+        {
+            return NodeFactory.DevirtualizationManager.CanReferenceConstructedTypeOrCanonicalFormOfType(type);
         }
 
         public DelegateCreationInfo GetDelegateCtor(TypeDesc delegateType, MethodDesc target, TypeDesc constrainedType, bool followVirtualDispatch)
@@ -261,7 +266,7 @@ public bool NeedsRuntimeLookup(ReadyToRunHelperId lookupKind, object targetOfLoo
 
         public ReadyToRunHelperId GetLdTokenHelperForType(TypeDesc type)
         {
-            bool canConstructPerWholeProgramAnalysis = NodeFactory.DevirtualizationManager.CanConstructType(type);
+            bool canConstructPerWholeProgramAnalysis = NodeFactory.DevirtualizationManager.CanReferenceConstructedMethodTable(type);
             bool creationAllowed = ConstructedEETypeNode.CreationAllowed(type);
             return (canConstructPerWholeProgramAnalysis && creationAllowed)
                 ? ReadyToRunHelperId.TypeHandle

src/coreclr/tools/aot/ILCompiler.Compiler/Compiler/ILScanner.cs

@@ -413,7 +413,8 @@ public override DictionaryLayoutNode GetLayout(TypeSystemEntity methodOrType)
 
         private sealed class ScannedDevirtualizationManager : DevirtualizationManager
         {
-            private HashSet<TypeDesc> _constructedTypes = new HashSet<TypeDesc>();
+            private HashSet<TypeDesc> _constructedMethodTables = new HashSet<TypeDesc>();
+            private HashSet<TypeDesc> _canonConstructedMethodTables = new HashSet<TypeDesc>();
             private HashSet<TypeDesc> _canonConstructedTypes = new HashSet<TypeDesc>();
             private HashSet<TypeDesc> _unsealedTypes = new HashSet<TypeDesc>();
             private Dictionary<TypeDesc, HashSet<TypeDesc>> _implementators = new();
@@ -442,7 +443,12 @@ public ScannedDevirtualizationManager(NodeFactory factory, ImmutableArray<Depend
 
                     if (type != null)
                     {
-                        _constructedTypes.Add(type);
+                        _constructedMethodTables.Add(type);
+                        TypeDesc canonForm = type.ConvertToCanonForm(CanonicalFormKind.Specific);
+                        if (canonForm != type)
+                        {
+                            _canonConstructedMethodTables.Add(canonForm);
+                        }
 
                         if (type.IsInterface)
                         {
@@ -687,7 +693,11 @@ protected override MethodDesc ResolveVirtualMethod(MethodDesc declMethod, DefTyp
                 return result;
             }
 
-            public override bool CanConstructType(TypeDesc type) => _constructedTypes.Contains(type);
+            public override bool CanReferenceConstructedMethodTable(TypeDesc type)
+                => _constructedMethodTables.Contains(type);
+
+            public override bool CanReferenceConstructedTypeOrCanonicalFormOfType(TypeDesc type)
+                => _constructedMethodTables.Contains(type) || _canonConstructedMethodTables.Contains(type);
 
             public override TypeDesc[] GetImplementingClasses(TypeDesc type)
             {

src/coreclr/tools/aot/ILCompiler.ReadyToRun/JitInterface/CorInfoImpl.ReadyToRun.cs

@@ -3280,7 +3280,7 @@ private void updateEntryPointForTailCall(ref CORINFO_CONST_LOOKUP entryPoint)
         private int getExactClasses(CORINFO_CLASS_STRUCT_* baseType, int maxExactClasses, CORINFO_CLASS_STRUCT_** exactClsRet)
         {
             // Not implemented for R2R yet
-            return 0;
+            return -1;
         }
 
         private bool getStaticFieldContent(CORINFO_FIELD_STRUCT_* fieldHandle, byte* buffer, int bufferSize, int valueOffset, bool ignoreMovableObjects)