diff --git a/src/compiler.ts b/src/compiler.ts
index 154b7006..304c5f38 100644
--- a/src/compiler.ts
+++ b/src/compiler.ts
@@ -6665,34 +6665,135 @@ export class Compiler extends DiagnosticEmitter {
     var module = this.module;
     // NOTE that this differs from TypeScript in that the rhs is a type, not an expression. at the
     // time of implementation, this seemed more useful because dynamic rhs expressions are not
-    // possible in AS anyway.
+    // possible in AS anyway. also note that the code generated below must preserve side-effects of
+    // the LHS expression even when the result is a constant, i.e. return a block dropping `expr`.
     var expr = this.compileExpressionRetainType(expression.expression, this.options.usizeType, WrapMode.NONE);
     var actualType = this.currentType;
-    var expectedType = this.resolver.resolveType(
-      expression.isType,
-      this.currentFlow.actualFunction
-    );
+    var expectedType = this.resolver.resolveType(expression.isType, this.currentFlow.actualFunction);
     this.currentType = Type.bool;
     if (!expectedType) return module.createUnreachable();
 
-    // instanceof <basicType> must be exact
+    // instanceof <basic> - must be exact
     if (!expectedType.is(TypeFlags.REFERENCE)) {
-      return module.createI32(actualType == expectedType ? 1 : 0);
+      return module.createBlock(null, [
+        this.convertExpression(expr, actualType, Type.void, ConversionKind.EXPLICIT, WrapMode.NONE, expression.expression),
+        module.createI32(actualType == expectedType ? 1 : 0)
+      ], NativeType.I32);
     }
-    // <nullable> instanceof <nonNullable> must be != 0
-    if (
-      actualType.is(TypeFlags.NULLABLE) && !expectedType.is(TypeFlags.NULLABLE) &&
-      actualType.nonNullableType.isAssignableTo(expectedType)
-    ) {
-      return module.createBinary(
-        actualType.is(TypeFlags.LONG)
-          ? BinaryOp.NeI64
-          : BinaryOp.NeI32,
-        expr,
-        actualType.toNativeZero(module)
-      );
+
+    // <basic> instanceof <reference> - always false
+    if (!actualType.is(TypeFlags.REFERENCE)) {
+      return module.createBlock(null, [
+        this.convertExpression(expr, actualType, Type.void, ConversionKind.EXPLICIT, WrapMode.NONE, expression.expression),
+        module.createI32(0)
+      ], NativeType.I32);
     }
-    return module.createI32(actualType.isAssignableTo(expectedType) ? 1 : 0);
+
+    // both LHS and RHS are references now
+    var nativeSizeType = actualType.toNativeType();
+
+    // <nullable> instanceof <nonNullable> - LHS must be != 0
+    if (actualType.is(TypeFlags.NULLABLE) && !expectedType.is(TypeFlags.NULLABLE)) {
+
+      // downcast - check statically
+      if (actualType.nonNullableType.isAssignableTo(expectedType)) {
+        return module.createBinary(
+          nativeSizeType == NativeType.I64
+            ? BinaryOp.NeI64
+            : BinaryOp.NeI32,
+          expr,
+          actualType.toNativeZero(module)
+        );
+      }
+
+      // upcast - check dynamically
+      if (expectedType.isAssignableTo(actualType)) {
+        let program = this.program;
+        this.needsInstanceOf = true;
+        if (!(actualType.isUnmanaged || expectedType.isUnmanaged)) {
+          let flow = this.currentFlow;
+          let tempLocal = flow.getAndFreeTempLocal(actualType, false);
+          this.needsInstanceOf = true;
+          return module.createIf(
+            module.createUnary(
+              nativeSizeType == NativeType.I64
+                ? UnaryOp.EqzI64
+                : UnaryOp.EqzI32,
+              module.createTeeLocal(tempLocal.index, expr),
+            ),
+            module.createI32(0),
+            module.createCall(BuiltinSymbols.runtime_instanceof, [
+              module.createLoad(4, false,
+                module.createBinary(BinaryOp.SubI32,
+                  module.createGetLocal(tempLocal.index, nativeSizeType),
+                  module.createI32(program.runtimeHeaderSize)
+                ),
+                NativeType.I32
+              ),
+              module.createI32(expectedType.classReference!.ensureId())
+            ], NativeType.I32)
+          );
+        } else {
+          this.error(
+            DiagnosticCode.Operation_not_supported,
+            expression.range
+          );
+        }
+      }
+
+    // either none or both nullable
+    } else {
+
+      // downcast - check statically
+      if (actualType.isAssignableTo(expectedType)) {
+        return module.createBlock(null, [
+          this.convertExpression(expr, actualType, Type.void, ConversionKind.EXPLICIT, WrapMode.NONE, expression.expression),
+          module.createI32(1)
+        ], NativeType.I32);
+
+      // upcast - check dynamically
+      } else if (expectedType.isAssignableTo(actualType)) {
+        let program = this.program;
+        if (!(actualType.isUnmanaged || expectedType.isUnmanaged)) {
+          // FIXME: the temp local and the if can be removed here once flows
+          // perform null checking, which would error earlier when checking
+          // uninitialized (thus zero) `var a: A` to be an instance of something.
+          let flow = this.currentFlow;
+          let tempLocal = flow.getAndFreeTempLocal(actualType, false);
+          this.needsInstanceOf = true;
+          return module.createIf(
+            module.createUnary(
+              nativeSizeType == NativeType.I64
+                ? UnaryOp.EqzI64
+                : UnaryOp.EqzI32,
+              module.createTeeLocal(tempLocal.index, expr),
+            ),
+            module.createI32(0),
+            module.createCall(BuiltinSymbols.runtime_instanceof, [
+              module.createLoad(4, false,
+                module.createBinary(BinaryOp.SubI32,
+                  module.createGetLocal(tempLocal.index, nativeSizeType),
+                  module.createI32(program.runtimeHeaderSize)
+                ),
+                NativeType.I32
+              ),
+              module.createI32(expectedType.classReference!.ensureId())
+            ], NativeType.I32)
+          );
+        } else {
+          this.error(
+            DiagnosticCode.Operation_not_supported,
+            expression.range
+          );
+        }
+      }
+    }
+
+    // false
+    return module.createBlock(null, [
+      this.convertExpression(expr, actualType, Type.void, ConversionKind.EXPLICIT, WrapMode.NONE, expression.expression),
+      module.createI32(0)
+    ], NativeType.I32);
   }
 
   compileLiteralExpression(
diff --git a/src/types.ts b/src/types.ts
index 09b9e164..35551e1d 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -160,6 +160,12 @@ export class Type {
     return false;
   }
 
+  /** Tests if this is a class type explicitly annotated as unmanaged. */
+  get isUnmanaged(): bool {
+    var classReference = this.classReference;
+    return classReference !== null && classReference.hasDecorator(DecoratorFlags.UNMANAGED);
+  }
+
   /** Computes the sign-extending shift in the target type. */
   computeSmallIntegerShift(targetType: Type): u32 {
     return targetType.size - this.size;
diff --git a/std/assembly/runtime.ts b/std/assembly/runtime.ts
index bbdbf54f..0d167a78 100644
--- a/std/assembly/runtime.ts
+++ b/std/assembly/runtime.ts
@@ -128,4 +128,15 @@ export namespace runtime {
     if (source) memory.copy(buffer, source, bufferSize);
     return array;
   }
+
+  // @ts-ignore: decorator
+  @unsafe
+  export function instanceOf(ref: usize, id: u32): bool {
+    return ref
+      ? __runtime_instanceof(
+          changetype<HEADER>(ref - HEADER_SIZE).classId,
+          id
+        )
+      : false;
+  }
 }
diff --git a/tests/compiler/gc.optimized.wat b/tests/compiler/gc.optimized.wat
index 14d0a109..959e4bd0 100644
--- a/tests/compiler/gc.optimized.wat
+++ b/tests/compiler/gc.optimized.wat
@@ -55,6 +55,7 @@
  (export "runtime.register" (func $~lib/runtime/runtime.register))
  (export ".setargc" (func $~lib/setargc))
  (export "runtime.makeArray" (func $~lib/runtime/runtime.makeArray|trampoline))
+ (export "runtime.instanceOf" (func $~lib/runtime/runtime.instanceOf))
  (export "gc.implemented" (global $~lib/gc/gc.implemented))
  (export "gc.collect" (func $~lib/gc/gc.collect))
  (export "gc.retain" (func $~lib/gc/gc.retain))
@@ -2361,10 +2362,56 @@
   end
   local.get $4
  )
- (func $null (; 28 ;) (type $FUNCSIG$v)
+ (func $~lib/runtime/runtime.instanceOf (; 28 ;) (type $FUNCSIG$iii) (param $0 i32) (param $1 i32) (result i32)
+  local.get $0
+  if (result i32)
+   local.get $0
+   i32.const 16
+   i32.sub
+   i32.load
+   local.get $1
+   call $~lib/runtime/__runtime_instanceof
+  else   
+   i32.const 0
+  end
+ )
+ (func $~lib/runtime/__runtime_instanceof (; 29 ;) (type $FUNCSIG$iii) (param $0 i32) (param $1 i32) (result i32)
+  block $nope
+   block $~lib/arraybuffer/ArrayBuffer
+    block $~lib/set/Set<usize>
+     block $~lib/string/String
+      block $gc/Ref
+       local.get $0
+       i32.const 1
+       i32.sub
+       br_table $gc/Ref $~lib/string/String $~lib/set/Set<usize> $~lib/arraybuffer/ArrayBuffer $nope
+      end
+      local.get $1
+      i32.const 1
+      i32.eq
+      return
+     end
+     local.get $1
+     i32.const 2
+     i32.eq
+     return
+    end
+    local.get $1
+    i32.const 3
+    i32.eq
+    return
+   end
+   local.get $1
+   i32.const 4
+   i32.eq
+   return
+  end
+  i32.const 0
+ )
+ (func $null (; 30 ;) (type $FUNCSIG$v)
   nop
  )
- (func $~lib/runtime/runtime.makeArray|trampoline (; 29 ;) (type $FUNCSIG$iiiii) (param $0 i32) (param $1 i32) (param $2 i32) (param $3 i32) (result i32)
+ (func $~lib/runtime/runtime.makeArray|trampoline (; 31 ;) (type $FUNCSIG$iiiii) (param $0 i32) (param $1 i32) (param $2 i32) (param $3 i32) (result i32)
   block $1of1
    block $0of1
     block $outOfRange
@@ -2384,7 +2431,7 @@
   local.get $3
   call $~lib/runtime/runtime.makeArray
  )
- (func $~lib/setargc (; 30 ;) (type $FUNCSIG$vi) (param $0 i32)
+ (func $~lib/setargc (; 32 ;) (type $FUNCSIG$vi) (param $0 i32)
   local.get $0
   global.set $~lib/argc
  )
diff --git a/tests/compiler/gc.untouched.wat b/tests/compiler/gc.untouched.wat
index 80ab5637..77b3114a 100644
--- a/tests/compiler/gc.untouched.wat
+++ b/tests/compiler/gc.untouched.wat
@@ -54,6 +54,7 @@
  (export "runtime.register" (func $~lib/runtime/runtime.register))
  (export ".setargc" (func $~lib/setargc))
  (export "runtime.makeArray" (func $~lib/runtime/runtime.makeArray|trampoline))
+ (export "runtime.instanceOf" (func $~lib/runtime/runtime.instanceOf))
  (export "gc.implemented" (global $~lib/gc/gc.implemented))
  (export "gc.collect" (func $~lib/gc/gc.collect))
  (export "gc.retain" (func $~lib/gc/gc.retain))
@@ -2972,7 +2973,20 @@
   end
   local.get $4
  )
- (func $start (; 33 ;) (type $FUNCSIG$v)
+ (func $~lib/runtime/runtime.instanceOf (; 33 ;) (type $FUNCSIG$iii) (param $0 i32) (param $1 i32) (result i32)
+  local.get $0
+  if (result i32)
+   local.get $0
+   global.get $~lib/util/runtime/HEADER_SIZE
+   i32.sub
+   i32.load
+   local.get $1
+   call $~lib/runtime/__runtime_instanceof
+  else   
+   i32.const 0
+  end
+ )
+ (func $start (; 34 ;) (type $FUNCSIG$v)
   global.get $~lib/memory/HEAP_BASE
   i32.const 7
   i32.add
@@ -2987,9 +3001,41 @@
   call $~lib/set/Set<usize>#constructor
   global.set $~lib/gc/ROOT
  )
- (func $null (; 34 ;) (type $FUNCSIG$v)
+ (func $~lib/runtime/__runtime_instanceof (; 35 ;) (type $FUNCSIG$iii) (param $0 i32) (param $1 i32) (result i32)
+  block $nope
+   block $~lib/arraybuffer/ArrayBuffer
+    block $~lib/set/Set<usize>
+     block $~lib/string/String
+      block $gc/Ref
+       local.get $0
+       br_table $nope $gc/Ref $~lib/string/String $~lib/set/Set<usize> $~lib/arraybuffer/ArrayBuffer $nope
+      end
+      local.get $1
+      i32.const 1
+      i32.eq
+      return
+     end
+     local.get $1
+     i32.const 2
+     i32.eq
+     return
+    end
+    local.get $1
+    i32.const 3
+    i32.eq
+    return
+   end
+   local.get $1
+   i32.const 4
+   i32.eq
+   return
+  end
+  i32.const 0
+  return
  )
- (func $~lib/runtime/runtime.makeArray|trampoline (; 35 ;) (type $FUNCSIG$iiiii) (param $0 i32) (param $1 i32) (param $2 i32) (param $3 i32) (result i32)
+ (func $null (; 36 ;) (type $FUNCSIG$v)
+ )
+ (func $~lib/runtime/runtime.makeArray|trampoline (; 37 ;) (type $FUNCSIG$iiiii) (param $0 i32) (param $1 i32) (param $2 i32) (param $3 i32) (result i32)
   block $1of1
    block $0of1
     block $outOfRange
@@ -3009,7 +3055,7 @@
   local.get $3
   call $~lib/runtime/runtime.makeArray
  )
- (func $~lib/setargc (; 36 ;) (type $FUNCSIG$vi) (param $0 i32)
+ (func $~lib/setargc (; 38 ;) (type $FUNCSIG$vi) (param $0 i32)
   local.get $0
   global.set $~lib/argc
  )
diff --git a/tests/compiler/instanceof.ts b/tests/compiler/instanceof.ts
index 64d96b03..b5b10329 100644
--- a/tests/compiler/instanceof.ts
+++ b/tests/compiler/instanceof.ts
@@ -15,7 +15,7 @@ assert(!(I instanceof A));
 assert(!(f instanceof A));
 assert(!(F instanceof A));
 
-assert(!(a instanceof B));
+// assert(!(a instanceof B)); // dynamic upcast, checked in runtime/instanceof
 assert(  b instanceof B );
 assert(!(i instanceof B));
 assert(!(I instanceof B));
diff --git a/tests/compiler/instanceof.untouched.wat b/tests/compiler/instanceof.untouched.wat
index ed9a4ef7..65e10c40 100644
--- a/tests/compiler/instanceof.untouched.wat
+++ b/tests/compiler/instanceof.untouched.wat
@@ -20,23 +20,75 @@
  (export "table" (table $0))
  (start $start)
  (func $instanceof/isI32<i32> (; 1 ;) (type $FUNCSIG$ii) (param $0 i32) (result i32)
-  i32.const 1
-  return
+  block (result i32)
+   local.get $0
+   drop
+   i32.const 1
+  end
+  if
+   i32.const 1
+   return
+  else   
+   i32.const 0
+   return
+  end
+  unreachable
+  unreachable
  )
  (func $instanceof/isI32<f64> (; 2 ;) (type $FUNCSIG$id) (param $0 f64) (result i32)
-  i32.const 0
-  return
+  block (result i32)
+   local.get $0
+   drop
+   i32.const 0
+  end
+  if
+   i32.const 1
+   return
+  else   
+   i32.const 0
+   return
+  end
+  unreachable
+  unreachable
  )
  (func $instanceof/isI32<u32> (; 3 ;) (type $FUNCSIG$ii) (param $0 i32) (result i32)
-  i32.const 0
-  return
+  block (result i32)
+   local.get $0
+   drop
+   i32.const 0
+  end
+  if
+   i32.const 1
+   return
+  else   
+   i32.const 0
+   return
+  end
+  unreachable
+  unreachable
  )
  (func $instanceof/isI32<u16> (; 4 ;) (type $FUNCSIG$ii) (param $0 i32) (result i32)
-  i32.const 0
-  return
+  block (result i32)
+   local.get $0
+   drop
+   i32.const 0
+  end
+  if
+   i32.const 1
+   return
+  else   
+   i32.const 0
+   return
+  end
+  unreachable
+  unreachable
  )
  (func $start:instanceof (; 5 ;) (type $FUNCSIG$v)
-  i32.const 1
+  block (result i32)
+   global.get $instanceof/a
+   drop
+   i32.const 1
+  end
   i32.eqz
   if
    i32.const 0
@@ -46,7 +98,11 @@
    call $~lib/env/abort
    unreachable
   end
-  i32.const 1
+  block (result i32)
+   global.get $instanceof/b
+   drop
+   i32.const 1
+  end
   i32.eqz
   if
    i32.const 0
@@ -56,7 +112,11 @@
    call $~lib/env/abort
    unreachable
   end
-  i32.const 0
+  block (result i32)
+   global.get $instanceof/i
+   drop
+   i32.const 0
+  end
   i32.eqz
   i32.eqz
   if
@@ -67,7 +127,11 @@
    call $~lib/env/abort
    unreachable
   end
-  i32.const 0
+  block (result i32)
+   global.get $instanceof/I
+   drop
+   i32.const 0
+  end
   i32.eqz
   i32.eqz
   if
@@ -78,7 +142,11 @@
    call $~lib/env/abort
    unreachable
   end
-  i32.const 0
+  block (result i32)
+   global.get $instanceof/f
+   drop
+   i32.const 0
+  end
   i32.eqz
   i32.eqz
   if
@@ -89,7 +157,11 @@
    call $~lib/env/abort
    unreachable
   end
-  i32.const 0
+  block (result i32)
+   global.get $instanceof/F
+   drop
+   i32.const 0
+  end
   i32.eqz
   i32.eqz
   if
@@ -100,18 +172,11 @@
    call $~lib/env/abort
    unreachable
   end
-  i32.const 0
-  i32.eqz
-  i32.eqz
-  if
-   i32.const 0
-   i32.const 16
-   i32.const 18
-   i32.const 0
-   call $~lib/env/abort
-   unreachable
+  block (result i32)
+   global.get $instanceof/b
+   drop
+   i32.const 1
   end
-  i32.const 1
   i32.eqz
   if
    i32.const 0
@@ -121,7 +186,11 @@
    call $~lib/env/abort
    unreachable
   end
-  i32.const 0
+  block (result i32)
+   global.get $instanceof/i
+   drop
+   i32.const 0
+  end
   i32.eqz
   i32.eqz
   if
@@ -132,7 +201,11 @@
    call $~lib/env/abort
    unreachable
   end
-  i32.const 0
+  block (result i32)
+   global.get $instanceof/I
+   drop
+   i32.const 0
+  end
   i32.eqz
   i32.eqz
   if
@@ -143,7 +216,11 @@
    call $~lib/env/abort
    unreachable
   end
-  i32.const 0
+  block (result i32)
+   global.get $instanceof/f
+   drop
+   i32.const 0
+  end
   i32.eqz
   i32.eqz
   if
@@ -154,7 +231,11 @@
    call $~lib/env/abort
    unreachable
   end
-  i32.const 0
+  block (result i32)
+   global.get $instanceof/F
+   drop
+   i32.const 0
+  end
   i32.eqz
   i32.eqz
   if
@@ -165,7 +246,11 @@
    call $~lib/env/abort
    unreachable
   end
-  i32.const 0
+  block (result i32)
+   global.get $instanceof/a
+   drop
+   i32.const 0
+  end
   i32.eqz
   i32.eqz
   if
@@ -176,7 +261,11 @@
    call $~lib/env/abort
    unreachable
   end
-  i32.const 0
+  block (result i32)
+   global.get $instanceof/b
+   drop
+   i32.const 0
+  end
   i32.eqz
   i32.eqz
   if
@@ -187,7 +276,11 @@
    call $~lib/env/abort
    unreachable
   end
-  i32.const 1
+  block (result i32)
+   global.get $instanceof/i
+   drop
+   i32.const 1
+  end
   i32.eqz
   if
    i32.const 0
@@ -197,7 +290,11 @@
    call $~lib/env/abort
    unreachable
   end
-  i32.const 0
+  block (result i32)
+   global.get $instanceof/I
+   drop
+   i32.const 0
+  end
   i32.eqz
   i32.eqz
   if
@@ -208,7 +305,11 @@
    call $~lib/env/abort
    unreachable
   end
-  i32.const 0
+  block (result i32)
+   global.get $instanceof/f
+   drop
+   i32.const 0
+  end
   i32.eqz
   i32.eqz
   if
@@ -219,7 +320,11 @@
    call $~lib/env/abort
    unreachable
   end
-  i32.const 0
+  block (result i32)
+   global.get $instanceof/F
+   drop
+   i32.const 0
+  end
   i32.eqz
   i32.eqz
   if
@@ -230,7 +335,11 @@
    call $~lib/env/abort
    unreachable
   end
-  i32.const 0
+  block (result i32)
+   global.get $instanceof/a
+   drop
+   i32.const 0
+  end
   i32.eqz
   i32.eqz
   if
@@ -241,7 +350,11 @@
    call $~lib/env/abort
    unreachable
   end
-  i32.const 0
+  block (result i32)
+   global.get $instanceof/b
+   drop
+   i32.const 0
+  end
   i32.eqz
   i32.eqz
   if
@@ -252,7 +365,11 @@
    call $~lib/env/abort
    unreachable
   end
-  i32.const 0
+  block (result i32)
+   global.get $instanceof/i
+   drop
+   i32.const 0
+  end
   i32.eqz
   i32.eqz
   if
@@ -263,7 +380,11 @@
    call $~lib/env/abort
    unreachable
   end
-  i32.const 1
+  block (result i32)
+   global.get $instanceof/I
+   drop
+   i32.const 1
+  end
   i32.eqz
   if
    i32.const 0
@@ -273,7 +394,11 @@
    call $~lib/env/abort
    unreachable
   end
-  i32.const 0
+  block (result i32)
+   global.get $instanceof/f
+   drop
+   i32.const 0
+  end
   i32.eqz
   i32.eqz
   if
@@ -284,7 +409,11 @@
    call $~lib/env/abort
    unreachable
   end
-  i32.const 0
+  block (result i32)
+   global.get $instanceof/F
+   drop
+   i32.const 0
+  end
   i32.eqz
   i32.eqz
   if
@@ -295,7 +424,11 @@
    call $~lib/env/abort
    unreachable
   end
-  i32.const 0
+  block (result i32)
+   global.get $instanceof/a
+   drop
+   i32.const 0
+  end
   i32.eqz
   i32.eqz
   if
@@ -306,7 +439,11 @@
    call $~lib/env/abort
    unreachable
   end
-  i32.const 0
+  block (result i32)
+   global.get $instanceof/b
+   drop
+   i32.const 0
+  end
   i32.eqz
   i32.eqz
   if
@@ -317,7 +454,11 @@
    call $~lib/env/abort
    unreachable
   end
-  i32.const 0
+  block (result i32)
+   global.get $instanceof/i
+   drop
+   i32.const 0
+  end
   i32.eqz
   i32.eqz
   if
@@ -328,7 +469,11 @@
    call $~lib/env/abort
    unreachable
   end
-  i32.const 0
+  block (result i32)
+   global.get $instanceof/I
+   drop
+   i32.const 0
+  end
   i32.eqz
   i32.eqz
   if
@@ -339,7 +484,11 @@
    call $~lib/env/abort
    unreachable
   end
-  i32.const 1
+  block (result i32)
+   global.get $instanceof/f
+   drop
+   i32.const 1
+  end
   i32.eqz
   if
    i32.const 0
@@ -349,7 +498,11 @@
    call $~lib/env/abort
    unreachable
   end
-  i32.const 0
+  block (result i32)
+   global.get $instanceof/F
+   drop
+   i32.const 0
+  end
   i32.eqz
   i32.eqz
   if
@@ -360,7 +513,11 @@
    call $~lib/env/abort
    unreachable
   end
-  i32.const 0
+  block (result i32)
+   global.get $instanceof/a
+   drop
+   i32.const 0
+  end
   i32.eqz
   i32.eqz
   if
@@ -371,7 +528,11 @@
    call $~lib/env/abort
    unreachable
   end
-  i32.const 0
+  block (result i32)
+   global.get $instanceof/b
+   drop
+   i32.const 0
+  end
   i32.eqz
   i32.eqz
   if
@@ -382,7 +543,11 @@
    call $~lib/env/abort
    unreachable
   end
-  i32.const 0
+  block (result i32)
+   global.get $instanceof/i
+   drop
+   i32.const 0
+  end
   i32.eqz
   i32.eqz
   if
@@ -393,7 +558,11 @@
    call $~lib/env/abort
    unreachable
   end
-  i32.const 0
+  block (result i32)
+   global.get $instanceof/I
+   drop
+   i32.const 0
+  end
   i32.eqz
   i32.eqz
   if
@@ -404,7 +573,11 @@
    call $~lib/env/abort
    unreachable
   end
-  i32.const 0
+  block (result i32)
+   global.get $instanceof/f
+   drop
+   i32.const 0
+  end
   i32.eqz
   i32.eqz
   if
@@ -415,7 +588,11 @@
    call $~lib/env/abort
    unreachable
   end
-  i32.const 1
+  block (result i32)
+   global.get $instanceof/F
+   drop
+   i32.const 1
+  end
   i32.eqz
   if
    i32.const 0
@@ -485,7 +662,11 @@
    call $~lib/env/abort
    unreachable
   end
-  i32.const 1
+  block (result i32)
+   global.get $instanceof/an
+   drop
+   i32.const 1
+  end
   i32.eqz
   if
    i32.const 0
@@ -509,7 +690,11 @@
    call $~lib/env/abort
    unreachable
   end
-  i32.const 1
+  block (result i32)
+   global.get $instanceof/an
+   drop
+   i32.const 1
+  end
   i32.eqz
   if
    i32.const 0
diff --git a/tests/compiler/runtime/instanceof.optimized.wat b/tests/compiler/runtime/instanceof.optimized.wat
index c3ece108..fe71599a 100644
--- a/tests/compiler/runtime/instanceof.optimized.wat
+++ b/tests/compiler/runtime/instanceof.optimized.wat
@@ -1,24 +1,212 @@
 (module
  (type $FUNCSIG$viiii (func (param i32 i32 i32 i32)))
- (type $FUNCSIG$v (func))
+ (type $FUNCSIG$ii (func (param i32) (result i32)))
  (type $FUNCSIG$iii (func (param i32 i32) (result i32)))
+ (type $FUNCSIG$vi (func (param i32)))
+ (type $FUNCSIG$viiddddd (func (param i32 i32 f64 f64 f64 f64 f64)))
+ (type $FUNCSIG$v (func))
+ (type $FUNCSIG$i (func (result i32)))
  (import "env" "abort" (func $~lib/env/abort (param i32 i32 i32 i32)))
+ (import "env" "trace" (func $~lib/env/trace (param i32 i32 f64 f64 f64 f64 f64)))
  (memory $0 1)
- (data (i32.const 8) "\02\00\00\00*\00\00\00r\00u\00n\00t\00i\00m\00e\00/\00i\00n\00s\00t\00a\00n\00c\00e\00o\00f\00.\00t\00s")
+ (data (i32.const 8) "\02\00\00\00*")
+ (data (i32.const 24) "r\00u\00n\00t\00i\00m\00e\00/\00i\00n\00s\00t\00a\00n\00c\00e\00o\00f\00.\00t\00s")
+ (data (i32.const 72) "\02\00\00\00\1e")
+ (data (i32.const 88) "~\00l\00i\00b\00/\00r\00u\00n\00t\00i\00m\00e\00.\00t\00s")
+ (data (i32.const 120) "\02\00\00\00\16")
+ (data (i32.const 136) "g\00c\00.\00r\00e\00g\00i\00s\00t\00e\00r")
  (table $0 1 funcref)
  (elem (i32.const 0) $null)
+ (global $gc/_dummy/register_count (mut i32) (i32.const 0))
+ (global $gc/_dummy/register_ref (mut i32) (i32.const 0))
+ (global $~lib/allocator/arena/startOffset (mut i32) (i32.const 0))
+ (global $~lib/allocator/arena/offset (mut i32) (i32.const 0))
+ (global $runtime/instanceof/animal (mut i32) (i32.const 0))
+ (global $runtime/instanceof/cat (mut i32) (i32.const 0))
+ (global $runtime/instanceof/blackcat (mut i32) (i32.const 0))
+ (global $runtime/instanceof/nullableAnimal (mut i32) (i32.const 0))
+ (global $runtime/instanceof/nullableCat (mut i32) (i32.const 0))
+ (global $runtime/instanceof/nullableBlackcat (mut i32) (i32.const 0))
+ (global $runtime/instanceof/nullAnimal (mut i32) (i32.const 0))
+ (global $runtime/instanceof/nullCat (mut i32) (i32.const 0))
+ (global $runtime/instanceof/nullBlackcat (mut i32) (i32.const 0))
+ (global $~lib/started (mut i32) (i32.const 0))
+ (global $~lib/capabilities i32 (i32.const 2))
  (export "memory" (memory $0))
  (export "table" (table $0))
- (start $start)
- (func $start:runtime/instanceof (; 1 ;) (type $FUNCSIG$v)
+ (export "main" (func $runtime/instanceof/main))
+ (export ".capabilities" (global $~lib/capabilities))
+ (func $~lib/allocator/arena/__mem_allocate (; 2 ;) (type $FUNCSIG$ii) (param $0 i32) (result i32)
+  (local $1 i32)
+  (local $2 i32)
+  (local $3 i32)
+  local.get $0
+  i32.const 1073741824
+  i32.gt_u
+  if
+   unreachable
+  end
+  global.get $~lib/allocator/arena/offset
+  local.tee $1
+  local.get $0
+  i32.const 1
+  local.get $0
+  i32.const 1
+  i32.gt_u
+  select
+  i32.add
+  i32.const 7
+  i32.add
+  i32.const -8
+  i32.and
+  local.tee $0
+  current_memory
+  local.tee $2
+  i32.const 16
+  i32.shl
+  i32.gt_u
+  if
+   local.get $2
+   local.get $0
+   local.get $1
+   i32.sub
+   i32.const 65535
+   i32.add
+   i32.const -65536
+   i32.and
+   i32.const 16
+   i32.shr_u
+   local.tee $3
+   local.get $2
+   local.get $3
+   i32.gt_s
+   select
+   grow_memory
+   i32.const 0
+   i32.lt_s
+   if
+    local.get $3
+    grow_memory
+    i32.const 0
+    i32.lt_s
+    if
+     unreachable
+    end
+   end
+  end
+  local.get $0
+  global.set $~lib/allocator/arena/offset
+  local.get $1
+ )
+ (func $~lib/runtime/runtime.allocate (; 3 ;) (type $FUNCSIG$i) (result i32)
+  (local $0 i32)
+  i32.const 16
+  call $~lib/allocator/arena/__mem_allocate
+  local.tee $0
+  i32.const -1520547049
+  i32.store
+  local.get $0
+  i32.const 0
+  i32.store offset=4
+  local.get $0
+  i32.const 0
+  i32.store offset=8
+  local.get $0
+  i32.const 0
+  i32.store offset=12
+  local.get $0
+  i32.const 16
+  i32.add
+ )
+ (func $gc/_dummy/__ref_register (; 4 ;) (type $FUNCSIG$vi) (param $0 i32)
+  i32.const 136
+  i32.const 1
+  local.get $0
+  f64.convert_i32_u
+  f64.const 0
+  f64.const 0
+  f64.const 0
+  f64.const 0
+  call $~lib/env/trace
+  global.get $gc/_dummy/register_count
+  i32.const 1
+  i32.add
+  global.set $gc/_dummy/register_count
+  local.get $0
+  global.set $gc/_dummy/register_ref
+ )
+ (func $~lib/runtime/runtime.register (; 5 ;) (type $FUNCSIG$iii) (param $0 i32) (param $1 i32) (result i32)
+  (local $2 i32)
+  local.get $0
+  i32.const 160
+  i32.le_u
+  if
+   i32.const 0
+   i32.const 88
+   i32.const 107
+   i32.const 6
+   call $~lib/env/abort
+   unreachable
+  end
+  local.get $0
+  i32.const 16
+  i32.sub
+  local.tee $2
+  i32.load
+  i32.const -1520547049
+  i32.ne
+  if
+   i32.const 0
+   i32.const 88
+   i32.const 109
+   i32.const 6
+   call $~lib/env/abort
+   unreachable
+  end
+  local.get $2
+  local.get $1
+  i32.store
+  local.get $0
+  call $gc/_dummy/__ref_register
+  local.get $0
+ )
+ (func $runtime/instanceof/Animal#constructor (; 6 ;) (type $FUNCSIG$ii) (param $0 i32) (result i32)
+  local.get $0
+  if (result i32)
+   local.get $0
+  else   
+   call $~lib/runtime/runtime.allocate
+   i32.const 1
+   call $~lib/runtime/runtime.register
+  end
+ )
+ (func $runtime/instanceof/Cat#constructor (; 7 ;) (type $FUNCSIG$ii) (param $0 i32) (result i32)
+  local.get $0
+  if (result i32)
+   local.get $0
+  else   
+   call $~lib/runtime/runtime.allocate
+   i32.const 3
+   call $~lib/runtime/runtime.register
+  end
+  call $runtime/instanceof/Animal#constructor
+ )
+ (func $runtime/instanceof/BlackCat#constructor (; 8 ;) (type $FUNCSIG$i) (result i32)
+  call $~lib/runtime/runtime.allocate
+  i32.const 4
+  call $~lib/runtime/runtime.register
+  call $runtime/instanceof/Cat#constructor
+ )
+ (func $start:runtime/instanceof (; 9 ;) (type $FUNCSIG$v)
+  (local $0 i32)
   i32.const 1
   i32.const 1
   call $~lib/runtime/__runtime_instanceof
   i32.eqz
   if
    i32.const 0
-   i32.const 16
-   i32.const 7
+   i32.const 24
+   i32.const 8
    i32.const 0
    call $~lib/env/abort
    unreachable
@@ -29,8 +217,8 @@
   i32.eqz
   if
    i32.const 0
-   i32.const 16
-   i32.const 14
+   i32.const 24
+   i32.const 15
    i32.const 0
    call $~lib/env/abort
    unreachable
@@ -41,8 +229,8 @@
   i32.eqz
   if
    i32.const 0
-   i32.const 16
-   i32.const 21
+   i32.const 24
+   i32.const 22
    i32.const 0
    call $~lib/env/abort
    unreachable
@@ -53,8 +241,8 @@
   i32.eqz
   if
    i32.const 0
-   i32.const 16
-   i32.const 28
+   i32.const 24
+   i32.const 29
    i32.const 0
    call $~lib/env/abort
    unreachable
@@ -65,8 +253,8 @@
   i32.eqz
   if
    i32.const 0
-   i32.const 16
-   i32.const 35
+   i32.const 24
+   i32.const 36
    i32.const 0
    call $~lib/env/abort
    unreachable
@@ -76,8 +264,8 @@
   call $~lib/runtime/__runtime_instanceof
   if
    i32.const 0
-   i32.const 16
-   i32.const 42
+   i32.const 24
+   i32.const 43
    i32.const 0
    call $~lib/env/abort
    unreachable
@@ -87,8 +275,8 @@
   call $~lib/runtime/__runtime_instanceof
   if
    i32.const 0
-   i32.const 16
-   i32.const 49
+   i32.const 24
+   i32.const 50
    i32.const 0
    call $~lib/env/abort
    unreachable
@@ -98,17 +286,466 @@
   call $~lib/runtime/__runtime_instanceof
   if
    i32.const 0
+   i32.const 24
+   i32.const 57
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  i32.const 160
+  global.set $~lib/allocator/arena/startOffset
+  global.get $~lib/allocator/arena/startOffset
+  global.set $~lib/allocator/arena/offset
+  i32.const 0
+  call $runtime/instanceof/Animal#constructor
+  global.set $runtime/instanceof/animal
+  i32.const 0
+  call $runtime/instanceof/Cat#constructor
+  global.set $runtime/instanceof/cat
+  call $runtime/instanceof/BlackCat#constructor
+  global.set $runtime/instanceof/blackcat
+  global.get $runtime/instanceof/animal
+  local.tee $0
+  if (result i32)
+   local.get $0
    i32.const 16
-   i32.const 56
+   i32.sub
+   i32.load
+   i32.const 3
+   call $~lib/runtime/__runtime_instanceof
+  else   
+   i32.const 0
+  end
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 69
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/animal
+  local.tee $0
+  if (result i32)
+   local.get $0
+   i32.const 16
+   i32.sub
+   i32.load
+   i32.const 4
+   call $~lib/runtime/__runtime_instanceof
+  else   
+   i32.const 0
+  end
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 70
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/cat
+  local.tee $0
+  if (result i32)
+   local.get $0
+   i32.const 16
+   i32.sub
+   i32.load
+   i32.const 3
+   call $~lib/runtime/__runtime_instanceof
+  else   
+   i32.const 0
+  end
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 73
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/cat
+  local.tee $0
+  if (result i32)
+   local.get $0
+   i32.const 16
+   i32.sub
+   i32.load
+   i32.const 4
+   call $~lib/runtime/__runtime_instanceof
+  else   
+   i32.const 0
+  end
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 74
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/blackcat
+  local.tee $0
+  if (result i32)
+   local.get $0
+   i32.const 16
+   i32.sub
+   i32.load
+   i32.const 3
+   call $~lib/runtime/__runtime_instanceof
+  else   
+   i32.const 0
+  end
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 77
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/blackcat
+  local.tee $0
+  if (result i32)
+   local.get $0
+   i32.const 16
+   i32.sub
+   i32.load
+   i32.const 4
+   call $~lib/runtime/__runtime_instanceof
+  else   
+   i32.const 0
+  end
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 78
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  i32.const 0
+  call $runtime/instanceof/Animal#constructor
+  global.set $runtime/instanceof/nullableAnimal
+  i32.const 0
+  call $runtime/instanceof/Cat#constructor
+  global.set $runtime/instanceof/nullableCat
+  call $runtime/instanceof/BlackCat#constructor
+  global.set $runtime/instanceof/nullableBlackcat
+  global.get $runtime/instanceof/nullableAnimal
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 84
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/nullableAnimal
+  local.tee $0
+  if (result i32)
+   local.get $0
+   i32.const 16
+   i32.sub
+   i32.load
+   i32.const 3
+   call $~lib/runtime/__runtime_instanceof
+  else   
+   i32.const 0
+  end
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 85
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/nullableAnimal
+  local.tee $0
+  if (result i32)
+   local.get $0
+   i32.const 16
+   i32.sub
+   i32.load
+   i32.const 4
+   call $~lib/runtime/__runtime_instanceof
+  else   
+   i32.const 0
+  end
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 86
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/nullableCat
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 88
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/nullableCat
+  local.tee $0
+  if (result i32)
+   local.get $0
+   i32.const 16
+   i32.sub
+   i32.load
+   i32.const 3
+   call $~lib/runtime/__runtime_instanceof
+  else   
+   i32.const 0
+  end
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 89
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/nullableCat
+  local.tee $0
+  if (result i32)
+   local.get $0
+   i32.const 16
+   i32.sub
+   i32.load
+   i32.const 4
+   call $~lib/runtime/__runtime_instanceof
+  else   
+   i32.const 0
+  end
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 90
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/nullableBlackcat
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 92
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/nullableBlackcat
+  local.tee $0
+  if (result i32)
+   local.get $0
+   i32.const 16
+   i32.sub
+   i32.load
+   i32.const 3
+   call $~lib/runtime/__runtime_instanceof
+  else   
+   i32.const 0
+  end
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 93
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/nullableBlackcat
+  local.tee $0
+  if (result i32)
+   local.get $0
+   i32.const 16
+   i32.sub
+   i32.load
+   i32.const 4
+   call $~lib/runtime/__runtime_instanceof
+  else   
+   i32.const 0
+  end
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 94
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/nullAnimal
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 100
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/nullAnimal
+  local.tee $0
+  if (result i32)
+   local.get $0
+   i32.const 16
+   i32.sub
+   i32.load
+   i32.const 3
+   call $~lib/runtime/__runtime_instanceof
+  else   
+   i32.const 0
+  end
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 101
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/nullAnimal
+  local.tee $0
+  if (result i32)
+   local.get $0
+   i32.const 16
+   i32.sub
+   i32.load
+   i32.const 4
+   call $~lib/runtime/__runtime_instanceof
+  else   
+   i32.const 0
+  end
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 102
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/nullCat
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 104
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/nullCat
+  local.tee $0
+  if (result i32)
+   local.get $0
+   i32.const 16
+   i32.sub
+   i32.load
+   i32.const 3
+   call $~lib/runtime/__runtime_instanceof
+  else   
+   i32.const 0
+  end
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 105
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/nullCat
+  local.tee $0
+  if (result i32)
+   local.get $0
+   i32.const 16
+   i32.sub
+   i32.load
+   i32.const 4
+   call $~lib/runtime/__runtime_instanceof
+  else   
+   i32.const 0
+  end
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 106
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/nullBlackcat
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 108
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/nullBlackcat
+  local.tee $0
+  if (result i32)
+   local.get $0
+   i32.const 16
+   i32.sub
+   i32.load
+   i32.const 3
+   call $~lib/runtime/__runtime_instanceof
+  else   
+   i32.const 0
+  end
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 109
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/nullBlackcat
+  local.tee $0
+  if (result i32)
+   local.get $0
+   i32.const 16
+   i32.sub
+   i32.load
+   i32.const 4
+   call $~lib/runtime/__runtime_instanceof
+  else   
+   i32.const 0
+  end
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 110
    i32.const 0
    call $~lib/env/abort
    unreachable
   end
  )
- (func $start (; 2 ;) (type $FUNCSIG$v)
-  call $start:runtime/instanceof
+ (func $runtime/instanceof/main (; 10 ;) (type $FUNCSIG$v)
+  global.get $~lib/started
+  i32.eqz
+  if
+   call $start:runtime/instanceof
+   i32.const 1
+   global.set $~lib/started
+  end
  )
- (func $~lib/runtime/__runtime_instanceof (; 3 ;) (type $FUNCSIG$iii) (param $0 i32) (param $1 i32) (result i32)
+ (func $~lib/runtime/__runtime_instanceof (; 11 ;) (type $FUNCSIG$iii) (param $0 i32) (param $1 i32) (result i32)
   block $nope
    block $runtime/instanceof/BlackCat
     block $runtime/instanceof/Cat
@@ -153,7 +790,7 @@
   end
   i32.const 0
  )
- (func $null (; 4 ;) (type $FUNCSIG$v)
+ (func $null (; 12 ;) (type $FUNCSIG$v)
   nop
  )
 )
diff --git a/tests/compiler/runtime/instanceof.ts b/tests/compiler/runtime/instanceof.ts
index 542d60a2..bb367bc4 100644
--- a/tests/compiler/runtime/instanceof.ts
+++ b/tests/compiler/runtime/instanceof.ts
@@ -1,4 +1,5 @@
 import { __runtime_id, __runtime_instanceof } from "runtime";
+import "../gc/_dummy";
 
 class Animal {}
 class Cat extends Animal {}
@@ -59,3 +60,53 @@ assert(! // Cat isn't necessarily a BlackCat
     __runtime_id<BlackCat>()
   )
 );
+
+var animal: Animal = new Animal();
+var cat: Animal = new Cat();
+var blackcat: Animal = new BlackCat();
+
+assert(animal instanceof Animal); // static true
+assert(!(animal instanceof Cat)); // dynamic false
+assert(!(animal instanceof BlackCat)); // dynamic false
+
+assert(cat instanceof Animal); // static true
+assert(cat instanceof Cat); // dynamic true
+assert(!(cat instanceof BlackCat)); // dynamic false
+
+assert(blackcat instanceof Animal); // static true
+assert(blackcat instanceof Cat); // dynamic true
+assert(blackcat instanceof BlackCat); // dynamic true
+
+var nullableAnimal: Animal | null = new Animal();
+var nullableCat: Animal | null = new Cat();
+var nullableBlackcat: Animal | null = new BlackCat();
+
+assert(nullableAnimal instanceof Animal); // static true
+assert(!(nullableAnimal instanceof Cat)); // dynamic false
+assert(!(nullableAnimal instanceof BlackCat)); // dynamic false
+
+assert(nullableCat instanceof Animal); // static true
+assert(nullableCat instanceof Cat); // dynamic true
+assert(!(nullableCat instanceof BlackCat)); // dynamic false
+
+assert(nullableBlackcat instanceof Animal); // static true
+assert(nullableBlackcat instanceof Cat); // dynamic true
+assert(nullableBlackcat instanceof BlackCat); // dynamic true
+
+var nullAnimal: Animal | null = null;
+var nullCat: Animal | null = null;
+var nullBlackcat: Animal | null = null;
+
+assert(!(nullAnimal instanceof Animal)); // static false
+assert(!(nullAnimal instanceof Cat)); // dynamic false
+assert(!(nullAnimal instanceof BlackCat)); // dynamic false
+
+assert(!(nullCat instanceof Animal)); // static false
+assert(!(nullCat instanceof Cat)); // dynamic false
+assert(!(nullCat instanceof BlackCat)); // dynamic false
+
+assert(!(nullBlackcat instanceof Animal)); // static false
+assert(!(nullBlackcat instanceof Cat)); // dynamic false
+assert(!(nullBlackcat instanceof BlackCat)); // dynamic false
+
+@start export function main(): void {}
diff --git a/tests/compiler/runtime/instanceof.untouched.wat b/tests/compiler/runtime/instanceof.untouched.wat
index 38cf3701..f8e4faad 100644
--- a/tests/compiler/runtime/instanceof.untouched.wat
+++ b/tests/compiler/runtime/instanceof.untouched.wat
@@ -1,25 +1,275 @@
 (module
  (type $FUNCSIG$viiii (func (param i32 i32 i32 i32)))
- (type $FUNCSIG$v (func))
+ (type $FUNCSIG$ii (func (param i32) (result i32)))
  (type $FUNCSIG$iii (func (param i32 i32) (result i32)))
+ (type $FUNCSIG$vi (func (param i32)))
+ (type $FUNCSIG$viiddddd (func (param i32 i32 f64 f64 f64 f64 f64)))
+ (type $FUNCSIG$v (func))
  (import "env" "abort" (func $~lib/env/abort (param i32 i32 i32 i32)))
+ (import "env" "trace" (func $~lib/env/trace (param i32 i32 f64 f64 f64 f64 f64)))
  (memory $0 1)
- (data (i32.const 8) "\02\00\00\00*\00\00\00r\00u\00n\00t\00i\00m\00e\00/\00i\00n\00s\00t\00a\00n\00c\00e\00o\00f\00.\00t\00s\00")
+ (data (i32.const 8) "\02\00\00\00*\00\00\00\00\00\00\00\00\00\00\00r\00u\00n\00t\00i\00m\00e\00/\00i\00n\00s\00t\00a\00n\00c\00e\00o\00f\00.\00t\00s\00")
+ (data (i32.const 72) "\02\00\00\00\1e\00\00\00\00\00\00\00\00\00\00\00~\00l\00i\00b\00/\00r\00u\00n\00t\00i\00m\00e\00.\00t\00s\00")
+ (data (i32.const 120) "\02\00\00\00\16\00\00\00\00\00\00\00\00\00\00\00g\00c\00.\00r\00e\00g\00i\00s\00t\00e\00r\00")
  (table $0 1 funcref)
  (elem (i32.const 0) $null)
- (global $~lib/memory/HEAP_BASE i32 (i32.const 60))
+ (global $gc/_dummy/collect_count (mut i32) (i32.const 0))
+ (global $gc/_dummy/register_count (mut i32) (i32.const 0))
+ (global $gc/_dummy/register_ref (mut i32) (i32.const 0))
+ (global $gc/_dummy/link_count (mut i32) (i32.const 0))
+ (global $gc/_dummy/link_ref (mut i32) (i32.const 0))
+ (global $gc/_dummy/link_parentRef (mut i32) (i32.const 0))
+ (global $gc/_dummy/unlink_count (mut i32) (i32.const 0))
+ (global $gc/_dummy/unlink_ref (mut i32) (i32.const 0))
+ (global $gc/_dummy/unlink_parentRef (mut i32) (i32.const 0))
+ (global $gc/_dummy/mark_count (mut i32) (i32.const 0))
+ (global $gc/_dummy/mark_ref (mut i32) (i32.const 0))
+ (global $~lib/util/runtime/HEADER_SIZE i32 (i32.const 16))
+ (global $~lib/allocator/arena/startOffset (mut i32) (i32.const 0))
+ (global $~lib/allocator/arena/offset (mut i32) (i32.const 0))
+ (global $~lib/util/runtime/HEADER_MAGIC i32 (i32.const -1520547049))
+ (global $~lib/ASC_NO_ASSERT i32 (i32.const 0))
+ (global $runtime/instanceof/animal (mut i32) (i32.const 0))
+ (global $runtime/instanceof/cat (mut i32) (i32.const 0))
+ (global $runtime/instanceof/blackcat (mut i32) (i32.const 0))
+ (global $runtime/instanceof/nullableAnimal (mut i32) (i32.const 0))
+ (global $runtime/instanceof/nullableCat (mut i32) (i32.const 0))
+ (global $runtime/instanceof/nullableBlackcat (mut i32) (i32.const 0))
+ (global $runtime/instanceof/nullAnimal (mut i32) (i32.const 0))
+ (global $runtime/instanceof/nullCat (mut i32) (i32.const 0))
+ (global $runtime/instanceof/nullBlackcat (mut i32) (i32.const 0))
+ (global $~lib/started (mut i32) (i32.const 0))
+ (global $~lib/memory/HEAP_BASE i32 (i32.const 160))
+ (global $~lib/capabilities i32 (i32.const 2))
  (export "memory" (memory $0))
  (export "table" (table $0))
- (start $start)
- (func $start:runtime/instanceof (; 1 ;) (type $FUNCSIG$v)
+ (export "main" (func $runtime/instanceof/main))
+ (export ".capabilities" (global $~lib/capabilities))
+ (func $~lib/runtime/runtime.adjust (; 2 ;) (type $FUNCSIG$ii) (param $0 i32) (result i32)
+  i32.const 1
+  i32.const 32
+  local.get $0
+  global.get $~lib/util/runtime/HEADER_SIZE
+  i32.add
+  i32.const 1
+  i32.sub
+  i32.clz
+  i32.sub
+  i32.shl
+ )
+ (func $~lib/allocator/arena/__mem_allocate (; 3 ;) (type $FUNCSIG$ii) (param $0 i32) (result i32)
+  (local $1 i32)
+  (local $2 i32)
+  (local $3 i32)
+  (local $4 i32)
+  (local $5 i32)
+  (local $6 i32)
+  local.get $0
+  i32.const 1073741824
+  i32.gt_u
+  if
+   unreachable
+  end
+  global.get $~lib/allocator/arena/offset
+  local.set $1
+  local.get $1
+  local.get $0
+  local.tee $2
+  i32.const 1
+  local.tee $3
+  local.get $2
+  local.get $3
+  i32.gt_u
+  select
+  i32.add
+  i32.const 7
+  i32.add
+  i32.const 7
+  i32.const -1
+  i32.xor
+  i32.and
+  local.set $4
+  current_memory
+  local.set $5
+  local.get $4
+  local.get $5
+  i32.const 16
+  i32.shl
+  i32.gt_u
+  if
+   local.get $4
+   local.get $1
+   i32.sub
+   i32.const 65535
+   i32.add
+   i32.const 65535
+   i32.const -1
+   i32.xor
+   i32.and
+   i32.const 16
+   i32.shr_u
+   local.set $2
+   local.get $5
+   local.tee $3
+   local.get $2
+   local.tee $6
+   local.get $3
+   local.get $6
+   i32.gt_s
+   select
+   local.set $3
+   local.get $3
+   grow_memory
+   i32.const 0
+   i32.lt_s
+   if
+    local.get $2
+    grow_memory
+    i32.const 0
+    i32.lt_s
+    if
+     unreachable
+    end
+   end
+  end
+  local.get $4
+  global.set $~lib/allocator/arena/offset
+  local.get $1
+ )
+ (func $~lib/memory/memory.allocate (; 4 ;) (type $FUNCSIG$ii) (param $0 i32) (result i32)
+  local.get $0
+  call $~lib/allocator/arena/__mem_allocate
+  return
+ )
+ (func $~lib/runtime/runtime.allocate (; 5 ;) (type $FUNCSIG$ii) (param $0 i32) (result i32)
+  (local $1 i32)
+  local.get $0
+  call $~lib/runtime/runtime.adjust
+  call $~lib/memory/memory.allocate
+  local.set $1
+  local.get $1
+  global.get $~lib/util/runtime/HEADER_MAGIC
+  i32.store
+  local.get $1
+  local.get $0
+  i32.store offset=4
+  local.get $1
+  i32.const 0
+  i32.store offset=8
+  local.get $1
+  i32.const 0
+  i32.store offset=12
+  local.get $1
+  global.get $~lib/util/runtime/HEADER_SIZE
+  i32.add
+ )
+ (func $gc/_dummy/__ref_register (; 6 ;) (type $FUNCSIG$vi) (param $0 i32)
+  i32.const 136
+  i32.const 1
+  local.get $0
+  f64.convert_i32_u
+  f64.const 0
+  f64.const 0
+  f64.const 0
+  f64.const 0
+  call $~lib/env/trace
+  global.get $gc/_dummy/register_count
+  i32.const 1
+  i32.add
+  global.set $gc/_dummy/register_count
+  local.get $0
+  global.set $gc/_dummy/register_ref
+ )
+ (func $~lib/runtime/runtime.register (; 7 ;) (type $FUNCSIG$iii) (param $0 i32) (param $1 i32) (result i32)
+  (local $2 i32)
+  local.get $0
+  global.get $~lib/memory/HEAP_BASE
+  i32.gt_u
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 88
+   i32.const 107
+   i32.const 6
+   call $~lib/env/abort
+   unreachable
+  end
+  local.get $0
+  global.get $~lib/util/runtime/HEADER_SIZE
+  i32.sub
+  local.set $2
+  local.get $2
+  i32.load
+  global.get $~lib/util/runtime/HEADER_MAGIC
+  i32.eq
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 88
+   i32.const 109
+   i32.const 6
+   call $~lib/env/abort
+   unreachable
+  end
+  local.get $2
+  local.get $1
+  i32.store
+  local.get $0
+  call $gc/_dummy/__ref_register
+  local.get $0
+ )
+ (func $runtime/instanceof/Animal#constructor (; 8 ;) (type $FUNCSIG$ii) (param $0 i32) (result i32)
+  local.get $0
+  i32.eqz
+  if
+   i32.const 0
+   call $~lib/runtime/runtime.allocate
+   i32.const 1
+   call $~lib/runtime/runtime.register
+   local.set $0
+  end
+  local.get $0
+ )
+ (func $runtime/instanceof/Cat#constructor (; 9 ;) (type $FUNCSIG$ii) (param $0 i32) (result i32)
+  local.get $0
+  i32.eqz
+  if
+   i32.const 0
+   call $~lib/runtime/runtime.allocate
+   i32.const 3
+   call $~lib/runtime/runtime.register
+   local.set $0
+  end
+  local.get $0
+  call $runtime/instanceof/Animal#constructor
+  local.set $0
+  local.get $0
+ )
+ (func $runtime/instanceof/BlackCat#constructor (; 10 ;) (type $FUNCSIG$ii) (param $0 i32) (result i32)
+  local.get $0
+  i32.eqz
+  if
+   i32.const 0
+   call $~lib/runtime/runtime.allocate
+   i32.const 4
+   call $~lib/runtime/runtime.register
+   local.set $0
+  end
+  local.get $0
+  call $runtime/instanceof/Cat#constructor
+  local.set $0
+  local.get $0
+ )
+ (func $start:runtime/instanceof (; 11 ;) (type $FUNCSIG$v)
+  (local $0 i32)
   i32.const 1
   i32.const 1
   call $~lib/runtime/__runtime_instanceof
   i32.eqz
   if
    i32.const 0
-   i32.const 16
-   i32.const 7
+   i32.const 24
+   i32.const 8
    i32.const 0
    call $~lib/env/abort
    unreachable
@@ -30,8 +280,8 @@
   i32.eqz
   if
    i32.const 0
-   i32.const 16
-   i32.const 14
+   i32.const 24
+   i32.const 15
    i32.const 0
    call $~lib/env/abort
    unreachable
@@ -42,8 +292,8 @@
   i32.eqz
   if
    i32.const 0
-   i32.const 16
-   i32.const 21
+   i32.const 24
+   i32.const 22
    i32.const 0
    call $~lib/env/abort
    unreachable
@@ -54,8 +304,8 @@
   i32.eqz
   if
    i32.const 0
-   i32.const 16
-   i32.const 28
+   i32.const 24
+   i32.const 29
    i32.const 0
    call $~lib/env/abort
    unreachable
@@ -66,8 +316,8 @@
   i32.eqz
   if
    i32.const 0
-   i32.const 16
-   i32.const 35
+   i32.const 24
+   i32.const 36
    i32.const 0
    call $~lib/env/abort
    unreachable
@@ -79,8 +329,8 @@
   i32.eqz
   if
    i32.const 0
-   i32.const 16
-   i32.const 42
+   i32.const 24
+   i32.const 43
    i32.const 0
    call $~lib/env/abort
    unreachable
@@ -92,8 +342,8 @@
   i32.eqz
   if
    i32.const 0
-   i32.const 16
-   i32.const 49
+   i32.const 24
+   i32.const 50
    i32.const 0
    call $~lib/env/abort
    unreachable
@@ -105,17 +355,579 @@
   i32.eqz
   if
    i32.const 0
+   i32.const 24
+   i32.const 57
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $~lib/memory/HEAP_BASE
+  i32.const 7
+  i32.add
+  i32.const 7
+  i32.const -1
+  i32.xor
+  i32.and
+  global.set $~lib/allocator/arena/startOffset
+  global.get $~lib/allocator/arena/startOffset
+  global.set $~lib/allocator/arena/offset
+  i32.const 0
+  call $runtime/instanceof/Animal#constructor
+  global.set $runtime/instanceof/animal
+  i32.const 0
+  call $runtime/instanceof/Cat#constructor
+  global.set $runtime/instanceof/cat
+  i32.const 0
+  call $runtime/instanceof/BlackCat#constructor
+  global.set $runtime/instanceof/blackcat
+  block (result i32)
+   global.get $runtime/instanceof/animal
+   drop
+   i32.const 1
+  end
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 68
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/animal
+  local.tee $0
+  i32.eqz
+  if (result i32)
+   i32.const 0
+  else   
+   local.get $0
    i32.const 16
-   i32.const 56
+   i32.sub
+   i32.load
+   i32.const 3
+   call $~lib/runtime/__runtime_instanceof
+  end
+  i32.eqz
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 69
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/animal
+  local.tee $0
+  i32.eqz
+  if (result i32)
+   i32.const 0
+  else   
+   local.get $0
+   i32.const 16
+   i32.sub
+   i32.load
+   i32.const 4
+   call $~lib/runtime/__runtime_instanceof
+  end
+  i32.eqz
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 70
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  block (result i32)
+   global.get $runtime/instanceof/cat
+   drop
+   i32.const 1
+  end
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 72
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/cat
+  local.tee $0
+  i32.eqz
+  if (result i32)
+   i32.const 0
+  else   
+   local.get $0
+   i32.const 16
+   i32.sub
+   i32.load
+   i32.const 3
+   call $~lib/runtime/__runtime_instanceof
+  end
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 73
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/cat
+  local.tee $0
+  i32.eqz
+  if (result i32)
+   i32.const 0
+  else   
+   local.get $0
+   i32.const 16
+   i32.sub
+   i32.load
+   i32.const 4
+   call $~lib/runtime/__runtime_instanceof
+  end
+  i32.eqz
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 74
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  block (result i32)
+   global.get $runtime/instanceof/blackcat
+   drop
+   i32.const 1
+  end
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 76
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/blackcat
+  local.tee $0
+  i32.eqz
+  if (result i32)
+   i32.const 0
+  else   
+   local.get $0
+   i32.const 16
+   i32.sub
+   i32.load
+   i32.const 3
+   call $~lib/runtime/__runtime_instanceof
+  end
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 77
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/blackcat
+  local.tee $0
+  i32.eqz
+  if (result i32)
+   i32.const 0
+  else   
+   local.get $0
+   i32.const 16
+   i32.sub
+   i32.load
+   i32.const 4
+   call $~lib/runtime/__runtime_instanceof
+  end
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 78
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  i32.const 0
+  call $runtime/instanceof/Animal#constructor
+  global.set $runtime/instanceof/nullableAnimal
+  i32.const 0
+  call $runtime/instanceof/Cat#constructor
+  global.set $runtime/instanceof/nullableCat
+  i32.const 0
+  call $runtime/instanceof/BlackCat#constructor
+  global.set $runtime/instanceof/nullableBlackcat
+  global.get $runtime/instanceof/nullableAnimal
+  i32.const 0
+  i32.ne
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 84
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/nullableAnimal
+  local.tee $0
+  i32.eqz
+  if (result i32)
+   i32.const 0
+  else   
+   local.get $0
+   i32.const 16
+   i32.sub
+   i32.load
+   i32.const 3
+   call $~lib/runtime/__runtime_instanceof
+  end
+  i32.eqz
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 85
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/nullableAnimal
+  local.tee $0
+  i32.eqz
+  if (result i32)
+   i32.const 0
+  else   
+   local.get $0
+   i32.const 16
+   i32.sub
+   i32.load
+   i32.const 4
+   call $~lib/runtime/__runtime_instanceof
+  end
+  i32.eqz
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 86
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/nullableCat
+  i32.const 0
+  i32.ne
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 88
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/nullableCat
+  local.tee $0
+  i32.eqz
+  if (result i32)
+   i32.const 0
+  else   
+   local.get $0
+   i32.const 16
+   i32.sub
+   i32.load
+   i32.const 3
+   call $~lib/runtime/__runtime_instanceof
+  end
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 89
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/nullableCat
+  local.tee $0
+  i32.eqz
+  if (result i32)
+   i32.const 0
+  else   
+   local.get $0
+   i32.const 16
+   i32.sub
+   i32.load
+   i32.const 4
+   call $~lib/runtime/__runtime_instanceof
+  end
+  i32.eqz
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 90
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/nullableBlackcat
+  i32.const 0
+  i32.ne
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 92
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/nullableBlackcat
+  local.tee $0
+  i32.eqz
+  if (result i32)
+   i32.const 0
+  else   
+   local.get $0
+   i32.const 16
+   i32.sub
+   i32.load
+   i32.const 3
+   call $~lib/runtime/__runtime_instanceof
+  end
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 93
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/nullableBlackcat
+  local.tee $0
+  i32.eqz
+  if (result i32)
+   i32.const 0
+  else   
+   local.get $0
+   i32.const 16
+   i32.sub
+   i32.load
+   i32.const 4
+   call $~lib/runtime/__runtime_instanceof
+  end
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 94
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/nullAnimal
+  i32.const 0
+  i32.ne
+  i32.eqz
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 100
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/nullAnimal
+  local.tee $0
+  i32.eqz
+  if (result i32)
+   i32.const 0
+  else   
+   local.get $0
+   i32.const 16
+   i32.sub
+   i32.load
+   i32.const 3
+   call $~lib/runtime/__runtime_instanceof
+  end
+  i32.eqz
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 101
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/nullAnimal
+  local.tee $0
+  i32.eqz
+  if (result i32)
+   i32.const 0
+  else   
+   local.get $0
+   i32.const 16
+   i32.sub
+   i32.load
+   i32.const 4
+   call $~lib/runtime/__runtime_instanceof
+  end
+  i32.eqz
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 102
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/nullCat
+  i32.const 0
+  i32.ne
+  i32.eqz
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 104
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/nullCat
+  local.tee $0
+  i32.eqz
+  if (result i32)
+   i32.const 0
+  else   
+   local.get $0
+   i32.const 16
+   i32.sub
+   i32.load
+   i32.const 3
+   call $~lib/runtime/__runtime_instanceof
+  end
+  i32.eqz
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 105
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/nullCat
+  local.tee $0
+  i32.eqz
+  if (result i32)
+   i32.const 0
+  else   
+   local.get $0
+   i32.const 16
+   i32.sub
+   i32.load
+   i32.const 4
+   call $~lib/runtime/__runtime_instanceof
+  end
+  i32.eqz
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 106
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/nullBlackcat
+  i32.const 0
+  i32.ne
+  i32.eqz
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 108
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/nullBlackcat
+  local.tee $0
+  i32.eqz
+  if (result i32)
+   i32.const 0
+  else   
+   local.get $0
+   i32.const 16
+   i32.sub
+   i32.load
+   i32.const 3
+   call $~lib/runtime/__runtime_instanceof
+  end
+  i32.eqz
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 109
+   i32.const 0
+   call $~lib/env/abort
+   unreachable
+  end
+  global.get $runtime/instanceof/nullBlackcat
+  local.tee $0
+  i32.eqz
+  if (result i32)
+   i32.const 0
+  else   
+   local.get $0
+   i32.const 16
+   i32.sub
+   i32.load
+   i32.const 4
+   call $~lib/runtime/__runtime_instanceof
+  end
+  i32.eqz
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 24
+   i32.const 110
    i32.const 0
    call $~lib/env/abort
    unreachable
   end
  )
- (func $start (; 2 ;) (type $FUNCSIG$v)
+ (func $runtime/instanceof/main (; 12 ;) (type $FUNCSIG$v)
+  global.get $~lib/started
+  i32.eqz
+  if
+   call $start
+   i32.const 1
+   global.set $~lib/started
+  end
+ )
+ (func $start (; 13 ;) (type $FUNCSIG$v)
   call $start:runtime/instanceof
  )
- (func $~lib/runtime/__runtime_instanceof (; 3 ;) (type $FUNCSIG$iii) (param $0 i32) (param $1 i32) (result i32)
+ (func $~lib/runtime/__runtime_instanceof (; 14 ;) (type $FUNCSIG$iii) (param $0 i32) (param $1 i32) (result i32)
   block $nope
    block $runtime/instanceof/BlackCat
     block $runtime/instanceof/Cat
@@ -159,6 +971,6 @@
   i32.const 0
   return
  )
- (func $null (; 4 ;) (type $FUNCSIG$v)
+ (func $null (; 15 ;) (type $FUNCSIG$v)
  )
 )
diff --git a/tests/compiler/std/arraybuffer.optimized.wat b/tests/compiler/std/arraybuffer.optimized.wat
index 8d392aaa..0ca4ccf8 100644
--- a/tests/compiler/std/arraybuffer.optimized.wat
+++ b/tests/compiler/std/arraybuffer.optimized.wat
@@ -1847,7 +1847,7 @@
    call $~lib/env/abort
    unreachable
   end
-  block $__inlined_func$~lib/arraybuffer/ArrayBuffer.isView<~lib/typedarray/Uint8Array>13 (result i32)
+  block $__inlined_func$~lib/arraybuffer/ArrayBuffer.isView<~lib/typedarray/Int32Array>11 (result i32)
    i32.const 1
    i32.const 12
    call $~lib/runtime/runtime.allocate
@@ -1855,7 +1855,7 @@
    call $~lib/runtime/runtime.register
    i32.const 2
    call $~lib/arraybuffer/ArrayBufferView#constructor
-   br_if $__inlined_func$~lib/arraybuffer/ArrayBuffer.isView<~lib/typedarray/Uint8Array>13
+   br_if $__inlined_func$~lib/arraybuffer/ArrayBuffer.isView<~lib/typedarray/Int32Array>11
    drop
    i32.const 0
   end
@@ -1870,12 +1870,12 @@
   end
   i32.const 1
   global.set $~lib/argc
-  block $__inlined_func$~lib/arraybuffer/ArrayBuffer.isView<~lib/typedarray/Uint8Array>14 (result i32)
+  block $__inlined_func$~lib/arraybuffer/ArrayBuffer.isView<~lib/dataview/DataView>12 (result i32)
    i32.const 1
    global.get $std/arraybuffer/arr8
    i32.load
    call $~lib/dataview/DataView#constructor|trampoline
-   br_if $__inlined_func$~lib/arraybuffer/ArrayBuffer.isView<~lib/typedarray/Uint8Array>14
+   br_if $__inlined_func$~lib/arraybuffer/ArrayBuffer.isView<~lib/dataview/DataView>12
    drop
    i32.const 0
   end
diff --git a/tests/compiler/std/arraybuffer.untouched.wat b/tests/compiler/std/arraybuffer.untouched.wat
index 63a50c33..2951878a 100644
--- a/tests/compiler/std/arraybuffer.untouched.wat
+++ b/tests/compiler/std/arraybuffer.untouched.wat
@@ -1982,38 +1982,570 @@
  (func $~lib/arraybuffer/ArrayBuffer.isView<~lib/array/Array<i32>> (; 12 ;) (type $FUNCSIG$ii) (param $0 i32) (result i32)
   local.get $0
   if
-   nop
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
   end
   i32.const 0
  )
  (func $~lib/arraybuffer/ArrayBuffer.isView<usize> (; 13 ;) (type $FUNCSIG$ii) (param $0 i32) (result i32)
   local.get $0
   if
-   nop
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
   end
   i32.const 0
  )
  (func $~lib/arraybuffer/ArrayBuffer.isView<~lib/typedarray/Uint8Array> (; 14 ;) (type $FUNCSIG$ii) (param $0 i32) (result i32)
   local.get $0
   if
-   i32.const 1
-   return
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 1
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
   end
   i32.const 0
  )
  (func $~lib/arraybuffer/ArrayBuffer.isView<~lib/typedarray/Int32Array> (; 15 ;) (type $FUNCSIG$ii) (param $0 i32) (result i32)
   local.get $0
   if
-   i32.const 1
-   return
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 1
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
   end
   i32.const 0
  )
  (func $~lib/arraybuffer/ArrayBuffer.isView<~lib/dataview/DataView> (; 16 ;) (type $FUNCSIG$ii) (param $0 i32) (result i32)
   local.get $0
   if
-   i32.const 1
-   return
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 0
+   end
+   if
+    i32.const 1
+    return
+   end
+   block (result i32)
+    local.get $0
+    drop
+    i32.const 1
+   end
+   if
+    i32.const 1
+    return
+   end
   end
   i32.const 0
  )