Improve support for reftypes (#168)

* Add nullref as an externally specifiable type

Spec commit: WebAssembly/reference-types@d4bc208

* Add typed select operator for reference types proposal

The reftypes proposal adds a typed select operator to simplify type checking
for reference types. The existing select operator is henceforth limited to
operate on the original MVP types only.

Spec change: WebAssembly/reference-types#43

Author: eqrion

src/binary_reader.rs

@@ -188,6 +188,7 @@ impl<'a> BinaryReader<'a> {
             -0x05 => Ok(Type::V128),
             -0x10 => Ok(Type::AnyFunc),
             -0x11 => Ok(Type::AnyRef),
+            -0x12 => Ok(Type::NullRef),
             -0x20 => Ok(Type::Func),
             -0x40 => Ok(Type::EmptyBlockType),
             _ => Err(BinaryReaderError {
@@ -987,6 +988,18 @@ impl<'a> BinaryReader<'a> {
             },
             0x1a => Operator::Drop,
             0x1b => Operator::Select,
+            0x1c => {
+                let results = self.read_var_u32()?;
+                if results != 1 {
+                    return Err(BinaryReaderError {
+                        message: "bad number of results",
+                        offset: self.position,
+                    });
+                }
+                Operator::TypedSelect {
+                    ty: self.read_type()?,
+                }
+            }
             0x20 => Operator::LocalGet {
                 local_index: self.read_var_u32()?,
             },

src/operators_validator.rs

@@ -26,9 +26,9 @@ use crate::primitives::{
 pub(crate) fn is_subtype_supertype(subtype: Type, supertype: Type) -> bool {
     match supertype {
         Type::AnyRef => {
-            subtype == Type::AnyRef || subtype == Type::AnyFunc || subtype == Type::Null
+            subtype == Type::AnyRef || subtype == Type::AnyFunc || subtype == Type::NullRef
         }
-        Type::AnyFunc => subtype == Type::AnyFunc || subtype == Type::Null,
+        Type::AnyFunc => subtype == Type::AnyFunc || subtype == Type::NullRef,
         _ => subtype == supertype,
     }
 }
@@ -624,28 +624,35 @@ impl OperatorValidator {
         self.check_frame_size(3)?;
         let func_state = &self.func_state;
         let last_block = func_state.last_block();
-        Ok(if last_block.is_stack_polymorphic() {
+
+        let ty = if last_block.is_stack_polymorphic() {
             match func_state.stack_types.len() - last_block.stack_starts_at {
-                0 => None,
+                0 => return Ok(None),
                 1 => {
                     self.check_operands_1(Type::I32)?;
-                    None
+                    return Ok(None);
                 }
                 2 => {
                     self.check_operands_1(Type::I32)?;
-                    Some(func_state.stack_types[func_state.stack_types.len() - 2])
+                    func_state.stack_types[func_state.stack_types.len() - 2]
                 }
                 _ => {
                     let ty = func_state.stack_types[func_state.stack_types.len() - 3];
                     self.check_operands_2(ty, Type::I32)?;
-                    Some(ty)
+                    ty
                 }
             }
         } else {
             let ty = func_state.stack_types[func_state.stack_types.len() - 3];
             self.check_operands_2(ty, Type::I32)?;
-            Some(ty)
-        })
+            ty
+        };
+
+        if !ty.is_valid_for_old_select() {
+            return Err("invalid type for select");
+        }
+
+        Ok(Some(ty))
     }
 
     pub(crate) fn process_operator(
@@ -761,6 +768,10 @@ impl OperatorValidator {
                 let ty = self.check_select()?;
                 self.func_state.change_frame_after_select(ty)?;
             }
+            Operator::TypedSelect { ty } => {
+                self.check_operands(&[Type::I32, ty, ty])?;
+                self.func_state.change_frame_after_select(Some(ty))?;
+            }
             Operator::LocalGet { local_index } => {
                 if local_index as usize >= self.func_state.local_types.len() {
                     return Err("local index out of bounds");
@@ -1323,7 +1334,7 @@ impl OperatorValidator {
             }
             Operator::RefNull => {
                 self.check_reference_types_enabled()?;
-                self.func_state.change_frame_with_type(0, Type::Null)?;
+                self.func_state.change_frame_with_type(0, Type::NullRef)?;
             }
             Operator::RefIsNull => {
                 self.check_reference_types_enabled()?;

src/primitives.rs

@@ -78,9 +78,18 @@ pub enum Type {
     V128,
     AnyFunc,
     AnyRef,
+    NullRef,
     Func,
     EmptyBlockType,
-    Null,
+}
+
+impl Type {
+    pub(crate) fn is_valid_for_old_select(&self) -> bool {
+        match self {
+            Type::I32 | Type::I64 | Type::F32 | Type::F64 => true,
+            _ => false,
+        }
+    }
 }
 
 /// Either a value type or a function type.
@@ -246,6 +255,7 @@ pub enum Operator<'a> {
     CallIndirect { index: u32, table_index: u32 },
     Drop,
     Select,
+    TypedSelect { ty: Type },
     LocalGet { local_index: u32 },
     LocalSet { local_index: u32 },
     LocalTee { local_index: u32 },

src/validator.rs

@@ -195,7 +195,7 @@ impl<'a> ValidatingParser<'a> {
     fn check_value_type(&self, ty: Type) -> ValidatorResult<'a, ()> {
         match ty {
             Type::I32 | Type::I64 | Type::F32 | Type::F64 => Ok(()),
-            Type::Null | Type::AnyFunc | Type::AnyRef => {
+            Type::NullRef | Type::AnyFunc | Type::AnyRef => {
                 if !self.config.operator_config.enable_reference_types {
                     return self.create_error("reference types support is not enabled");
                 }
@@ -316,7 +316,7 @@ impl<'a> ValidatingParser<'a> {
                 if !self.config.operator_config.enable_reference_types {
                     return self.create_error("reference types support is not enabled");
                 }
-                Type::Null
+                Type::NullRef
             }
             Operator::V128Const { .. } => {
                 if !self.config.operator_config.enable_simd {