[vm] Replace unreachable 'throw' code with Never/Stop

Change core library '_throwNew' helpers to return Never.

Since https://dart-review.googlesource.com/c/sdk/+/405604 flow graph
builder automatically appends Stop and closes fragment after appending
a call with return type Never. So additional 'throw' after calling
'_throwNew' are no longer needed and can be removed.

Also, cleanup unused FlowGraphBuilder::ThrowTypeError() and
replace 'throw' with shorter Stop at the end of 'case'.

TEST=ci

Change-Id: I0a5109fd693c0463c53e70e5dd261f8080bf1132
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/406080
Reviewed-by: Slava Egorov <[email protected]>
Commit-Queue: Alexander Markov <[email protected]>

Author: alexmarkov

runtime/vm/compiler/frontend/kernel_binary_flowgraph.cc

@@ -679,7 +679,7 @@ Fragment StreamingFlowGraphBuilder::BuildFunctionBody(
   } else if (dart_function.is_external()) {
     body += ThrowNoSuchMethodError(TokenPosition::kNoSource, dart_function,
                                    /*incompatible_arguments=*/false);
-    body += ThrowException(TokenPosition::kNoSource);  // Close graph.
+    ASSERT(body.is_closed());
   } else if (has_body) {
     body += BuildStatement();
   }
@@ -1699,10 +1699,6 @@ Fragment StreamingFlowGraphBuilder::StringInterpolateSingle(
   return flow_graph_builder_->StringInterpolateSingle(position);
 }
 
-Fragment StreamingFlowGraphBuilder::ThrowTypeError() {
-  return flow_graph_builder_->ThrowTypeError();
-}
-
 Fragment StreamingFlowGraphBuilder::LoadInstantiatorTypeArguments() {
   return flow_graph_builder_->LoadInstantiatorTypeArguments();
 }
@@ -2113,7 +2109,7 @@ Fragment StreamingFlowGraphBuilder::BuildVariableGetImpl(
           already_assigned += flow_graph_builder_->ThrowLateInitializationError(
               position, "_throwLocalAssignedDuringInitialization",
               variable->name());
-          already_assigned += Goto(join);
+          ASSERT(already_assigned.is_closed());
         }
       } else {
         // Late non-final variable. Store the initializer result.
@@ -2126,7 +2122,7 @@ Fragment StreamingFlowGraphBuilder::BuildVariableGetImpl(
       Fragment initialize(is_uninitialized);
       initialize += flow_graph_builder_->ThrowLateInitializationError(
           position, "_throwLocalNotInitialized", variable->name());
-      initialize += Goto(join);
+      ASSERT(initialize.is_closed());
     }
   }
 
@@ -2193,7 +2189,7 @@ Fragment StreamingFlowGraphBuilder::BuildVariableSetImpl(
       Fragment already_initialized(is_initialized);
       already_initialized += flow_graph_builder_->ThrowLateInitializationError(
           position, "_throwLocalAlreadyInitialized", variable->name());
-      already_initialized += Goto(join);
+      ASSERT(already_initialized.is_closed());
     }
 
     instructions = Fragment(instructions.entry, join);
@@ -4641,11 +4637,6 @@ Fragment StreamingFlowGraphBuilder::BuildAssertStatement(
       Class::ZoneHandle(Z, Library::LookupCoreClass(Symbols::AssertionError()));
   ASSERT(!klass.IsNull());
 
-  // Build equivalent of `throw _AssertionError._throwNew(start, end, message)`
-  // expression. We build throw (even through _throwNew already throws) because
-  // call is not a valid last instruction for the block. Blocks can only
-  // terminate with explicit control flow instructions (Branch, Goto, Return
-  // or Throw).
   Fragment otherwise_fragment(otherwise);
   if (CompilerState::Current().is_aot()) {
     // When in AOT, figure out start line, end line, line fragment needed for
@@ -4701,8 +4692,8 @@ Fragment StreamingFlowGraphBuilder::BuildAssertStatement(
     otherwise_fragment +=
         StaticCall(condition_start_offset, target, 3, ICData::kStatic);
   }
-  otherwise_fragment += ThrowException(TokenPosition::kNoSource);
   otherwise_fragment += Drop();
+  ASSERT(otherwise_fragment.is_closed());
 
   return Fragment(instructions.entry, then);
 }
@@ -4982,16 +4973,9 @@ Fragment StreamingFlowGraphBuilder::BuildSwitchCase(SwitchHelper* helper,
     body_fragment += Drop();
   }
 
-  // TODO(http://dartbug.com/50595): The CFE does not insert breaks for
-  // unterminated cases which never reach the end of their control flow.
-  // If the CFE inserts synthesized breaks, we can add an assert here instead.
   if (!is_default && body_fragment.is_open() &&
       (case_index < (helper->case_count() - 1))) {
-    const auto& error =
-        String::ZoneHandle(Z, Symbols::New(thread(), "Unreachable code."));
-    body_fragment += Constant(error);
-    body_fragment += ThrowException(TokenPosition::kNoSource);
-    body_fragment += Drop();
+    body_fragment += B->Stop("Unreachable end of case");
   }
 
   // If there is an implicit fall-through we have one [SwitchCase] and

runtime/vm/compiler/frontend/kernel_binary_flowgraph.h

@@ -226,7 +226,6 @@ class StreamingFlowGraphBuilder : public KernelReaderHelper {
   Fragment StoreStaticField(TokenPosition position, const Field& field);
   Fragment StringInterpolate(TokenPosition position);
   Fragment StringInterpolateSingle(TokenPosition position);
-  Fragment ThrowTypeError();
   Fragment LoadInstantiatorTypeArguments();
   Fragment LoadFunctionTypeArguments();
   Fragment InstantiateType(const AbstractType& type);

runtime/vm/compiler/frontend/kernel_to_il.cc

@@ -528,6 +528,7 @@ Fragment FlowGraphBuilder::ThrowLateInitializationError(
       StaticCall(TokenPosition::Synthetic(position.Pos()), throw_new,
                  /* argument_count = */ 1, ICData::kStatic);
   instructions += Drop();
+  ASSERT(instructions.is_closed());
 
   return instructions;
 }
@@ -566,7 +567,7 @@ Fragment FlowGraphBuilder::StoreLateField(const Field& field,
       already_initialized += ThrowLateInitializationError(
           position, "_throwFieldAlreadyInitialized",
           String::ZoneHandle(Z, field.name()));
-      already_initialized += Goto(join);
+      ASSERT(already_initialized.is_closed());
     }
 
     instructions = Fragment(instructions.entry, join);
@@ -711,47 +712,6 @@ Fragment FlowGraphBuilder::StringInterpolate(TokenPosition position) {
   return instructions;
 }
 
-Fragment FlowGraphBuilder::ThrowTypeError() {
-  const Class& klass =
-      Class::ZoneHandle(Z, Library::LookupCoreClass(Symbols::TypeError()));
-  ASSERT(!klass.IsNull());
-  GrowableHandlePtrArray<const String> pieces(Z, 3);
-  pieces.Add(Symbols::TypeError());
-  pieces.Add(Symbols::Dot());
-  pieces.Add(H.DartSymbolObfuscate("_create"));
-
-  const Function& constructor = Function::ZoneHandle(
-      Z, klass.LookupConstructorAllowPrivate(
-             String::ZoneHandle(Z, Symbols::FromConcatAll(thread_, pieces))));
-  ASSERT(!constructor.IsNull());
-
-  const String& url = H.DartString(
-      parsed_function_->function().ToLibNamePrefixedQualifiedCString(),
-      Heap::kOld);
-
-  Fragment instructions;
-
-  // Create instance of _TypeError
-  instructions += AllocateObject(TokenPosition::kNoSource, klass, 0);
-  LocalVariable* instance = MakeTemporary();
-
-  // Call _TypeError._create constructor.
-  instructions += LoadLocal(instance);                             // this
-  instructions += Constant(url);                                   // url
-  instructions += NullConstant();                                  // line
-  instructions += IntConstant(0);                                  // column
-  instructions += Constant(H.DartSymbolPlain("Malformed type."));  // message
-
-  instructions += StaticCall(TokenPosition::kNoSource, constructor,
-                             /* argument_count = */ 5, ICData::kStatic);
-  instructions += Drop();
-
-  // Throw the exception
-  instructions += ThrowException(TokenPosition::kNoSource);
-
-  return instructions;
-}
-
 Fragment FlowGraphBuilder::ThrowNoSuchMethodError(TokenPosition position,
                                                   const Function& target,
                                                   bool incompatible_arguments,
@@ -818,6 +778,7 @@ Fragment FlowGraphBuilder::ThrowNoSuchMethodError(TokenPosition position,
   instructions += NullConstant();  // argumentNames
   instructions += StaticCall(position, throw_function, /* argument_count = */ 7,
                              ICData::kNoRebind);
+  ASSERT(instructions.is_closed());
   return instructions;
 }
 
@@ -2859,7 +2820,7 @@ FlowGraph* FlowGraphBuilder::BuildGraphOfRecordFieldGetter(
   throw_nsm += ThrowNoSuchMethodError(TokenPosition::kNoSource, function,
                                       /*incompatible_arguments=*/false,
                                       /*receiver_pushed=*/true);
-  throw_nsm += ThrowException(TokenPosition::kNoSource);  // Close graph.
+  ASSERT(throw_nsm.is_closed());
 
   // There is no prologue code for a record field getter.
   PrologueInfo prologue_info(-1, -1);

runtime/vm/compiler/frontend/kernel_to_il.h

@@ -259,7 +259,6 @@ class FlowGraphBuilder : public BaseFlowGraphBuilder {
                                   intptr_t type_args_len = 0);
   Fragment StringInterpolateSingle(TokenPosition position);
   Fragment StringInterpolate(TokenPosition position);
-  Fragment ThrowTypeError();
 
   // [incompatible_arguments] should be true if the NSM is due to a mismatch
   // between the provided arguments and the function signature.

sdk/lib/_internal/vm/lib/errors_patch.dart

@@ -41,13 +41,17 @@ class _AssertionError extends Error implements AssertionError {
   // out of the script. It expects a Dart stack frame from class
   // _AssertionError. Thus we need a Dart stub that calls the native code.
   @pragma("vm:entry-point", "call")
-  static _throwNew(int assertionStart, int assertionEnd, Object? message) {
+  static Never _throwNew(
+    int assertionStart,
+    int assertionEnd,
+    Object? message,
+  ) {
     _doThrowNew(assertionStart, assertionEnd, message);
   }
 
   @pragma("vm:entry-point", "call")
   @pragma("vm:external-name", "AssertionError_throwNewSource")
-  external static _throwNewSource(
+  external static Never _throwNewSource(
     String failedAssertion,
     String? scriptUrl,
     int line,
@@ -56,7 +60,7 @@ class _AssertionError extends Error implements AssertionError {
   );
 
   @pragma("vm:external-name", "AssertionError_throwNew")
-  external static _doThrowNew(
+  external static Never _doThrowNew(
     int assertionStart,
     int assertionEnd,
     Object? message,
@@ -110,7 +114,7 @@ class _TypeError extends Error implements TypeError {
 
   @pragma("vm:entry-point", "call")
   @pragma("vm:external-name", "TypeError_throwNew")
-  external static _throwNew(
+  external static Never _throwNew(
     int location,
     Object srcValue,
     _Type dstType,
@@ -136,7 +140,7 @@ class _InternalError {
 @patch
 @pragma("vm:entry-point")
 class UnsupportedError {
-  static _throwNew(String msg) {
+  static Never _throwNew(String msg) {
     throw new UnsupportedError(msg);
   }
 }
@@ -145,7 +149,7 @@ class UnsupportedError {
 @pragma("vm:entry-point")
 class StateError {
   @pragma("vm:entry-point")
-  static _throwNew(String msg) {
+  static Never _throwNew(String msg) {
     throw new StateError(msg);
   }
 }
@@ -163,15 +167,15 @@ class NoSuchMethodError {
 
   NoSuchMethodError._withInvocation(this._receiver, this._invocation);
 
-  static void _throwNewInvocation(Object? receiver, Invocation invocation) {
+  static Never _throwNewInvocation(Object? receiver, Invocation invocation) {
     throw new NoSuchMethodError.withInvocation(receiver, invocation);
   }
 
   // The compiler emits a call to _throwNew when it cannot resolve a static
   // method at compile time. The receiver is actually the literal class of the
   // unresolved method.
   @pragma("vm:entry-point", "call")
-  static void _throwNew(
+  static Never _throwNew(
     Object receiver,
     String memberName,
     int invocationType,

sdk/lib/_internal/vm/lib/internal_patch.dart

@@ -205,22 +205,22 @@ bool isSentinel(dynamic value) => throw UnsupportedError('isSentinel');
 @patch
 class LateError {
   @pragma("vm:entry-point")
-  static _throwFieldAlreadyInitialized(String fieldName) {
+  static Never _throwFieldAlreadyInitialized(String fieldName) {
     throw new LateError.fieldAI(fieldName);
   }
 
   @pragma("vm:entry-point")
-  static _throwLocalNotInitialized(String localName) {
+  static Never _throwLocalNotInitialized(String localName) {
     throw new LateError.localNI(localName);
   }
 
   @pragma("vm:entry-point")
-  static _throwLocalAlreadyInitialized(String localName) {
+  static Never _throwLocalAlreadyInitialized(String localName) {
     throw new LateError.localAI(localName);
   }
 
   @pragma("vm:entry-point")
-  static _throwLocalAssignedDuringInitialization(String localName) {
+  static Never _throwLocalAssignedDuringInitialization(String localName) {
     throw new LateError.localADI(localName);
   }
 }