[vm/ffi] Fix constant Finalizables

The kernel builder relied on only seeing expressions emitted by the
finalizable transform (variable get and this) as arguments to the fence.

However, other transforms can run later. In this case the expression
was turned into a constant.

This CL changes the implementation to accept any expression rather than
accepting a subset of expressions.

TEST=tests/ffi/regress_49402_test.dart

Closes: https://github.com/dart-lang/sdk/issues/49402
Change-Id: I1a962a5b7a38099eb5c3bbf5a5a8145b16727d97
Cq-Include-Trybots: luci.dart.try:vm-kernel-precomp-linux-debug-x64-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/250744
Commit-Queue: Daco Harkes <dacoharkes@google.com>
Reviewed-by: Slava Egorov <vegorov@google.com>

runtime/vm/compiler/frontend/kernel_binary_flowgraph.cc

@@ -1169,7 +1169,9 @@ Fragment StreamingFlowGraphBuilder::BuildStatementAt(intptr_t kernel_offset) {
   return BuildStatement();  // read statement.
 }
 
-Fragment StreamingFlowGraphBuilder::BuildExpression(TokenPosition* position) {
+Fragment StreamingFlowGraphBuilder::BuildExpression(
+    TokenPosition* position,
+    bool allow_late_uninitialized) {
   ++num_ast_nodes_;
   uint8_t payload = 0;
   Tag tag = ReadTag(&payload);  // read tag.
@@ -1177,9 +1179,9 @@ Fragment StreamingFlowGraphBuilder::BuildExpression(TokenPosition* position) {
     case kInvalidExpression:
       return BuildInvalidExpression(position);
     case kVariableGet:
-      return BuildVariableGet(position);
+      return BuildVariableGet(position, allow_late_uninitialized);
     case kSpecializedVariableGet:
-      return BuildVariableGet(payload, position);
+      return BuildVariableGet(payload, position, allow_late_uninitialized);
     case kVariableSet:
       return BuildVariableSet(position);
     case kSpecializedVariableSet:
@@ -5818,26 +5820,12 @@ Fragment StreamingFlowGraphBuilder::BuildReachabilityFence() {
   ASSERT(positional_count == 1);
 
   // The CFE transform only generates a subset of argument expressions:
-  // either variable get or `this`.
-  uint8_t payload = 0;
-  Tag tag = ReadTag(&payload);
+  // either variable get or `this`. However, subsequent transforms can
+  // generate different expressions, including: constant expressions.
+  // So, build an arbitrary expression here instead.
   TokenPosition* position = nullptr;
   const bool allow_late_uninitialized = true;
-  Fragment code;
-  switch (tag) {
-    case kVariableGet:
-      code = BuildVariableGet(position, allow_late_uninitialized);
-      break;
-    case kSpecializedVariableGet:
-      code = BuildVariableGet(payload, position, allow_late_uninitialized);
-      break;
-    case kThisExpression:
-      code = BuildThisExpression(position);
-      break;
-    default:
-      // The transformation should not be generating anything else.
-      FATAL1("Unexpected tag %i", tag);
-  }
+  Fragment code = BuildExpression(position, allow_late_uninitialized);
 
   const intptr_t named_args_len = ReadListLength();
   ASSERT(named_args_len == 0);

runtime/vm/compiler/frontend/kernel_binary_flowgraph.h

@@ -70,7 +70,8 @@ class StreamingFlowGraphBuilder : public KernelReaderHelper {
   Fragment BuildInitializers(const Class& parent_class);
   FlowGraph* BuildGraphOfFunction(bool constructor);
 
-  Fragment BuildExpression(TokenPosition* position = nullptr);
+  Fragment BuildExpression(TokenPosition* position = nullptr,
+                           bool allow_late_uninitialized = false);
   Fragment BuildStatement(TokenPosition* position = nullptr);
   Fragment BuildStatementWithBranchCoverage(TokenPosition* position = nullptr);
 

tests/ffi/regress_49402_test.dart

@@ -0,0 +1,13 @@
+// Copyright (c) 2022, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:ffi';
+
+class A implements Finalizable {}
+
+void b([A? a]) {}
+
+void main() {
+  b();
+}

tests/ffi_2/regress_49402_test.dart

@@ -0,0 +1,15 @@
+// Copyright (c) 2022, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// @dart=2.9
+
+import 'dart:ffi';
+
+class A implements Finalizable {}
+
+void b([A a]) {}
+
+void main() {
+  b();
+}