dart2dart: Implementation of simple try/catch.

pkg/compiler/lib/src/cps_ir/cps_ir_builder_visitor.dart

Index: pkg/compiler/lib/src/cps_ir/cps_ir_builder_visitor.dart
diff --git a/pkg/compiler/lib/src/cps_ir/cps_ir_builder_visitor.dart b/pkg/compiler/lib/src/cps_ir/cps_ir_builder_visitor.dart
index ea7bf3a1fe80c63bc88acd16a83908be7d0c4354..95030b63f4262584433946b16f7591d724965d90 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_ir_builder_visitor.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_ir_builder_visitor.dart
@@ -451,6 +451,147 @@ abstract class IrBuilderVisitor extends ResolvedVisitor<ir.Primitive>
     return null;
   }
 
+  ir.Primitive visitTryStatement(ast.TryStatement node) {
+    assert(this.irBuilder.isOpen);
+    // Try/catch is not yet implemented in the JS backend.
+    if (this.irBuilder.tryStatements == null) {
+      return giveup(node, 'try/catch in the JS backend');
+    }
+    // Multiple catch blocks are not yet implemented.
+    if (node.catchBlocks.isEmpty ||
+        node.catchBlocks.nodes.tail == null) {
+      return giveup(node, 'not exactly one catch block');
+    }
+    // 'on T' catch blocks are not yet implemented.
+    if ((node.catchBlocks.nodes.head as ast.CatchBlock).onKeyword != null) {
+      return giveup(node, '"on T" catch block');
+    }
+    // Finally blocks are not yet implemented.
+    if (node.finallyBlock != null) {
+      return giveup(node, 'try/finally');
+    }
+
+    // Catch handlers are in scope for their body.  The CPS translation of
+    // [[try tryBlock catch (e) catchBlock; successor]] is:
+    //
+    // let cont join(v0, v1, ...) = [[successor]] in
+    //   let mutable m0 = x0 in
+    //     let mutable m1 = x1 in
+    //       ...
+    //       let handler catch_(e) =
+    //         let prim p0 = GetMutable(m0) in
+    //           let prim p1 = GetMutable(m1) in
+    //             ...
+    //             [[catchBlock]]
+    //             join(p0, p1, ...)
+    //       in
+    //         [[tryBlock]]
+    //         let prim p0' = GetMutable(m0) in
+    //           let prim p1' = GetMutable(m1) in
+    //             ...
+    //             join(p0', p1', ...)
+    //
+    // In other words, both the try and catch block are in the scope of the
+    // join-point continuation, and they are both in the scope of a sequence
+    // of mutable bindings for the variables assigned in the try.  The join-
+    // point continuation is not in the scope of these mutable bindings.
+    // The tryBlock is in the scope of a binding for the catch handler.  Each
+    // instruction (specifically, each call) in the tryBlock is in the dynamic
+    // scope of the handler.  The mutable bindings are dereferenced at the end
+    // of the try block and at the beginning of the catch block, so th

[floitsch, 2015/02/16 14:54:07]

the

+    // variables are unboxed in the catch block and at the join point.

[floitsch, 2015/02/16 14:54:07]

I guess they are also unboxed before calls to "continue" or "break". Not sure if
it requires an update to the comment.

[Kevin Millikin (Google), 2015/02/24 11:59:25]

It does require an update to the comment, and a fix to the implementation
because it's not correct as is.

+
+    IrBuilder tryCatchBuilder = irBuilder.makeDelimitedBuilder();
+    TryStatementInfo tryInfo = tryCatchBuilder.tryStatements[node];
+    // Variables that are boxed due to being captured in a closure are boxed
+    // for their entire lifetime, and so they do not need to be boxed on entry
+    // to any try block.  They are only removed here because we cannot
+    // identify all of them in the same pass where we identify the variables
+    // assigned in the try (the may be captured by a closure after the try
+    // statement).
+    tryInfo.boxedOnEntry.removeAll(tryCatchBuilder.mutableCapturedVariables);

[floitsch, 2015/02/16 14:54:07]

I'm not a fan of these kind of side-effects in a builder-visitor.

[Kevin Millikin (Google), 2015/02/24 11:59:25]

Hmmm.  The other approach that we use a lot is to have some kind of predicate
that says "if it's in this set and not in this other set, then box it".  I can
do that instead but I'm not sure it's obviously better.

+    for (LocalVariableElement variable in tryInfo.boxedOnEntry) {
+      assert(!tryCatchBuilder.isInMutableVariable(variable));
+      ir.Primitive value = tryCatchBuilder.buildLocalGet(variable);
+      tryCatchBuilder.makeMutableVariable(variable);
+      tryCatchBuilder.declareLocalVariable(variable, initialValue: value);
+    }
+
+    IrBuilder catchBuilder = tryCatchBuilder.makeDelimitedBuilder();
+    IrBuilder tryBuilder = tryCatchBuilder.makeDelimitedBuilder();
+    List<ir.Parameter> joinParameters =
+        new List<ir.Parameter>.generate(irBuilder.environment.length, (i) {
+      return new ir.Parameter(irBuilder.environment.index2variable[i]);
+    });
+    ir.Continuation joinContinuation = new ir.Continuation(joinParameters);
+    withBuilder(tryBuilder, () {
+      visit(node.tryBlock);
+    });
+    if (tryBuilder.isOpen) {
+      for (LocalVariableElement variable in tryInfo.boxedOnEntry) {
+        assert(tryBuilder.isInMutableVariable(variable));
+        ir.Primitive value = tryBuilder.buildLocalGet(variable);
+        tryBuilder.environment.update(variable, value);
+      }
+      assert(tryBuilder.environment.length >= irBuilder.environment.length);
+      ir.InvokeContinuation jump = new ir.InvokeContinuation.uninitialized();
+      jump.continuation = new ir.Reference(joinContinuation);
+      jump.arguments = new List<ir.Reference>.generate(
+          irBuilder.environment.length, (i) {
+        return new ir.Reference(tryBuilder.environment[i]);
+      });
+      tryBuilder.add(jump);
+      tryBuilder._current = null;

[asgerf, 2015/02/20 10:10:07]

Would it make sense to extract these 8 lines into
tryBuilder.jumpTo(joinContinuation)?

And similarly catchBuilder.jumpTo(joinContinuation) below.

[Kevin Millikin (Google), 2015/02/24 11:59:25]

Yes.  It also occurs when breaking from a labeled (non-loop) statement.

+    }
+
+    for (LocalVariableElement variable in tryInfo.boxedOnEntry) {
+      assert(catchBuilder.isInMutableVariable(variable));
+      ir.Primitive value = catchBuilder.buildLocalGet(variable);
+      // Note that we remove the variable from the set of mutable variables
+      // here (and not above for the try body).  This is because the set of
+      // mutable variables is global for the whole function and not local to
+      // a delimited builder.
+      catchBuilder.removeMutableVariable(variable);
+      catchBuilder.environment.update(variable, value);
+    }
+    ast.CatchBlock catchClause = node.catchBlocks.nodes.head;
+    assert(catchClause.exception != null);
+    List<ir.Parameter> catchParameters =
+        <ir.Parameter>[new ir.Parameter(elements[catchClause.exception])];
+    catchBuilder.environment.extend(elements[catchClause.exception] as Local,

[karlklose, 2015/02/16 10:15:48]

Why do you cast to Local here and below?

[Kevin Millikin (Google), 2015/02/24 11:59:25]

Otherwise the editor reports "The argument type 'Element' cannot be assigned to
the parameter type 'Local'".

As far as I can tell (this comes from a NodeList so I've got to try to reason
about where it was put into that list) this is always a LocalVariableElement,
which is an Element and a Local.  But Element and Local are not related.

+                                    catchParameters[0]);
+    if (catchClause.trace != null) {
+      catchParameters.add(new ir.Parameter(elements[catchClause.trace]));
+      catchBuilder.environment.extend(elements[catchClause.trace] as Local,
+                                      catchParameters[1]);
+    }
+    withBuilder(catchBuilder, () {
+      visit(catchClause.block);
+    });
+    if (catchBuilder.isOpen) {
+      assert(catchBuilder.environment.length >= irBuilder.environment.length);
+      ir.InvokeContinuation jump = new ir.InvokeContinuation.uninitialized();
+      jump.continuation = new ir.Reference(joinContinuation);
+      jump.arguments = new List<ir.Reference>.generate(
+          irBuilder.environment.length, (i) {
+            return new ir.Reference(catchBuilder.environment[i]);
+          });
+      catchBuilder.add(jump);
+      catchBuilder._current = null;
+    }
+    ir.Continuation catchContinuation = new ir.Continuation(catchParameters);
+    catchContinuation.body = catchBuilder._root;
+
+    tryCatchBuilder.add(new ir.LetHandler(catchContinuation, tryBuilder._root));
+    tryCatchBuilder._current = null;
+
+    irBuilder.add(new ir.LetCont(joinContinuation, tryCatchBuilder._root));
+    for (int i = 0; i < irBuilder.environment.length; ++i) {
+      irBuilder.environment.index2value[i] = joinParameters[i];
+    }
+    return null;
+  }
+
   // ==== Expressions ====
   ir.Primitive visitConditional(ast.Conditional node) {
     return irBuilder.buildConditional(
@@ -960,8 +1101,7 @@ dynamic giveup(ast.Node node, [String reason]) {
 /// sees a feature that is currently unsupport by that builder. In particular,
 /// loop variables captured in a for-loop initializer, condition, or update
 /// expression are unsupported.
-class DartCapturedVariables extends ast.Visitor
-                             implements DartCapturedVariableInfo {
+class DartCapturedVariables extends ast.Visitor {
   final TreeElements elements;
   DartCapturedVariables(this.elements);
 
@@ -969,6 +1109,12 @@ class DartCapturedVariables extends ast.Visitor
   bool insideInitializer = false;
   Set<Local> capturedVariables = new Set<Local>();
 
+  Map<ast.TryStatement, TryStatementInfo> tryStatements =
+      <ast.TryStatement, TryStatementInfo>{};
+
+  TryStatementInfo currentTryInfo;
+  bool get inTryStatement => currentTryInfo != null;
+
   void markAsCaptured(Local local) {
     capturedVariables.add(local);
   }
@@ -1016,18 +1162,28 @@ class DartCapturedVariables extends ast.Visitor
   visitSendSet(ast.SendSet node) {
     handleSend(node);
     Element element = elements[node];
-    // Initializers in an initializer-list can communicate via parameters.
-    // If a parameter is stored in an initializer list we box it.
-    if (insideInitializer &&
-        Elements.isLocal(element) &&
-        element.isParameter) {
+    if (Elements.isLocal(element)) {
       LocalElement local = element;
-      // TODO(sigurdm): Fix this.
-      // Though these variables do not outlive the activation of the function,
-      // they still need to be boxed.  As a simplification, we treat them as if
-      // they are captured by a closure (i.e., they do outlive the activation of
-      // the function).
-      markAsCaptured(local);
+      if (insideInitializer) {
+        assert(local.isParameter);
+        // Initializers in an initializer-list can communicate via parameters.
+        // If a parameter is stored in an initializer list we box it.
+        // TODO(sigurdm): Fix this.
+        // Though these variables do not outlive the activation of the
+        // function, they still need to be boxed.  As a simplification, we
+        // treat them as if they are captured by a closure (i.e., they do
+        // outlive the activation of the function).
+        markAsCaptured(local);
+      } else if (inTryStatement) {
+        assert(local.isParameter || local.isVariable);
+        if (!currentTryInfo.declared.contains(local)) {
+          // If a variable is assigned in a try and not declared in that try
+          // then it has to be boxed on entry to the try.
+          // Later we will remove such variables that will be boxed in an
+          // enclosing try.
+          currentTryInfo.boxedOnEntry.add(local);
+        }
+      }
     }
     node.visitChildren(this);
   }
@@ -1046,6 +1202,35 @@ class DartCapturedVariables extends ast.Visitor
     visit(node.body);
     currentFunction = oldFunction;
   }
+
+  visitTryStatement(ast.TryStatement node) {
+    TryStatementInfo outer = currentTryInfo;
+    tryStatements[node] = currentTryInfo = new TryStatementInfo(outer);
+    visit(node.tryBlock);
+    if (outer == null) {
+      // For each top-level try, compute the variables boxed on entry to it
+      // and each nested try in a top-down manner.
+      currentTryInfo.computeVariablesBoxedOnEntry(null);
+    }
+    currentTryInfo = outer;
+
+    visit(node.catchBlocks);
+    if (node.finallyBlock != null) visit(node.finallyBlock);
+  }
+
+  visitVariableDefinitions(ast.VariableDefinitions node) {
+    if (inTryStatement) {
+      for (ast.Node definition in node.definitions.nodes) {
+        LocalVariableElement local = elements[definition];
+        assert(local != null);
+        // In the closure conversion pass we check for isInitializingFormal,
+        // but I'm not sure it can arise.
+        assert(!local.isInitializingFormal);
+        currentTryInfo.declared.add(local);
+      }
+    }
+    node.visitChildren(this);
+  }
 }
 
 /// IR builder specific to the Dart backend, coupled to the [DartIrBuilder].