kernel -> ssa: implement if-statements

pkg/compiler/lib/src/ssa/ssa_branch_builder.dart

+// Copyright (c) 2016, 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 '../compiler.dart';
+import '../io/source_information.dart';
+import '../js_backend/js_backend.dart';
+import '../tree/tree.dart' as ast;
+
+import 'graph_builder.dart';
+import 'locals_handler.dart';
+import 'nodes.dart';
+
+class SsaBranch {
+  final SsaBranchBuilder branchBuilder;
+  final HBasicBlock block;
+  LocalsHandler startLocals;
+  LocalsHandler exitLocals;
+  SubGraph graph;
+
+  SsaBranch(this.branchBuilder) : block = new HBasicBlock();
+}
+
+class SsaBranchBuilder {
+  final GraphBuilder builder;
+  final Compiler compiler;
+  final ast.Node diagnosticNode;
+
+  SsaBranchBuilder(this.builder, this.compiler, [this.diagnosticNode]);
+
+  void checkNotAborted() {
+    if (builder.isAborted()) {
+      compiler.unimplemented(diagnosticNode, "aborted control flow");
+    }
+  }
+
+  void buildCondition(
+      void visitCondition(),
+      SsaBranch conditionBranch,
+      SsaBranch thenBranch,
+      SsaBranch elseBranch,
+      SourceInformation sourceInformation) {
+    startBranch(conditionBranch);
+    visitCondition();
+    checkNotAborted();
+    assert(identical(builder.current, builder.lastOpenedBlock));
+    HInstruction conditionValue = builder.popBoolified();
+    HIf branch = new HIf(conditionValue)..sourceInformation = sourceInformation;
+    HBasicBlock conditionExitBlock = builder.current;
+    builder.close(branch);
+    conditionBranch.exitLocals = builder.localsHandler;
+    conditionExitBlock.addSuccessor(thenBranch.block);
+    conditionExitBlock.addSuccessor(elseBranch.block);
+    bool conditionBranchLocalsCanBeReused =
+    mergeLocals(conditionBranch, thenBranch, mayReuseFromLocals: true);
+    mergeLocals(conditionBranch, elseBranch,
+        mayReuseFromLocals: conditionBranchLocalsCanBeReused);
+
+    conditionBranch.graph =
+    new SubExpression(conditionBranch.block, conditionExitBlock);
+  }
+
+  /**
+   * Returns true if the locals of the [fromBranch] may be reused. A [:true:]
+   * return value implies that [mayReuseFromLocals] was set to [:true:].
+   */
+  bool mergeLocals(SsaBranch fromBranch, SsaBranch toBranch,
+      {bool mayReuseFromLocals}) {
+    LocalsHandler fromLocals = fromBranch.exitLocals;
+    if (toBranch.startLocals == null) {
+      if (mayReuseFromLocals) {
+        toBranch.startLocals = fromLocals;
+        return false;
+      } else {
+        toBranch.startLocals = new LocalsHandler.from(fromLocals);
+        return true;
+      }
+    } else {
+      toBranch.startLocals.mergeWith(fromLocals, toBranch.block);
+      return true;
+    }
+  }
+
+  void startBranch(SsaBranch branch) {
+    builder.graph.addBlock(branch.block);
+    builder.localsHandler = branch.startLocals;
+    builder.open(branch.block);
+  }
+
+  HInstruction buildBranch(SsaBranch branch, void visitBranch(),
+      SsaBranch joinBranch, bool isExpression) {
+    startBranch(branch);
+    visitBranch();
+    branch.graph = new SubGraph(branch.block, builder.lastOpenedBlock);
+    branch.exitLocals = builder.localsHandler;
+    if (!builder.isAborted()) {
+      builder.goto(builder.current, joinBranch.block);
+      mergeLocals(branch, joinBranch, mayReuseFromLocals: true);
+    }
+    if (isExpression) {
+      checkNotAborted();
+      return builder.pop();
+    }
+    return null;
+  }
+
+  handleIf(void visitCondition(), void visitThen(), void visitElse(),
+      {SourceInformation sourceInformation}) {
+    if (visitElse == null) {
+      // Make sure to have an else part to avoid a critical edge. A
+      // critical edge is an edge that connects a block with multiple
+      // successors to a block with multiple predecessors. We avoid
+      // such edges because they prevent inserting copies during code
+      // generation of phi instructions.
+      visitElse = () {};
+    }
+
+    _handleDiamondBranch(visitCondition, visitThen, visitElse,
+        isExpression: false, sourceInformation: sourceInformation);
+  }
+
+  handleConditional(void visitCondition(), void visitThen(), void visitElse()) {
+    assert(visitElse != null);
+    _handleDiamondBranch(visitCondition, visitThen, visitElse,
+        isExpression: true);
+  }
+
+  handleIfNull(void left(), void right()) {
+    // x ?? y is transformed into: x == null ? y : x
+    HInstruction leftExpression;
+    handleConditional(() {
+      left();
+      leftExpression = builder.pop();
+      builder.pushCheckNull(leftExpression);
+    }, right, () => builder.stack.add(leftExpression));
+  }
+
+  /// Creates the graph for '&&' or '||' operators.
+  ///
+  /// x && y is transformed into:
+  ///
+  ///     t0 = boolify(x);
+  ///     if (t0) {
+  ///       t1 = boolify(y);
+  ///     }
+  ///     result = phi(t1, false);
+  ///
+  /// x || y is transformed into:
+  ///
+  ///     t0 = boolify(x);
+  ///     if (not(t0)) {
+  ///       t1 = boolify(y);
+  ///     }
+  ///     result = phi(t1, true);
+  void handleLogicalBinary(void left(), void right(), {bool isAnd}) {
+    HInstruction boolifiedLeft;
+    HInstruction boolifiedRight;
+
+    void visitCondition() {
+      left();
+      boolifiedLeft = builder.popBoolified();
+      builder.stack.add(boolifiedLeft);
+      if (!isAnd) {
+        JavaScriptBackend backend = compiler.backend;
+        builder.push(new HNot(builder.pop(), backend.boolType));
+      }
+    }
+
+    void visitThen() {
+      right();
+      boolifiedRight = builder.popBoolified();
+    }
+
+    handleIf(visitCondition, visitThen, null);
+    HConstant notIsAnd = builder.graph.addConstantBool(!isAnd, compiler);
+    JavaScriptBackend backend = compiler.backend;
+    HPhi result = new HPhi.manyInputs(
+        null, <HInstruction>[boolifiedRight, notIsAnd], backend.dynamicType);
+    builder.current.addPhi(result);
+    builder.stack.add(result);
+  }
+
+  void _handleDiamondBranch(
+      void visitCondition(), void visitThen(), void visitElse(),
+      {bool isExpression, SourceInformation sourceInformation}) {
+    SsaBranch conditionBranch = new SsaBranch(this);
+    SsaBranch thenBranch = new SsaBranch(this);
+    SsaBranch elseBranch = new SsaBranch(this);
+    SsaBranch joinBranch = new SsaBranch(this);
+
+    conditionBranch.startLocals = builder.localsHandler;
+    builder.goto(builder.current, conditionBranch.block);
+
+    buildCondition(visitCondition, conditionBranch, thenBranch, elseBranch,
+        sourceInformation);
+    HInstruction thenValue =
+    buildBranch(thenBranch, visitThen, joinBranch, isExpression);
+    HInstruction elseValue =
+    buildBranch(elseBranch, visitElse, joinBranch, isExpression);
+
+    if (isExpression) {
+      assert(thenValue != null && elseValue != null);
+      JavaScriptBackend backend = compiler.backend;
+      HPhi phi = new HPhi.manyInputs(
+          null, <HInstruction>[thenValue, elseValue], backend.dynamicType);
+      joinBranch.block.addPhi(phi);
+      builder.stack.add(phi);
+    }
+
+    HBasicBlock joinBlock;
+    // If at least one branch did not abort, open the joinBranch.
+    if (!joinBranch.block.predecessors.isEmpty) {
+      startBranch(joinBranch);
+      joinBlock = joinBranch.block;
+    }
+
+    HIfBlockInformation info = new HIfBlockInformation(
+        new HSubExpressionBlockInformation(conditionBranch.graph),
+        new HSubGraphBlockInformation(thenBranch.graph),
+        new HSubGraphBlockInformation(elseBranch.graph));
+
+    HBasicBlock conditionStartBlock = conditionBranch.block;
+    conditionStartBlock.setBlockFlow(info, joinBlock);
+    SubGraph conditionGraph = conditionBranch.graph;
+    HIf branch = conditionGraph.end.last;
+    assert(branch is HIf);
+    branch.blockInformation = conditionStartBlock.blockFlow;
+  }
+}