[turbofan] Implicitly emit eager checkpoint at graph building.

This makes preparation of eager checkpoints within the graph builder
implicit. Every sub-expression visitation is now guaranteed to emit
valid checkpoints in AstContext.

R=jarin@chromium.org
BUG=v8:5021

Review-Url: https://codereview.chromium.org/2074703002
Cr-Commit-Position: refs/heads/master@{#37368}

src/compiler/ast-graph-builder.h

@@ -445,8 +445,7 @@ class AstGraphBuilder : public AstVisitor {
   // Dispatched from VisitForInStatement.
   void VisitForInAssignment(Expression* expr, Node* value,
                             const VectorSlotPair& feedback,
-                            BailoutId bailout_id_before,
-                            BailoutId bailout_id_after);
+                            BailoutId bailout_id);
 
   // Dispatched from VisitObjectLiteral.
   void VisitObjectLiteralAccessor(Node* home_object,

src/compiler/checkpoint-elimination.cc

@@ -30,14 +30,39 @@ bool IsRedundantCheckpoint(Node* node) {
 
 }  // namespace
 
-Reduction CheckpointElimination::Reduce(Node* node) {
-  if (node->opcode() != IrOpcode::kCheckpoint) return NoChange();
+Reduction CheckpointElimination::ReduceCheckpoint(Node* node) {
+  DCHECK_EQ(IrOpcode::kCheckpoint, node->opcode());
   if (IsRedundantCheckpoint(node)) {
     return Replace(NodeProperties::GetEffectInput(node));
   }
   return NoChange();
 }
 
+Reduction CheckpointElimination::ReduceReturn(Node* node) {
+  DCHECK_EQ(IrOpcode::kReturn, node->opcode());
+  Node* effect = NodeProperties::GetEffectInput(node);
+  if (effect->opcode() == IrOpcode::kCheckpoint && effect->OwnedBy(node)) {
+    // Any checkpoint that is wholly owned by a {Return} node can never be used
+    // for an actual bailout and can hence be cut out of the effect chain.
+    Node* replacement = NodeProperties::GetEffectInput(effect);
+    NodeProperties::ReplaceEffectInput(node, replacement);
+    return Changed(node);
+  }
+  return NoChange();
+}
+
+Reduction CheckpointElimination::Reduce(Node* node) {
+  switch (node->opcode()) {
+    case IrOpcode::kCheckpoint:
+      return ReduceCheckpoint(node);
+    case IrOpcode::kReturn:
+      return ReduceReturn(node);
+    default:
+      break;
+  }
+  return NoChange();
+}
+
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8

src/compiler/checkpoint-elimination.h

@@ -18,6 +18,10 @@ class CheckpointElimination final : public AdvancedReducer {
   ~CheckpointElimination() final {}
 
   Reduction Reduce(Node* node) final;
+
+ private:
+  Reduction ReduceCheckpoint(Node* node);
+  Reduction ReduceReturn(Node* node);
 };
 
 }  // namespace compiler

src/full-codegen/full-codegen.cc

@@ -875,7 +875,7 @@ void FullCodeGenerator::VisitDoExpression(DoExpression* expr) {
   Comment cmnt(masm_, "[ Do Expression");
   SetExpressionPosition(expr);
   VisitBlock(expr->block());
-  EmitVariableLoad(expr->result());
+  VisitInDuplicateContext(expr->result());
 }