[ignition] Fix building lookup graph when search depth is 0

src/compiler/bytecode-graph-builder.cc

Index: src/compiler/bytecode-graph-builder.cc
diff --git a/src/compiler/bytecode-graph-builder.cc b/src/compiler/bytecode-graph-builder.cc
index c07aa742cc11cb36ccc15599c646455c61523abb..198574a931b6c19d5a5184b3dc7eeeaed78cab86 100644
--- a/src/compiler/bytecode-graph-builder.cc
+++ b/src/compiler/bytecode-graph-builder.cc
@@ -896,8 +896,6 @@ BytecodeGraphBuilder::Environment* BytecodeGraphBuilder::CheckContextExtensions(
   // Output environment where the context has an extension
   Environment* slow_environment = nullptr;
 
-  DCHECK_GT(depth, 0u);
-
   // We only need to check up to the last-but-one depth, because the an eval in
   // the same scope as the variable itself has no way of shadowing it.
   for (uint32_t d = 0; d < depth; d++) {
@@ -931,7 +929,9 @@ BytecodeGraphBuilder::Environment* BytecodeGraphBuilder::CheckContextExtensions(
     }
   }
 
-  DCHECK_NOT_NULL(slow_environment);
+  // The depth can be zero, in which case no slow-path checks are built, and the
+  // slow path environment can be null.
+  DCHECK(depth == 0 || slow_environment != nullptr);
 
   return slow_environment;
 }
@@ -951,26 +951,30 @@ void BytecodeGraphBuilder::BuildLdaLookupContextSlot(TypeofMode typeof_mode) {
     environment()->BindAccumulator(NewNode(op, context));
     NewMerge();

[Michael Starzinger, 2016/10/04 09:43:10]

nit: Shouldn't we only create the singleton merge node in the case we have a
slow path (i.e. move it into the below conditional)?

[Leszek Swirski, 2016/10/04 10:46:55]

Done.

   }
-  Environment* fast_environment = environment();
 
-  // Slow path, do a runtime load lookup.
-  set_environment(slow_environment);
-  {
-    FrameStateBeforeAndAfter states(this);
+  // Only build the slow path if there were any slow-path checks.
+  if (slow_environment != nullptr) {
+    Environment* fast_environment = environment();
+
+    // Slow path, do a runtime load lookup.
+    set_environment(slow_environment);
+    {
+      FrameStateBeforeAndAfter states(this);
 
-    Node* name =
-        jsgraph()->Constant(bytecode_iterator().GetConstantForIndexOperand(0));
+      Node* name = jsgraph()->Constant(
+          bytecode_iterator().GetConstantForIndexOperand(0));
 
-    const Operator* op =
-        javascript()->CallRuntime(typeof_mode == TypeofMode::NOT_INSIDE_TYPEOF
-                                      ? Runtime::kLoadLookupSlot
-                                      : Runtime::kLoadLookupSlotInsideTypeof);
-    Node* value = NewNode(op, name);
-    environment()->BindAccumulator(value, &states);
-  }
+      const Operator* op =
+          javascript()->CallRuntime(typeof_mode == TypeofMode::NOT_INSIDE_TYPEOF
+                                        ? Runtime::kLoadLookupSlot
+                                        : Runtime::kLoadLookupSlotInsideTypeof);
+      Node* value = NewNode(op, name);
+      environment()->BindAccumulator(value, &states);
+    }
 
-  fast_environment->Merge(environment());
-  set_environment(fast_environment);
+    fast_environment->Merge(environment());
+    set_environment(fast_environment);
+  }
 }
 
 void BytecodeGraphBuilder::VisitLdaLookupContextSlot() {
@@ -996,26 +1000,30 @@ void BytecodeGraphBuilder::BuildLdaLookupGlobalSlot(TypeofMode typeof_mode) {
 
     NewMerge();

[Michael Starzinger, 2016/10/04 09:43:10]

nit: Likewise.

[Leszek Swirski, 2016/10/04 10:46:55]

Done.

   }
-  Environment* fast_environment = environment();
 
-  // Slow path, do a runtime load lookup.
-  set_environment(slow_environment);
-  {
-    FrameStateBeforeAndAfter states(this);
+  // Only build the slow path if there were any slow-path checks.
+  if (slow_environment != nullptr) {
+    Environment* fast_environment = environment();
 
-    Node* name =
-        jsgraph()->Constant(bytecode_iterator().GetConstantForIndexOperand(0));
+    // Slow path, do a runtime load lookup.
+    set_environment(slow_environment);
+    {
+      FrameStateBeforeAndAfter states(this);
 
-    const Operator* op =
-        javascript()->CallRuntime(typeof_mode == TypeofMode::NOT_INSIDE_TYPEOF
-                                      ? Runtime::kLoadLookupSlot
-                                      : Runtime::kLoadLookupSlotInsideTypeof);
-    Node* value = NewNode(op, name);
-    environment()->BindAccumulator(value, &states);
-  }
+      Node* name = jsgraph()->Constant(
+          bytecode_iterator().GetConstantForIndexOperand(0));
 
-  fast_environment->Merge(environment());
-  set_environment(fast_environment);
+      const Operator* op =
+          javascript()->CallRuntime(typeof_mode == TypeofMode::NOT_INSIDE_TYPEOF
+                                        ? Runtime::kLoadLookupSlot
+                                        : Runtime::kLoadLookupSlotInsideTypeof);
+      Node* value = NewNode(op, name);
+      environment()->BindAccumulator(value, &states);
+    }
+
+    fast_environment->Merge(environment());
+    set_environment(fast_environment);
+  }
 }
 
 void BytecodeGraphBuilder::VisitLdaLookupGlobalSlot() {