[maglev] Implement LdaContextSlot

In the simplest way possible.

Bug: v8:7700
Change-Id: I155aaf85192b75c89617820d6f127a2ae04c7d9b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3599484
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Victor Gomes <victorgomes@chromium.org>
Commit-Queue: Victor Gomes <victorgomes@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80089}

src/maglev/maglev-graph-builder.cc

@@ -289,8 +289,30 @@ void MaglevGraphBuilder::VisitLdaFalse() {
 void MaglevGraphBuilder::VisitLdaConstant() {
   SetAccumulator(GetConstant(GetRefOperand<HeapObject>(0)));
 }
-MAGLEV_UNIMPLEMENTED_BYTECODE(LdaContextSlot)
-MAGLEV_UNIMPLEMENTED_BYTECODE(LdaImmutableContextSlot)
+
+void MaglevGraphBuilder::VisitLdaContextSlot() {
+  ValueNode* context = LoadRegisterTagged(0);
+  int slot_index = iterator_.GetIndexOperand(1);
+  int depth = iterator_.GetUnsignedImmediateOperand(2);
+
+  for (int i = 0; i < depth; ++i) {
+    context = AddNewNode<LoadField>(
+        {context}, LoadSimpleFieldHandler(FieldIndex::ForInObjectOffset(
+                       Context::OffsetOfElementAt(Context::PREVIOUS_INDEX),
+                       FieldIndex::kTagged)));
+  }
+
+  // TODO(leszeks): Passing a LoadHandler to LoadField here is a bit of
+  // a hack, maybe we should have a LoadRawOffset or similar.
+  SetAccumulator(AddNewNode<LoadField>(
+      {context},
+      LoadSimpleFieldHandler(FieldIndex::ForInObjectOffset(
+          Context::OffsetOfElementAt(slot_index), FieldIndex::kTagged))));
+}
+void MaglevGraphBuilder::VisitLdaImmutableContextSlot() {
+  // TODO(leszeks): Consider context specialising.
+  VisitLdaCurrentContextSlot();
+}
 void MaglevGraphBuilder::VisitLdaCurrentContextSlot() {
   ValueNode* context = GetContext();
   int slot_index = iterator_.GetIndexOperand(0);
@@ -306,6 +328,7 @@ void MaglevGraphBuilder::VisitLdaImmutableCurrentContextSlot() {
   // TODO(leszeks): Consider context specialising.
   VisitLdaCurrentContextSlot();
 }
+
 void MaglevGraphBuilder::VisitStar() {
   MoveNodeBetweenRegisters(interpreter::Register::virtual_accumulator(),
                            iterator_.GetRegisterOperand(0));

test/mjsunit/maglev/mutable-context-access.js

+// Copyright 2022 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --allow-natives-syntax --maglev --no-stress-opt
+
+function factory() {
+  var x = 1;
+  x = 2;
+  return function foo() {
+    return x;
+  }
+}
+
+let foo = factory();
+
+%PrepareFunctionForOptimization(foo);
+assertEquals(2, foo());
+assertEquals(2, foo());
+
+%OptimizeMaglevOnNextCall(foo);
+assertEquals(2, foo());
+
+
+function nested_factory() {
+  var x = 1;
+  x = 2;
+  return (function() {
+    var z = 3;
+    return function foo(){
+      // Add two values from different contexts to force an
+      // LdaImmutableCurrentContextSlot
+      return x+z;
+    }
+  })();
+}
+
+foo = nested_factory();
+
+%PrepareFunctionForOptimization(foo);
+assertEquals(5, foo());
+assertEquals(5, foo());
+
+%OptimizeMaglevOnNextCall(foo);
+assertEquals(5, foo());