WIP context-allocation for “this” in arrow functions

src/x64/full-codegen-x64.cc

Index: src/x64/full-codegen-x64.cc
diff --git a/src/x64/full-codegen-x64.cc b/src/x64/full-codegen-x64.cc
index 24747ee9e85b11d7e3fed1b50e70db0d2073e9b2..ed24025888bede63f2083f3b6e516831e1779c21 100644
--- a/src/x64/full-codegen-x64.cc
+++ b/src/x64/full-codegen-x64.cc
@@ -113,8 +113,10 @@ void FullCodeGenerator::Generate() {
 
   // Sloppy mode functions and builtins need to replace the receiver with the
   // global proxy when called as functions (without an explicit receiver
-  // object).
-  if (info->strict_mode() == SLOPPY && !info->is_native()) {
+  // object). Arrow functions need to replace the receiver with the "this"
+  // binding from the context, so this does not need to be done for those.
+  if (info->strict_mode() == SLOPPY && !info->is_native() &&
+      !info->function()->is_arrow()) {
     Label ok;
     // +1 for return address.
     StackArgumentsAccessor args(rsp, info->scope()->num_parameters());
@@ -140,6 +142,19 @@ void FullCodeGenerator::Generate() {
   __ Prologue(info->IsCodePreAgingActive());
   info->AddNoFrameRange(0, masm_->pc_offset());
 
+  // For arrow functions the value of the receiver is stored in the context,
+  // so in the case of a function that actually uses "this", the receiver is
+  // replaced by the one resulting from looking it up in the context.
+  if (info->function()->is_arrow() && info->scope()->uses_this()) {
+    DCHECK(info->scope()->scope_type() == ARROW_SCOPE);
+    Comment cmnt(masm_, "[ Patch receiver for arrow function");
+    StackArgumentsAccessor args(rbp, info->scope()->num_parameters());
+    __ Push(rsi);  // Context
+    __ Push(isolate()->factory()->this_string());
+    __ CallRuntime(Runtime::kLoadLookupSlot, 2);
+    __ movp(args.GetReceiverOperand(), rax);
+  }
+
   { Comment cmnt(masm_, "[ Allocate locals");
     int locals_count = info->scope()->num_stack_slots();
     // Generators allocate locals, if any, in context slots.
@@ -207,9 +222,12 @@ void FullCodeGenerator::Generate() {
     __ movp(Operand(rbp, StandardFrameConstants::kContextOffset), rax);
 
     // Copy any necessary parameters into the context.
+    DCHECK(info->scope()->receiver()->IsContextSlot() ==
+           info->scope()->inner_uses_this());
+
     int num_parameters = info->scope()->num_parameters();
-    for (int i = 0; i < num_parameters; i++) {
-      Variable* var = scope()->parameter(i);
+    for (int i = -1; i < num_parameters; i++) {
+      Variable* var = (i == -1) ? scope()->receiver() : scope()->parameter(i);
       if (var->IsContextSlot()) {
         int parameter_offset = StandardFrameConstants::kCallerSPOffset +
             (num_parameters - 1 - i) * kPointerSize;