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..41bac14e3761317b0c3d694a42d50bde3934b5a9 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 receiver
+  // of the parent scope, so this does not need to be done for those.

[wingo, 2015/01/15 10:09:06]

It's not necessarily the receiver from the parent scope -- the parent scope
could be a block scope, or another arrow function; anyway let's be more precise
with comments :)

[aperez, 2015/01/15 16:58:03]

Done.

+  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.
@@ -207,6 +222,30 @@ void FullCodeGenerator::Generate() {
     __ movp(Operand(rbp, StandardFrameConstants::kContextOffset), rax);
 
     // Copy any necessary parameters into the context.
+    if (info->scope()->inner_uses_this()) {

[wingo, 2015/01/15 10:09:06]

Let's factor this into the loop below

[aperez, 2015/01/15 16:58:03]

Done.

+      Comment cmnt(masm_, "[ Save receiver in context for arrow function");
+      DCHECK(info->scope()->receiver()->IsContextSlot());
+      // Put the "this" in the context after the rest of the function
+      // parameters, to avoid having to change the case that handles those.
+      int parameter_offset = StandardFrameConstants::kCallerSPOffset +
+          info->scope()->num_parameters() * kPointerSize;
+      __ movp(rax, Operand(rbp, parameter_offset));
+
+      // Store it in the context.
+      int context_offset = Context::SlotOffset(scope()->receiver()->index());
+      __ movp(Operand(rsi, context_offset), rax);
+
+      if (need_write_barrier) {
+        __ RecordWriteContextSlot(
+            rsi, context_offset, rax, rbx, kDontSaveFPRegs);
+      } else if (FLAG_debug_code) {
+        Label done;
+        __ JumpIfInNewSpace(rsi, rax, &done, Label::kNear);
+        __ Abort(kExpectedNewSpaceObject);
+        __ bind(&done);
+      }
+    }
+
     int num_parameters = info->scope()->num_parameters();
     for (int i = 0; i < num_parameters; i++) {
       Variable* var = scope()->parameter(i);
@@ -2721,7 +2760,8 @@ void FullCodeGenerator::EmitCallWithLoadIC(Call* expr) {
       PrepareForBailout(callee, NO_REGISTERS);
     }
     // Push undefined as receiver. This is patched in the method prologue if it
-    // is a sloppy mode method.
+    // is a sloppy mode method, or by the function prologue it is an arrow

[wingo, 2015/01/15 10:09:06]

"if it is"

[aperez, 2015/01/15 16:58:03]

Done.

+    // function.
     __ Push(isolate()->factory()->undefined_value());
   } else {
     // Load the function from the receiver.