[es6] implement default/optional parameters (WIP / comments)

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 9c7a41f44906a2e4c326d9b52d40f9fbc496899d..06ce9420f2926ce1254c43a7cf56e9ea98f9f4d5 100644
--- a/src/x64/full-codegen-x64.cc
+++ b/src/x64/full-codegen-x64.cc
@@ -312,6 +312,7 @@ void FullCodeGenerator::Generate() {
 
   } else {
     PrepareForBailoutForId(BailoutId::FunctionEntry(), NO_REGISTERS);
+
     { Comment cmnt(masm_, "[ Declarations");
       // For named function expressions, declare the function name as a
       // constant.
@@ -334,6 +335,11 @@ void FullCodeGenerator::Generate() {
        __ bind(&ok);
     }
 
+    if (info->function()->default_values()->length()) {
+      Comment cmnt(masm_, "[ Optional parameters");
+      EmitInitializeOptionalParameters(info->scope(), info->function());

[arv (Not doing code reviews), 2015/04/10 15:08:48]

I'm not sure this gets the TDZ right... maybe the tests will tell.

[caitp (gmail), 2015/04/10 15:28:00]

I think it's mostly right, actually

1 thing I'm not positive on, is reference to formals declared to the right of
the optional parameter (which are currently initialized first).

Here's a demo:

```
d8> function f(a = b) { "use strict"; let b = 1; return a; }
undefined
d8> f()
(d8):1: ReferenceError: b is not defined
function f(a = b) { "use strict"; let b = 1; return a; }
               ^
ReferenceError: b is not defined
    at f ((d8):1:16)
    at (d8):1:1

d8> function f(a = b) { "use strict"; var b = 1; return a; }
undefined
d8> f()
undefined
```

[arv (Not doing code reviews), 2015/04/10 15:58:16]

This looks wrong
Some rules:

 - The body is not in scope when the initializer is evaluated.
 - You cannot forward reference (the param has a TDZ)

I need to back that up with links to the spec though.

+    }
+
     { Comment cmnt(masm_, "[ Body");
       DCHECK(loop_depth() == 0);
       VisitStatements(function()->body());
@@ -4624,6 +4630,61 @@ void FullCodeGenerator::EmitCallJSRuntimeFunction(CallRuntime* expr) {
 }
 
 
+void FullCodeGenerator::EmitInitializeOptionalParameters(Scope* scope,
+                                                         FunctionLiteral* fn) {
+  ZoneList<Expression*>* defaults = fn->default_values();
+  if (defaults->is_empty()) return;
+
+  int num_parameters = scope->num_parameters();
+  int heap_slots = scope->num_heap_slots() - Context::MIN_CONTEXT_SLOTS;
+
+  int current_default = 0;
+
+  for (int i = 0; i < num_parameters; ++i) {
+    Variable* param = scope->parameter(i);
+
+    if (param->isRestParameter()) break;
+
+    if (param->isOptionalParameter()) {
+      Expression* default_value = defaults->at(current_default++);

[arv (Not doing code reviews), 2015/04/10 15:08:48]

So the default list is not a direct mapping from the params list?

function f(x, y = 1, z, w = 2) {}

params:   [x, y, z, w]
defaults: [1, 2]

?

That seems a bit strange.

[caitp (gmail), 2015/04/10 15:28:00]

I think we will want to combine them into one list, since parameter lists get a
lot more complicated for destructuring. There is code which depends on the
expressions in parameter lists to all be VariableProxy, and it might be a lot of
work to go through and fix all of those cases

But I'm not sure I want to start that yet

+
+      // If the default value is undefined, there is no point initializing it.
+      if (default_value->IsLiteral() &&
+          default_value->AsLiteral()->IsUndefined()) continue;

[arv (Not doing code reviews), 2015/04/10 15:08:49]

undefined is a mutable global binding. We cannot make this optimization.

[caitp (gmail), 2015/04/10 15:28:00]

hmm :( that's true

+
+      int parameter_offset = StandardFrameConstants::kCallerSPOffset +
+          (num_parameters - 1 - i) * kPointerSize;
+
+      Label next;
+
+      __ movp(rax, Operand(rbp, parameter_offset));
+
+      __ CompareRoot(rax, Heap::kUndefinedValueRootIndex);
+      __ j(not_equal, &next);
+
+      {
+        AccumulatorValueContext context(this);
+        Visit(default_value);
+      }
+
+      if (param->IsContextSlot() && heap_slots > 0) {
+        bool write_barrier = heap_slots > FastNewContextStub::kMaximumSlots;
+        int context_offset = Context::SlotOffset(param->index());
+        __ movp(Operand(rsi, context_offset), rax);
+        if (write_barrier) {
+          __ RecordWriteContextSlot(
+              rsi, context_offset, rax, rbx, kDontSaveFPRegs);
+        }
+      } else {
+        SetVar(param, rax, rbx, rdx);
+      }
+
+      __ bind(&next);
+    }
+  }
+}
+
+
 void FullCodeGenerator::VisitCallRuntime(CallRuntime* expr) {
   ZoneList<Expression*>* args = expr->arguments();
   int arg_count = args->length();