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

src/x64/full-codegen-x64.cc

 // Copyright 2012 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.
 
 #include "src/v8.h"
 
 #if V8_TARGET_ARCH_X64
 
 #include "src/code-factory.h"
 #include "src/code-stubs.h"
@@ skipping 294 matching lines @@
   }
 
   // Visit the declarations and body unless there is an illegal
   // redeclaration.
   if (scope()->HasIllegalRedeclaration()) {
     Comment cmnt(masm_, "[ Declarations");
     scope()->VisitIllegalRedeclaration(this);
 
   } else {
     PrepareForBailoutForId(BailoutId::FunctionEntry(), NO_REGISTERS);
+
     { Comment cmnt(masm_, "[ Declarations");
       // For named function expressions, declare the function name as a
       // constant.
       if (scope()->is_function_scope() && scope()->function() != NULL) {
         VariableDeclaration* function = scope()->function();
         DCHECK(function->proxy()->var()->mode() == CONST ||
                function->proxy()->var()->mode() == CONST_LEGACY);
         DCHECK(function->proxy()->var()->location() != Variable::UNALLOCATED);
         VisitVariableDeclaration(function);
       }
       VisitDeclarations(scope()->declarations());
     }
 
     { Comment cmnt(masm_, "[ Stack check");
       PrepareForBailoutForId(BailoutId::Declarations(), NO_REGISTERS);
        Label ok;
        __ CompareRoot(rsp, Heap::kStackLimitRootIndex);
        __ j(above_equal, &ok, Label::kNear);
        __ call(isolate()->builtins()->StackCheck(), RelocInfo::CODE_TARGET);
        __ 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());
       DCHECK(loop_depth() == 0);
     }
   }
 
   // Always emit a 'return undefined' in case control fell off the end of
   // the body.
   { Comment cmnt(masm_, "[ return <undefined>;");
@@ skipping 4270 matching lines @@
   int arg_count = args->length();
 
   // Record source position of the IC call.
   SetSourcePosition(expr->position());
   CallFunctionStub stub(isolate(), arg_count, NO_CALL_FUNCTION_FLAGS);
   __ movp(rdi, Operand(rsp, (arg_count + 1) * kPointerSize));
   __ CallStub(&stub);
 }
 
 
+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();
 
   if (expr->is_jsruntime()) {
     Comment cmnt(masm_, "[ CallRuntime");
 
     EmitLoadJSRuntimeFunction(expr);
 
     // Push the target function under the receiver.
@@ skipping 780 matching lines @@
   DCHECK_EQ(isolate->builtins()->OsrAfterStackCheck()->entry(),
             Assembler::target_address_at(call_target_address,
                                          unoptimized_code));
   return OSR_AFTER_STACK_CHECK;
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64