StubCallInterfaceDescriptor takes a context register.

src/code-stubs-hydrogen.cc

Index: src/code-stubs-hydrogen.cc
diff --git a/src/code-stubs-hydrogen.cc b/src/code-stubs-hydrogen.cc
index bdd168b690fb79d57f4333a72c6e2397a34b4ba0..53406913c6b751e0a496a5134bdc66c108feb02f 100644
--- a/src/code-stubs-hydrogen.cc
+++ b/src/code-stubs-hydrogen.cc
@@ -39,7 +39,8 @@ class CodeStubGraphBuilderBase : public HGraphBuilder {
         info_(stub, isolate),
         context_(NULL) {
     descriptor_ = stub->GetInterfaceDescriptor();
-    parameters_.Reset(new HParameter*[descriptor_->register_param_count()]);
+    int parameter_count = descriptor_->register_param_count() - 1;

[danno, 2014/07/16 13:19:59]

GetEnvironmentParameterCount

[mvstanton, 2014/07/17 09:04:04]

Done.

+    parameters_.Reset(new HParameter*[parameter_count]);
   }
   virtual bool BuildGraph();
 
@@ -116,7 +117,9 @@ bool CodeStubGraphBuilderBase::BuildGraph() {
     isolate()->GetHTracer()->TraceCompilation(&info_);
   }
 
-  int param_count = descriptor_->register_param_count();
+  // Every descriptor has a context register parameter, which we don't need
+  // to include here.
+  int param_count = descriptor_->register_param_count() - 1;

[danno, 2014/07/16 13:19:59]

Use GetEnvironmentLength(), then comment is unnecessary

[mvstanton, 2014/07/17 09:04:04]

Done. (but I used GetEnvironmentParameterCount() as suggested in the line 42
comment)

   HEnvironment* start_environment = graph()->start_environment();
   HBasicBlock* next_block = CreateBasicBlock(start_environment);
   Goto(next_block);
@@ -126,11 +129,13 @@ bool CodeStubGraphBuilderBase::BuildGraph() {
   bool runtime_stack_params = descriptor_->stack_parameter_count().is_valid();
   HInstruction* stack_parameter_count = NULL;
   for (int i = 0; i < param_count; ++i) {
-    Representation r = descriptor_->GetRegisterParameterRepresentation(i);
-    HParameter* param = Add<HParameter>(i, HParameter::REGISTER_PARAMETER, r);
+    int parameter_index = i+1;

[danno, 2014/07/16 13:19:59]

nit: make sure to clang format your patch. Comment about accounting for the
context parameter?

[mvstanton, 2014/07/17 09:04:04]

I got rid of this whole notion of "convert a parameter index to a descriptor
register parameter index" with the +1 thing. I just made a new method
GetEnvironmentParameterRegister() and GetEnvironmentParameterRepresentation().

After addressing the feedback to do something like that in deoptimizer.cc, it
made sense to come back and touch this too.

+    Representation r = descriptor_->GetParameterRepresentation(parameter_index);
+    HParameter* param = Add<HParameter>(parameter_index,
+                                        HParameter::REGISTER_PARAMETER, r);
     start_environment->Bind(i, param);
     parameters_[i] = param;
-    if (descriptor_->IsParameterCountRegister(i)) {
+    if (descriptor_->IsParameterCountRegister(parameter_index)) {
       param->set_type(HType::Smi());
       stack_parameter_count = param;
       arguments_length_ = stack_parameter_count;