StubCallInterfaceDescriptor takes a context register.

src/deoptimizer.cc

Index: src/deoptimizer.cc
diff --git a/src/deoptimizer.cc b/src/deoptimizer.cc
index a730b48d2e8c41c26c8346637f21cf2d7d21d4ff..30a7840358530b4d87424193aa09924a5bf0abc4 100644
--- a/src/deoptimizer.cc
+++ b/src/deoptimizer.cc
@@ -1554,10 +1554,13 @@ void Deoptimizer::DoComputeCompiledStubFrame(TranslationIterator* iterator,
   // The output frame must have room for all pushed register parameters
   // and the standard stack frame slots.  Include space for an argument
   // object to the callee and optionally the space to pass the argument
-  // object to the stub failure handler.
-  CHECK_GE(descriptor->register_param_count(), 0);
-  int height_in_bytes = kPointerSize * descriptor->register_param_count() +
-      sizeof(Arguments) + kPointerSize;
+  // object to the stub failure handler. The descriptor's first register
+  // parameter is the context register, which we deal with separately.
+  int param_count = descriptor->register_param_count() - 1;

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

Use GetEnvironmentParameterLength(), then comment is unnecessary

[mvstanton, 2014/07/17 09:04:05]

Done.

+  CHECK_GE(param_count, 0);
+
+  int height_in_bytes = kPointerSize * param_count + sizeof(Arguments) +
+      kPointerSize;
   int fixed_frame_size = StandardFrameConstants::kFixedFrameSize;
   int input_frame_size = input_->GetFrameSize();
   int output_frame_size = height_in_bytes + fixed_frame_size;
@@ -1699,11 +1702,14 @@ void Deoptimizer::DoComputeCompiledStubFrame(TranslationIterator* iterator,
 
   // Copy the register parameters to the failure frame.
   int arguments_length_offset = -1;
-  for (int i = 0; i < descriptor->register_param_count(); ++i) {
+  for (int i = 0; i < param_count; ++i) {
     output_frame_offset -= kPointerSize;
     DoTranslateCommand(iterator, 0, output_frame_offset);
 
-    if (!arg_count_known && descriptor->IsParameterCountRegister(i)) {
+    // In the frame we only have descriptor->ParameterRegisters [1..param_count]
+    // because we don't include the context register. Adjust our index into
+    // the descriptor parameter array accordingly.

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

Why not move this up to the loop iteration? e.g.

for (int i = 1; i < param_count; ++i) {

Otherwise, I think you might actually do one too many iterations here, since you
access param_count + 1, which is technically OOB

[mvstanton, 2014/07/17 09:04:05]

Well, I would have to make the terminating condition "i <= param_count", because
param count runs from [0, param_count] but the index in the descriptor runs from
[1, param_count+1] since we have to skip over the context register. I thought
about that and decided to keep the loop condition simple, and the complexity in
this notion that I need to translate to a descriptor parameter register. 

So I'd prefer to leave it this way, but I could go from 1 <= i <= param_count
too, let me know.

<EDIT>

On second thought, what a complex mess. I just introduced method
IsEnvironmentParameterCountRegister() which makes things simpler.

+    if (!arg_count_known && descriptor->IsParameterCountRegister(i + 1)) {
       arguments_length_offset = output_frame_offset;
     }
   }