Fix issues with Arm's use of embedded constant pools

src/arm/frames-arm.h

 // 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.
 
 #ifndef V8_ARM_FRAMES_ARM_H_
 #define V8_ARM_FRAMES_ARM_H_
 
 namespace v8 {
 namespace internal {
 
@@ skipping 51 matching lines @@
 
 
 // Number of registers for which space is reserved in safepoints. Must be a
 // multiple of 8.
 // TODO(regis): Only 8 registers may actually be sufficient. Revisit.
 const int kNumSafepointRegisters = 16;
 
 // Define the list of registers actually saved at safepoints.
 // Note that the number of saved registers may be smaller than the reserved
 // space, i.e. kNumSafepointSavedRegisters <= kNumSafepointRegisters.
-const RegList kSafepointSavedRegisters = kJSCallerSaved | kCalleeSaved;
-const int kNumSafepointSavedRegisters = kNumJSCallerSaved + kNumCalleeSaved;
+const RegList kSafepointSavedRegisters =
+    kJSCallerSaved |
+    (FLAG_enable_embedded_constant_pool ? kCalleeSaved & ~(1 << 8)
+                                        : kCalleeSaved);

[rmcilroy, 2015/06/08 11:11:11]

I'm guessing you are doing this because pp holds a non-tagged pointer? If so,
I'm wondering if there is a better way to do this - kCalleeSaved also includes
the fp which also holds a non-tagged pointer - could you avoid removing the pp
register from kSafepointSavedRegisters somehow by following what's happening for
fp? 

[MTBrandyberry, 2015/06/08 13:52:39]

Partly.  It's true that pp doesn't need to be saved in the safepoint since it
isn't tagged.  More important, however, is that pp is saved in the stack frame
where its value will be updated if GC code movement occurs.  Allowing it to
remain in the safepoint causes a potentially stale value to be restored.

fp does not have this same consideration and is currently included in the
safepoint.

[rmcilroy, 2015/06/09 10:10:46]

OK, but let's make this a bit easier to understand. Could you pull out some
constants to make this:

const RegList kSafepointSavedRegisters = kJSCallerSaved | kCalleeSaved &
~kConstantPointerRegMask;
const int kNumSafepointSavedRegisters = kNumJSCallerSaved + kNumCalleeSaved -
kNumConstantPoolPointerReg;

or similar.

[MTBrandyberry, 2015/06/09 13:26:38]

Done.

+const int kNumSafepointSavedRegisters =
+    kNumJSCallerSaved + (FLAG_enable_embedded_constant_pool
+                             ? kNumCalleeSaved - 1
+                             : kNumCalleeSaved);
 
 // ----------------------------------------------------
 
 
 class EntryFrameConstants : public AllStatic {
  public:
   static const int kCallerFPOffset =
       -(StandardFrameConstants::kFixedFrameSizeFromFp + kPointerSize);
 };
 
@@ skipping 64 matching lines @@
 
 inline Object* JavaScriptFrame::function_slot_object() const {
   const int offset = JavaScriptFrameConstants::kFunctionOffset;
   return Memory::Object_at(fp() + offset);
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_ARM_FRAMES_ARM_H_