Fix issues with Arm's use of embedded constant pools

src/arm/frames-arm.h

Index: src/arm/frames-arm.h
diff --git a/src/arm/frames-arm.h b/src/arm/frames-arm.h
index ea621c965b47e9322b7f6d0ddcb0eefb10db9c20..73d1ce1f7841c5ed8d1cf6f6ab4ded98e620a0bc 100644
--- a/src/arm/frames-arm.h
+++ b/src/arm/frames-arm.h
@@ -69,8 +69,14 @@ 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);
 
 // ----------------------------------------------------