Initial patch for a64.

src/deoptimizer.cc

Index: src/deoptimizer.cc
diff --git a/src/deoptimizer.cc b/src/deoptimizer.cc
index 723d3f692e6b3e45e415534d20d21e1eb977ee03..0eca4edf1b64f1d915bc2c3eca790d06e38d6c28 100644
--- a/src/deoptimizer.cc
+++ b/src/deoptimizer.cc
@@ -2402,6 +2402,9 @@ FrameDescription::FrameDescription(uint32_t frame_size,
       context_(kZapUint32) {
   // Zap all the registers.
   for (int r = 0; r < Register::kNumRegisters; r++) {
+    // TODO(jbramley): It isn't safe to use kZapUint32 here. If the register
+    // isn't used before the next safepoint, the GC will try to scan it as a
+    // tagged value. kZapUint32 looks like a valid tagged pointer, but it isn't.
     SetRegister(r, kZapUint32);
   }