diff --git a/runtime/arch/arm64/registers_arm64.h b/runtime/arch/arm64/registers_arm64.h
index 7b7a460..9ccab70 100644
--- a/runtime/arch/arm64/registers_arm64.h
+++ b/runtime/arch/arm64/registers_arm64.h
@@ -56,16 +56,15 @@
   X29 = 29,
   X30 = 30,
   X31 = 31,
-  SP  = 32,      // SP and XZR are encoded in instructions using the register
-                 // code 31, the context deciding which is used. We use a
-                 // different enum value to distinguish between the two.
-  TR  = X18,     // ART Thread Register - Managed Runtime (Caller Saved Reg)
-  ETR = X21,     // ART Thread Register - External Calls  (Callee Saved Reg)
-  IP0 = X16,     // Used as scratch by VIXL.
-  IP1 = X17,     // Used as scratch by ART JNI Assembler.
-  FP  = X29,
-  LR  = X30,
-  XZR = X31,
+  TR  = 18,     // ART Thread Register - Managed Runtime (Caller Saved Reg)
+  ETR = 21,     // ART Thread Register - External Calls  (Callee Saved Reg)
+  IP0 = 16,     // Used as scratch by VIXL.
+  IP1 = 17,     // Used as scratch by ART JNI Assembler.
+  FP  = 29,
+  LR  = 30,
+  SP  = 31,     // SP is X31 and overlaps with XRZ but we encode it as a
+                // special register, due to the different instruction semantics.
+  XZR = 32,
   kNumberOfCoreRegisters = 33,
   kNoRegister = -1,
 };
@@ -105,7 +104,7 @@
   W29 = 29,
   W30 = 30,
   W31 = 31,
-  WZR = W31,
+  WZR = 31,
   kNumberOfWRegisters = 32,
   kNoWRegister = -1,
 };