jni: Add read barrier fast path to jni compiler

Static method dispatch via JNI requires a read barrier
for the ArtMethod::GetDeclaringClass() load before adding it to the
JNI StackHandleScope.

We used to call ReadBarrierJni unconditionally but add a branch
to skip calling it if the GC is not currently in the marking phase.

Test: ART_USE_READ_BARRIER=true make test-art-host test-art-target
Bug: 30437917
Change-Id: I4f505ebde17c0a67209c7bb51b3f39e37a06373a
diff --git a/compiler/utils/arm64/jni_macro_assembler_arm64.h b/compiler/utils/arm64/jni_macro_assembler_arm64.h
index b9f6854..264e99a 100644
--- a/compiler/utils/arm64/jni_macro_assembler_arm64.h
+++ b/compiler/utils/arm64/jni_macro_assembler_arm64.h
@@ -168,6 +168,15 @@
   // and branch to a ExceptionSlowPath if it is.
   void ExceptionPoll(ManagedRegister scratch, size_t stack_adjust) OVERRIDE;
 
+  // Create a new label that can be used with Jump/Bind calls.
+  std::unique_ptr<JNIMacroLabel> CreateLabel() OVERRIDE;
+  // Emit an unconditional jump to the label.
+  void Jump(JNIMacroLabel* label) OVERRIDE;
+  // Emit a conditional jump to the label by applying a unary condition test to the register.
+  void Jump(JNIMacroLabel* label, JNIMacroUnaryCondition cond, ManagedRegister test) OVERRIDE;
+  // Code at this offset will serve as the target for the Jump call.
+  void Bind(JNIMacroLabel* label) OVERRIDE;
+
  private:
   class Arm64Exception {
    public:
@@ -222,6 +231,16 @@
   ArenaVector<std::unique_ptr<Arm64Exception>> exception_blocks_;
 };
 
+class Arm64JNIMacroLabel FINAL
+    : public JNIMacroLabelCommon<Arm64JNIMacroLabel,
+                                 vixl::aarch64::Label,
+                                 kArm64> {
+ public:
+  vixl::aarch64::Label* AsArm64() {
+    return AsPlatformLabel();
+  }
+};
+
 }  // namespace arm64
 }  // namespace art