Support `!`-style fast calls for Android

Summary:
Adds support for `!`-style fast calls in Android versions < 8.

For now, this comes with the following restrictions:

- has to be enabled with a macro
- only supports native functions that return and accept primitive Java types (`jint`, `jdouble`, etc.), and `void`
- this is supposed to map to `CriticalNative` as described in https://source.android.com/devices/tech/dalvik/improvements#faster-native-methods.

Inline documentation in art: http://androidxref.com/6.0.1_r10/xref/art/runtime/jni_internal.cc#2110

Possible follow ups:
- don’t prefix when the Android runtime is too new (to avoid the warning, and future error, see http://androidxref.com/8.1.0_r33/xref/art/runtime/jni_internal.cc#2355)
- shim `CriticalNative` (see https://android.googlesource.com/platform/libcore/+/master/dalvik/src/main/java/dalvik/annotation/optimization/CriticalNative.java)
- test whether we can use `CriticalNative` at all (docs claim classes must be on bootclasspath)
- suppport `FastNative`, i.e. instance methods that can also receive and return `jobject`

Reviewed By: cjhopman

Differential Revision: D9630538

fbshipit-source-id: 0ae86c909b192429d60f8eddb15528cc38610379

ReactAndroid/src/main/jni/first-party/fb/include/fb/fbjni/CoreClasses.h

@@ -235,11 +235,18 @@ class FBEXPORT JClass : public JavaClass<JClass, JObject, jclass> {
   ///     makeNativeMethod("nativeMethodWithExplicitDescriptor",
   ///                      "(Lcom/facebook/example/MyClass;)V",
   ///                      methodWithExplicitDescriptor),
+  ///     makeCriticalNativeMethod("criticalNativeMethodWithAutomaticDescriptor",
+  ///                              criticalNativeMethodWithAutomaticDescriptor),
+  ///     makeCriticalNativeMethod("criticalNativeMethodWithExplicitDescriptor",
+  ///                              "(IIF)Z",
+  ///                              criticalNativeMethodWithExplicitDescriptor),
   ///  });
   ///
   /// By default, C++ exceptions raised will be converted to Java exceptions.
   /// To avoid this and get the "standard" JNI behavior of a crash when a C++
   /// exception is crashing out of the JNI method, declare the method noexcept.
+  /// This does NOT apply to critical native methods, where exceptions causes
+  /// a crash.
   void registerNatives(std::initializer_list<NativeMethod> methods);
 
   /// Check to see if the class is assignable from another class

ReactAndroid/src/main/jni/first-party/fb/include/fb/fbjni/Registration-inl.h

@@ -142,6 +142,25 @@ inline std::string makeDescriptor(R (C::*)(Args... args)) {
   return jmethod_traits_from_cxx<R(Args...)>::descriptor();
 }
 
+template<typename R, typename ...Args>
+template<R(*func)(Args...)>
+R CriticalMethod<R(*)(Args...)>::call(alias_ref<jclass>, Args... args) {
+  static_assert(
+    IsJniPrimitive<R>() || std::is_void<R>(),
+    "Critical Native Methods may only return primitive JNI types, or void.");
+  static_assert(
+    AreJniPrimitives<Args...>(),
+    "Critical Native Methods may only use primitive JNI types as parameters");
+
+  return func(std::forward<Args>(args)...);
+}
+
+template<typename R, typename ...Args>
+template<R(*func)(Args...)>
+inline std::string CriticalMethod<R(*)(Args...)>::desc() {
+  return makeDescriptor(call<func>);
+}
+
 }
 
 }}

ReactAndroid/src/main/jni/first-party/fb/include/fb/fbjni/Registration.h

@@ -44,6 +44,18 @@ std::string makeDescriptor(R (*func)(alias_ref<C>, Args... args));
 template<typename R, typename C, typename... Args>
 std::string makeDescriptor(R (C::*method0)(Args... args));
 
+template<typename F>
+struct CriticalMethod;
+
+template<typename R, typename ...Args>
+struct CriticalMethod<R(*)(Args...)> {
+  template<R(*func)(Args...)>
+  static R call(alias_ref<jclass>, Args... args);
+
+  template<R(*func)(Args...)>
+  inline static std::string desc();
+};
+
 }
 
 // We have to use macros here, because the func needs to be used
@@ -66,6 +78,35 @@ std::string makeDescriptor(R (C::*method0)(Args... args));
 #define makeNativeMethodN(a, b, c, count, ...) makeNativeMethod ## count
 #define makeNativeMethod(...) makeNativeMethodN(__VA_ARGS__, 3, 2)(__VA_ARGS__)
 
+
+// FAST CALLS / CRITICAL CALLS
+// Android up to and including v7 supports "fast calls" by prefixing the method
+// signature with an exclamation mark.
+// Android v8+ supports fast calls by annotating methods:
+// https://source.android.com/devices/tech/dalvik/improvements#faster-native-methods
+
+// prefixes a JNI method signature as android "fast call".
+#if defined(__ANDROID__) && defined(FBJNI_WITH_FAST_CALLS)
+#define FBJNI_PREFIX_FAST_CALL(desc) (std::string{"!"} + desc)
+#else
+#define FBJNI_PREFIX_FAST_CALL(desc) (desc)
+#endif
+
+#define makeCriticalNativeMethod3(name, desc, func) \
+  makeNativeMethod3(                                \
+    name,                                           \
+    FBJNI_PREFIX_FAST_CALL(desc),                   \
+    ::facebook::jni::detail::CriticalMethod<decltype(&func)>::call<&func>)
+
+#define makeCriticalNativeMethod2(name, func)                                \
+  makeCriticalNativeMethod3(                                                 \
+    name,                                                                    \
+    ::facebook::jni::detail::CriticalMethod<decltype(&func)>::desc<&func>(), \
+    func)
+
+#define makeCriticalNativeMethodN(a, b, c, count, ...) makeCriticalNativeMethod ## count
+#define makeCriticalNativeMethod(...) makeCriticalNativeMethodN(__VA_ARGS__, 3, 2)(__VA_ARGS__)
+
 }}
 
 #include "Registration-inl.h"

ReactAndroid/src/main/jni/first-party/fb/include/fb/fbjni/TypeTraits.h

@@ -69,6 +69,25 @@ constexpr bool IsJniPrimitive() {
   return is_jni_primitive<T>::value;
 }
 
+/// Metafunction to determine whether a series of types are all primitive JNI types.
+template<typename ...Ts>
+struct are_jni_primitives;
+
+template<typename T, typename ...Ts>
+struct are_jni_primitives<T, Ts...> :
+  std::integral_constant<bool,
+    is_jni_primitive<T>::value && are_jni_primitives<Ts...>::value> {};
+
+template<>
+struct are_jni_primitives<> : std::integral_constant<bool, true> {};
+
+/// Helper to simplify use of are_jni_primitives
+template<typename ...Ts>
+constexpr bool AreJniPrimitives() {
+  return are_jni_primitives<Ts...>::value;
+}
+
+
 /// Metafunction to determine whether a type is a JNI array of primitives or not
 template <typename T>
 struct is_jni_primitive_array :