shrink UniqueInstance ctor

Summary:
It is monomorphized but called rarely. Best to optimize for size.

The `UniqueInstance` ctor is currently called in the constructor of each translation unit for each distinct instance of `SingletonThreadLocal`.

`UniqueInstance` may be extended to more use-cases so it is worthwhile optimizing for size.

Reviewed By: luciang

Differential Revision: D27602938

fbshipit-source-id: e4077830544516219485814ccc48805b7bb38e34

folly/detail/UniqueInstance.cpp

@@ -83,13 +83,9 @@ std::string render(Value value) {
 
 } // namespace
 
-void UniqueInstance::enforce(
-    Ptr tmpl,
-    Ptr const* ptrs,
-    std::uint32_t key_size,
-    std::uint32_t mapped_size,
-    Value& global) noexcept {
-  Value const local{tmpl, ptrs, key_size, mapped_size};
+void UniqueInstance::enforce(Arg& arg) noexcept {
+  auto const& local = arg.local;
+  auto& global = StaticSingletonManager::create<Value>(arg.global);
 
   if (!global.tmpl) {
     global = local;

folly/detail/UniqueInstance.h

@@ -19,6 +19,7 @@
 #include <cstdint>
 #include <typeinfo>
 
+#include <folly/CppAttributes.h>
 #include <folly/detail/StaticSingletonManager.h>
 
 namespace folly {
@@ -30,12 +31,14 @@ class UniqueInstance {
   explicit UniqueInstance(...) noexcept {}
 #else
   template <template <typename...> class Z, typename... Key, typename... Mapped>
-  FOLLY_EXPORT explicit UniqueInstance(
+  FOLLY_EXPORT FOLLY_ALWAYS_INLINE explicit UniqueInstance(
       tag_t<Z<Key..., Mapped...>>, tag_t<Key...>, tag_t<Mapped...>) noexcept {
-    Ptr const tmpl = &typeid(key_t<Z>);
+    static Ptr const tmpl = &typeid(key_t<Z>);
     static Ptr const ptrs[] = {&typeid(Key)..., &typeid(Mapped)...};
-    auto& global = createGlobal<Value, key_t<Z, Key...>>();
-    enforce(tmpl, ptrs, sizeof...(Key), sizeof...(Mapped), global);
+    static Arg arg{
+        {tmpl, ptrs, sizeof...(Key), sizeof...(Mapped)},
+        {tag<Value, key_t<Z, Key...>>}};
+    enforce(arg);
   }
 #endif
 
@@ -59,15 +62,12 @@ class UniqueInstance {
     std::uint32_t key_size;
     std::uint32_t mapped_size;
   };
+  struct Arg {
+    Value local;
+    StaticSingletonManager::ArgCreate<true> global;
+  };
 
-  //  Under Clang, this call signature shrinks the aligned and padded size of
-  //  call-sites, as compared to a call signature taking Value or Value const&.
-  static void enforce(
-      Ptr tmpl,
-      Ptr const* ptrs,
-      std::uint32_t key_size,
-      std::uint32_t mapped_size,
-      Value& global) noexcept;
+  static void enforce(Arg& arg) noexcept;
 };
 
 } // namespace detail