small-object optimization for SkFunction

src/core/SkFunction.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkFunction_DEFINED
 #define SkFunction_DEFINED
 
 // TODO: document
 
 #include "SkTypes.h"
+#include "SkTLogic.h"
 
 template <typename> class SkFunction;
 
 template <typename R, typename... Args>
 class SkFunction<R(Args...)> : SkNoncopyable {
 public:
     explicit SkFunction(R (*fn)(Args...)) : fVTable(GetFunctionPointerVTable()) {
         // We've been passed a function pointer.  We'll just store it.
         fFunction = reinterpret_cast<void*>(fn);
     }
 
     template <typename Fn>
-    explicit SkFunction(Fn fn) : fVTable(GetVTable<Fn>()) {
-        // We've got a functor.  The basic thing we can always do is copy it onto the heap.
+    explicit SkFunction(Fn fn, SK_WHEN_C((sizeof(Fn) > sizeof(void*)), void*) = nullptr)
+            : fVTable(GetOutlineVTable<Fn>()) {
+        // We've got a functor larger than a pointer.  We've go to copy it onto the heap.
         fFunction = SkNEW_ARGS(Fn, (fn));
     }
 
-    ~SkFunction() { fVTable.fDelete(fFunction); }
+    template <typename Fn>
+    explicit SkFunction(Fn fn, SK_WHEN_C((sizeof(Fn) <= sizeof(void*)), void*) = nullptr)
+            : fVTable(GetInlineVTable<Fn>()) {
+        // We've got a functor that fits in a pointer.  We copy it right inline.
+        SkNEW_PLACEMENT_ARGS(&fFunction, Fn, (fn));
+    }
+
+    ~SkFunction() { fVTable.fCleanUp(fFunction); }
 
     R operator()(Args... args) { return fVTable.fCall(fFunction, args...); }
 
 private:
     struct VTable {
         R (*fCall)(void*, Args...);
-        void (*fDelete)(void*);
+        void (*fCleanUp)(void*);
     };
 
+    // Used when fFunction is a function pointer of type R(*)(Args...).
     static const VTable& GetFunctionPointerVTable() {
         static const VTable vtable = {
             [](void* fn, Args... args) { return reinterpret_cast<R(*)(Args...)>(fn)(args...); },
-            [](void*) { /* Don't delete function pointers. */ },
+            [](void*) { /* Nothing to clean up for function pointers. */ }
         };
         return vtable;
     }
 
+    // Used when fFunction is a pointer to a functor of type Fn on the heap (we own it).
     template <typename Fn>
-    static const VTable& GetVTable() {
+    static const VTable& GetOutlineVTable() {
         static const VTable vtable = {
             [](void* fn, Args... args) { return (*static_cast<Fn*>(fn))(args...); },
             [](void* fn) { SkDELETE(static_cast<Fn*>(fn)); },
         };
         return vtable;
     }
 
-    void* fFunction;        // Either a function pointer, or a pointer to a functor.
+    // Used when fFunction _is_ a functor of type Fn, not a pointer to the functor.
+    template <typename Fn>
+    static const VTable& GetInlineVTable() {
+        static const VTable vtable = {
+            [](void* fn, Args... args) {
+                union { void* p; Fn f; } pun = { fn };
+                return pun.f(args...);
+            },
+            [](void* fn) {
+                union { void* p; Fn f; } pun = { fn };
+                pun.f.~Fn();
+                (void)(pun.f);   // Otherwise, when ~Fn() is trivial, MSVC complains pun is unused.
+            }
+        };
+        return vtable;
+    }
+
+
+    void* fFunction;        // A function pointer, a pointer to a functor, or an inlined functor.
     const VTable& fVTable;  // How to call, delete (and one day copy, move) fFunction.
 };
 
 // TODO:
 //   - is it worth moving fCall out of the VTable into SkFunction itself to avoid the indirection?
 //   - should constructors be implicit?
 //   - make SkFunction copyable
 //   - emulate std::forward for moveable functors (e.g. lambdas)
 //   - forward args too?
-//   - implement small-object optimization to store functors inline
 
 #endif//SkFunction_DEFINED