Make SkSmallAllocator obey the RAII invariants and be expandable

src/core/SkSmallAllocator.h

 /*
  * Copyright 2014 Google, Inc
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkSmallAllocator_DEFINED
 #define SkSmallAllocator_DEFINED
 
-#include "SkTDArray.h"
+#include "SkTArray.h"
 #include "SkTypes.h"
 
-#include <new>
 #include <utility>
 
 /*
  *  Template class for allocating small objects without additional heap memory
- *  allocations. kMaxObjects is a hard limit on the number of objects that can
- *  be allocated using this class. After that, attempts to create more objects
- *  with this class will assert and return nullptr.
+ *  allocations.
  *
  *  kTotalBytes is the total number of bytes provided for storage for all
  *  objects created by this allocator. If an object to be created is larger
  *  than the storage (minus storage already used), it will be allocated on the
  *  heap. This class's destructor will handle calling the destructor for each
  *  object it allocated and freeing its memory.
- *
- *  Current the class always aligns each allocation to 16-bytes to be safe, but future
- *  may reduce this to only the alignment that is required per alloc.
  */
-template<uint32_t kMaxObjects, size_t kTotalBytes>
+template<uint32_t kExpectedObjects, size_t kTotalBytes>
 class SkSmallAllocator : SkNoncopyable {
 public:
-    SkSmallAllocator()
-    : fStorageUsed(0)
-    , fNumObjects(0)
-    {}
-
     ~SkSmallAllocator() {
         // Destruct in reverse order, in case an earlier object points to a
         // later object.
-        while (fNumObjects > 0) {
-            fNumObjects--;
-            Rec* rec = &fRecs[fNumObjects];
-            rec->fKillProc(rec->fObj);
-            // Safe to do if fObj is in fStorage, since fHeapStorage will
-            // point to nullptr.
-            sk_free(rec->fHeapStorage);
+        while (fRecs.count() > 0) {
+            this->deleteLast();
         }
     }
 
     /*
      *  Create a new object of type T. Its lifetime will be handled by this
      *  SkSmallAllocator.
-     *  Note: If kMaxObjects have been created by this SkSmallAllocator, nullptr
-     *  will be returned.
      */
     template<typename T, typename... Args>
     T* createT(Args&&... args) {
-        void* buf = this->reserveT<T>();
-        if (nullptr == buf) {
-            return nullptr;
-        }
+        void* buf = this->reserve(sizeof(T), DefaultDestructor<T>);
         return new (buf) T(std::forward<Args>(args)...);
     }
 
     /*
-     *  Reserve a specified amount of space (must be enough space for one T).
-     *  The space will be in fStorage if there is room, or on the heap otherwise.
-     *  Either way, this class will call ~T() in its destructor and free the heap
-     *  allocation if necessary.
-     *  Unlike createT(), this method will not call the constructor of T.
+     * Create a new object of size using initer to initialize the memory. The initer function has
+     * the signature T* initer(void* storage). If initer is unable to initialize the memory it
+     * should return nullptr where SkSmallAllocator will free the memory.
      */
-    template<typename T> void* reserveT(size_t storageRequired = sizeof(T)) {
-        SkASSERT(fNumObjects < kMaxObjects);
-        SkASSERT(storageRequired >= sizeof(T));
-        if (kMaxObjects == fNumObjects) {
-            return nullptr;
+    template <typename T, typename Initer>
+    T* createWithIniterT(size_t size, Initer initer) {

[mtklein_C, 2016/11/09 20:47:57]

Can't we infer T?

template <typename Initer>
auto createWithIniter(size_t, Initer initer) -> decltype(initer(nullptr)) {
  ...
}

?

+        SkASSERT(size >= sizeof(T));
+
+        void* storage = this->reserve(size, DefaultDestructor<T>);
+        T* candidate = initer(storage);
+        if (!candidate) {
+            // Initializing didn't workout so free the memory.
+            this->freeLast();
         }
+
+        return candidate;
+    }
+
+    /*
+     * Free the last object allocated and call its destructor. This can be called multiple times
+     * removing objects from the pool in reverse order.
+     */
+    void deleteLast() {
+        SkASSERT(fRecs.count() > 0);
+        Rec& rec = fRecs.back();
+        rec.fDestructor(rec.fObj);
+        this->freeLast();
+    }
+
+private:
+    using Destructor = void(*)(void*);
+    struct Rec {
+        size_t     fStorageSize;  // 0 if allocated on heap
+        char*      fObj;
+        Destructor fDestructor;
+    };
+
+    // Used to call the destructor for allocated objects.
+    template<typename T>
+    static void DefaultDestructor(void* ptr) {
+        static_cast<T*>(ptr)->~T();
+    }
+
+    // Reserve storageRequired from fStorage if possible otherwise allocate on the heap.
+    void* reserve(size_t storageRequired, Destructor destructor) {
         const size_t storageRemaining = sizeof(fStorage) - fStorageUsed;
-        Rec* rec = &fRecs[fNumObjects];
+        Rec& rec = fRecs.push_back();
         if (storageRequired > storageRemaining) {
             // Allocate on the heap. Ideally we want to avoid this situation.
 
             // With the gm composeshader_bitmap2, storage required is 4476
             // and storage remaining is 3392. Increasing the base storage
             // causes google 3 tests to fail.
 
-            rec->fStorageSize = 0;
-            rec->fHeapStorage = sk_malloc_throw(storageRequired);
-            rec->fObj = static_cast<void*>(rec->fHeapStorage);
+            rec.fStorageSize = 0;
+            rec.fObj = new char [storageRequired];
         } else {
             // There is space in fStorage.
-            rec->fStorageSize = storageRequired;
-            rec->fHeapStorage = nullptr;
-            rec->fObj = static_cast<void*>(fStorage + fStorageUsed);
+            rec.fStorageSize = storageRequired;
+            rec.fObj = &fStorage[fStorageUsed];
             fStorageUsed += storageRequired;
         }
-        rec->fKillProc = DestroyT<T>;
-        fNumObjects++;
-        return rec->fObj;
+        rec.fDestructor = destructor;
+        return rec.fObj;
     }
 
-    /*
-     *  Free the memory reserved last without calling the destructor.
-     *  Can be used in a nested way, i.e. after reserving A and B, calling
-     *  freeLast once will free B and calling it again will free A.
-     */
     void freeLast() {
-        SkASSERT(fNumObjects > 0);
-        Rec* rec = &fRecs[fNumObjects - 1];
-        sk_free(rec->fHeapStorage);
-        fStorageUsed -= rec->fStorageSize;
-
-        fNumObjects--;
+        Rec& rec = fRecs.back();
+        if (0 == rec.fStorageSize) {
+            delete [] rec.fObj;
+        }
+        fStorageUsed -= rec.fStorageSize;
+        fRecs.pop_back();
     }
 
-private:
-    struct Rec {
-        size_t fStorageSize;  // 0 if allocated on heap
-        void*  fObj;
-        void*  fHeapStorage;
-        void   (*fKillProc)(void*);
-    };
-
-    // Used to call the destructor for allocated objects.
-    template<typename T>
-    static void DestroyT(void* ptr) {
-        static_cast<T*>(ptr)->~T();
-    }
-
-    alignas(16) char fStorage[kTotalBytes];
-    size_t           fStorageUsed;  // Number of bytes used so far.
-    uint32_t         fNumObjects;
-    Rec              fRecs[kMaxObjects];
+    size_t                                 fStorageUsed {0};  // Number of bytes used so far.
+    SkSTArray<kExpectedObjects, Rec, true> fRecs;
+    char                                   fStorage[kTotalBytes];
 };
 
 #endif // SkSmallAllocator_DEFINED