Make SkSmallAllocator obey the RAII invariants and be expandable

src/core/SkSmallAllocator.h

Index: src/core/SkSmallAllocator.h
diff --git a/src/core/SkSmallAllocator.h b/src/core/SkSmallAllocator.h
index 13b1505821ab588f99a8a28fe21fc0ddd9a16cb9..efe0bc7b4026de198d3b3c258188e482f2395bcd 100644
--- a/src/core/SkSmallAllocator.h
+++ b/src/core/SkSmallAllocator.h
@@ -8,132 +8,145 @@
 #ifndef SkSmallAllocator_DEFINED
 #define SkSmallAllocator_DEFINED
 
-#include "SkTDArray.h"
+#include "SkTArray.h"
 #include "SkTypes.h"
 
-#include <new>
+#include <functional>
+#include <type_traits>
 #include <utility>
 
+
+// max_align_t is needed to calculate the alignment for createWithIniterT when the T used is an
+// abstract type. The complication with max_align_t is that it is defined differently for
+// different builds.
+namespace {
+#if defined(SK_BUILD_FOR_WIN32) || defined(SK_BUILD_FOR_MAC)
+    // Use std::max_align_t for compiles that follow the standard.
+    #include <cstddef>
+    using SystemAlignment = std::max_align_t;
+#else
+    // Ubuntu compiles don't have std::max_align_t defined, but MSVC does not define max_align_t.
+    #include <stddef.h>
+    using SystemAlignment = max_align_t;
+#endif
+}
+
 /*
  *  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;
-        }
-        const size_t storageRemaining = sizeof(fStorage) - fStorageUsed;
-        Rec* rec = &fRecs[fNumObjects];
-        if (storageRequired > storageRemaining) {
-            // Allocate on the heap. Ideally we want to avoid this situation.
+    template <typename Initer>
+    auto createWithIniter(size_t size, Initer initer) -> decltype(initer(nullptr)) {
+        using ObjType = typename std::remove_pointer<decltype(initer(nullptr))>::type;
+        SkASSERT(size >= sizeof(ObjType));
 
-            // 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);
-        } else {
-            // There is space in fStorage.
-            rec->fStorageSize = storageRequired;
-            rec->fHeapStorage = nullptr;
-            rec->fObj = static_cast<void*>(fStorage + fStorageUsed);
-            fStorageUsed += storageRequired;
+        void* storage = this->reserve(size, DefaultDestructor<ObjType>);
+        auto candidate = initer(storage);
+        if (!candidate) {
+            // Initializing didn't workout so free the memory.
+            this->freeLast();
         }
-        rec->fKillProc = DestroyT<T>;
-        fNumObjects++;
-        return rec->fObj;
+
+        return candidate;
     }
 
     /*
-     *  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.
+     * Free the last object allocated and call its destructor. This can be called multiple times
+     * removing objects from the pool in reverse order.
      */
-    void freeLast() {
-        SkASSERT(fNumObjects > 0);
-        Rec* rec = &fRecs[fNumObjects - 1];
-        sk_free(rec->fHeapStorage);
-        fStorageUsed -= rec->fStorageSize;
-
-        fNumObjects--;
+    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
-        void*  fObj;
-        void*  fHeapStorage;
-        void   (*fKillProc)(void*);
+        char*      fObj;
+        Destructor fDestructor;
     };
 
     // Used to call the destructor for allocated objects.
     template<typename T>
-    static void DestroyT(void* ptr) {
+    static void DefaultDestructor(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];
+    static constexpr size_t kAlignment = alignof(SystemAlignment);
+
+    static constexpr size_t AlignSize(size_t size) {
+        return (size + kAlignment - 1) & ~(kAlignment - 1);
+    }
+
+    // Reserve storageRequired from fStorage if possible otherwise allocate on the heap.
+    void* reserve(size_t storageRequired, Destructor destructor) {
+        // Make sure that all allocations stay aligned by rounding the storageRequired up to the
+        // aligned value.
+        char* objectStart = fStorageEnd;
+        char* objectEnd = objectStart + AlignSize(storageRequired);
+        Rec& rec = fRecs.push_back();
+        if (objectEnd > &fStorage[kTotalBytes]) {
+            // Allocate on the heap. Ideally we want to avoid this situation.
+            rec.fObj = new char [storageRequired];
+        } else {
+            // There is space in fStorage.
+            rec.fObj = objectStart;
+            fStorageEnd = objectEnd;
+        }
+        rec.fDestructor = destructor;
+        return rec.fObj;
+    }
+
+    void freeLast() {
+        Rec& rec = fRecs.back();
+        if (std::less<char*>()(rec.fObj, fStorage)
+            || !std::less<char*>()(rec.fObj, &fStorage[kTotalBytes])) {
+            delete [] rec.fObj;
+        } else {
+            fStorageEnd = rec.fObj;
+        }
+        fRecs.pop_back();
+    }
+
+    SkSTArray<kExpectedObjects, Rec, true> fRecs;
+    char*                                  fStorageEnd {fStorage};
+    // Since char have an alignment of 1, it should be forced onto an alignment the compiler
+    // expects which is the alignment of std::max_align_t.
+    alignas (kAlignment) char              fStorage[kTotalBytes];
 };
 
 #endif // SkSmallAllocator_DEFINED