Partially restore small-T optimization for very small (empty) T.

src/record/SkRecord.h

Index: src/record/SkRecord.h
diff --git a/src/record/SkRecord.h b/src/record/SkRecord.h
index b1e4ae24f8da69b5e60aecad189573dbd448c9fd..5d5c122c3ab8ee40b4c6a80c13b309c1b7e97bc5 100644
--- a/src/record/SkRecord.h
+++ b/src/record/SkRecord.h
@@ -77,7 +77,7 @@ public:
         }
 
         fTypes[fCount] = T::kType;
-        return fRecords[fCount++].set(this->alloc<T>());
+        return fRecords[fCount++].set(this->allocCommand<T>());
     }
 
     // Replace the i-th command with a new command of type T.
@@ -91,7 +91,7 @@ public:
         this->mutate(i, destroyer);
 
         fTypes[i] = T::kType;
-        return fRecords[i].set(this->alloc<T>());
+        return fRecords[i].set(this->allocCommand<T>());
     }
 
     // Replace the i-th command with a new command of type T.
@@ -105,7 +105,7 @@ public:
         SkASSERT(proofOfAdoption == fRecords[i].ptr<Existing>());
 
         fTypes[i] = T::kType;
-        return fRecords[i].set(this->alloc<T>());
+        return fRecords[i].set(this->allocCommand<T>());
     }
 
 private:
@@ -162,6 +162,16 @@ private:
         uint8_t fType;
     };
 
+    template <typename T>
+    T* allocCommand() {
+        struct AddChar : public T { char unused; };
+        if (sizeof(AddChar) == sizeof(char)) {
+            // T is an empty struct.
+            return NULL;
+        }
+        return this->alloc<T>();
+    }

[bungeman-skia, 2014/05/07 14:35:24]

I understand what you're doing here, but eck. Can we just make an IsEmpty trait?

SkTLogic.h:
class SkTIsEmpty<T> {
    struct test : public T { char unused; };
public:
    static const bool value = sizeof(test) == sizeof(char);
};

Here:
template <typename T> SK_WHEN<SkTIsEmpty<T>, T*> allocCommand() {
    return NULL;
}
template <typename T> SK_WHEN<!SkTIsEmpty<T>, T*> allocCommand() {
    return this->alloc<T>();
}

You're already writing/using a template DSL here, this seems more in keeping
with the rest of the implementation.

Come to think of it, we should probably put out traits in a namespace or
something. And move SkTIsConst there also.

[mtklein, 2014/05/07 15:27:43]

Done.  Leaving rearranging / renaming the traits for later.

+
     // An untyped pointer to some bytes in fAlloc.  This is the interface for polymorphic dispatch:
     // visit() and mutate() work with the parallel fTypes array to do the work of a vtable.
     struct Record {