Rearrange SkRecord with small N in mind

src/core/SkRecord.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 SkRecord_DEFINED
 #define SkRecord_DEFINED
 
 #include "SkRecords.h"
 #include "SkTLogic.h"
 #include "SkTemplates.h"
 #include "SkVarAlloc.h"
 
-// SkRecord (REC-ord) represents a sequence of SkCanvas calls, saved for future use.
+// SkRecord represents a sequence of SkCanvas calls, saved for future use.
 // These future uses may include: replay, optimization, serialization, or combinations of those.
 //
 // Though an enterprising user may find calling alloc(), append(), visit(), and mutate() enough to
 // work with SkRecord, you probably want to look at SkRecorder which presents an SkCanvas interface
 // for creating an SkRecord, and SkRecordDraw which plays an SkRecord back into another SkCanvas.
 //
 // SkRecord often looks like it's compatible with any type T, but really it's compatible with any
 // type T which has a static const SkRecords::Type kType.  That is to say, SkRecord is compatible
 // only with SkRecords::* structs defined in SkRecords.h.  Your compiler will helpfully yell if you
 // get this wrong.
 
 class SkRecord : public SkNVRefCnt<SkRecord> {
     enum {
-        kFirstReserveCount = 64 / sizeof(void*),
+        // TODO: tune these two constants.
+        kInlineRecords      = 4, // Ideally our lower limit on recorded ops per picture.
+        kLgInlineAllocBytes = 8, // 256 bytes inline, then SkVarAlloc mallocs starting at 512 bytes.

[reed1, 2015/04/08 20:34:08]

the const says "8 bytes" but the comment says 256 bytes. I'm confused what space
kLgInlineAllocBytes is in.

[mtklein, 2015/04/08 20:36:39]

It's lg(inlineAllocBytes).

     };
 public:
-    SkRecord() : fCount(0), fReserved(0), fAlloc(8/*start block sizes at 256 bytes*/) {}
+    SkRecord()
+        : fCount(0)
+        , fReserved(kInlineRecords)
+        , fAlloc(kLgInlineAllocBytes+1, fInlineAlloc, sizeof(fInlineAlloc)) {}

[reed1, 2015/04/08 20:34:08]

Why the +1 ?

[mtklein, 2015/04/08 20:36:39]

*2.  We inline 1<<8 bytes, then the SkVarAlloc starts at 1<<9.

     ~SkRecord();
 
     // Returns the number of canvas commands in this SkRecord.
     unsigned count() const { return fCount; }
 
     // Visit the i-th canvas command with a functor matching this interface:
     //   template <typename T>
     //   R operator()(const T& record) { ... }
     // This operator() must be defined for at least all SkRecords::*.
     template <typename R, typename F>
     R visit(unsigned i, F& f) const {
         SkASSERT(i < this->count());
-        return fRecords[i].visit<R>(fTypes[i], f);
+        return fRecords[i].visit<R>(f);
     }
 
     // Mutate the i-th canvas command with a functor matching this interface:
     //   template <typename T>
     //   R operator()(T* record) { ... }
     // This operator() must be defined for at least all SkRecords::*.
     template <typename R, typename F>
     R mutate(unsigned i, F& f) {
         SkASSERT(i < this->count());
-        return fRecords[i].mutate<R>(fTypes[i], f);
+        return fRecords[i].mutate<R>(f);
     }
-    // TODO: It'd be nice to infer R from F for visit and mutate if we ever get std::result_of.
+
+    // TODO: It'd be nice to infer R from F for visit and mutate.
 
     // Allocate contiguous space for count Ts, to be freed when the SkRecord is destroyed.
     // Here T can be any class, not just those from SkRecords.  Throws on failure.
     template <typename T>
     T* alloc(size_t count = 1) {
-        // Bump up to the next pointer width if needed, so all allocations start pointer-aligned.
         return (T*)fAlloc.alloc(sizeof(T) * count, SK_MALLOC_THROW);
     }
 
     // Add a new command of type T to the end of this SkRecord.
     // You are expected to placement new an object of type T onto this pointer.
     template <typename T>
     T* append() {
         if (fCount == fReserved) {
             this->grow();
         }
-        fTypes[fCount] = T::kType;
         return fRecords[fCount++].set(this->allocCommand<T>());
     }
 
     // Replace the i-th command with a new command of type T.
     // You are expected to placement new an object of type T onto this pointer.
     // References to the original command are invalidated.
     template <typename T>
     T* replace(unsigned i) {
         SkASSERT(i < this->count());
 
         Destroyer destroyer;
         this->mutate<void>(i, destroyer);
 
-        fTypes[i] = T::kType;
         return fRecords[i].set(this->allocCommand<T>());
     }
 
     // Replace the i-th command with a new command of type T.
     // You are expected to placement new an object of type T onto this pointer.
     // You must show proof that you've already adopted the existing command.
     template <typename T, typename Existing>
     T* replace(unsigned i, const SkRecords::Adopted<Existing>& proofOfAdoption) {
         SkASSERT(i < this->count());
 
-        SkASSERT(Existing::kType == fTypes[i]);
-        SkASSERT(proofOfAdoption == fRecords[i].ptr<Existing>());
+        SkASSERT(Existing::kType == fRecords[i].fType);
+        SkASSERT(proofOfAdoption == (void*)fRecords[i].fPtr);
 
-        fTypes[i] = T::kType;
         return fRecords[i].set(this->allocCommand<T>());
     }
 
     // Does not return the bytes in any pointers embedded in the Records; callers
     // need to iterate with a visitor to measure those they care for.
     size_t bytesUsed() const;
 
 private:
-    // Implementation notes!
-    //
-    // Logically an SkRecord is structured as an array of pointers into a big chunk of memory where
+    // An SkRecord is structured as an array of pointers into a big chunk of memory where
     // records representing each canvas draw call are stored:
     //
     // fRecords:  [*][*][*]...
     //             |  |  |
     //             |  |  |
     //             |  |  +---------------------------------------+
     //             |  +-----------------+                        |
     //             |                    |                        |
     //             v                    v                        v
     //   fAlloc:  [SkRecords::DrawRect][SkRecords::DrawPosTextH][SkRecords::DrawRect]...
     //
-    // In the scheme above, the pointers in fRecords are void*: they have no type.  The type is not
-    // stored in fAlloc either; we just write raw data there.  But we need that type information.
-    // Here are some options:
-    //   1) use inheritance, virtuals, and vtables to make the fRecords pointers smarter
-    //   2) store the type data manually in fAlloc at the start of each record
-    //   3) store the type data manually somewhere with fRecords
-    //
-    // This code uses approach 3).  The implementation feels very similar to 1), but it's
-    // devirtualized instead of using the language's polymorphism mechanisms.  This lets us work
-    // with the types themselves (as SkRecords::Type), a sort of limited free RTTI; it lets us pay
-    // only 1 byte to store the type instead of a full pointer (4-8 bytes); and it leads to better
-    // decoupling between the SkRecords::* record types and the operations performed on them in
-    // visit() or mutate().  The recorded canvas calls don't have to have any idea about the
-    // operations performed on them.
-    //
-    // We store the types in a parallel fTypes array, mainly so that they can be tightly packed as
-    // single bytes.  This has the side effect of allowing very fast analysis passes over an
-    // SkRecord looking for just patterns of draw commands (or using this as a quick reject
-    // mechanism) though there's admittedly not a very good API exposed publically for this.
-    //
-    // The cost to append a T into this structure is 1 + sizeof(void*) + sizeof(T).
+    // We store the types of each of the pointers alongside the pointer.
+    // The cost to append a T to this structure is 8 + sizeof(T) bytes.
 
     // A mutator that can be used with replace to destroy canvas commands.
     struct Destroyer {
         template <typename T>
         void operator()(T* record) { record->~T(); }
     };
 
-    // Logically the same as SkRecords::Type, but packed into 8 bits.
-    struct Type8 {
-    public:
-        // This intentionally converts implicitly back and forth.
-        Type8(SkRecords::Type type) : fType(type) { SkASSERT(*this == type); }
-        operator SkRecords::Type () { return (SkRecords::Type)fType; }
-
-    private:
-        uint8_t fType;
-    };
-
-    // No point in allocating any more than one of an empty struct.
-    // We could just return NULL but it's sort of confusing to return NULL on success.
     template <typename T>
     SK_WHEN(SkTIsEmpty<T>, T*) allocCommand() {
         static T singleton = {};
         return &singleton;
     }
 
     template <typename T>
     SK_WHEN(!SkTIsEmpty<T>, T*) allocCommand() { return this->alloc<T>(); }
 
-    // Called when we've run out of room to record new commands.
     void grow();
 
-    // 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.
+    // A typed pointer to some bytes in fAlloc.  visit() and mutate() allow polymorphic dispatch.
     struct Record {
-    public:
         // Point this record to its data in fAlloc.  Returns ptr for convenience.
         template <typename T>
         T* set(T* ptr) {
-            fPtr = ptr;
+            fType = T::kType;
+            fPtr  = (uintptr_t)ptr;
             return ptr;
         }
 
-        // Get the data in fAlloc, assuming it's of type T.
-        template <typename T>
-        T* ptr() const { return (T*)fPtr; }
-
-        // Visit this record with functor F (see public API above) assuming the record we're
-        // pointing to has this type.
+        // Visit this record with functor F (see public API above).
         template <typename R, typename F>
-        R visit(Type8 type, F& f) const {
-        #define CASE(T) case SkRecords::T##_Type: return f(*this->ptr<SkRecords::T>());
-            switch(type) { SK_RECORD_TYPES(CASE) }
+        R visit(F& f) const {
+        #define CASE(T) case SkRecords::T##_Type: return f(*(const SkRecords::T*)fPtr);
+            switch(fType) { SK_RECORD_TYPES(CASE) }
         #undef CASE
             SkDEBUGFAIL("Unreachable");
             return R();
         }
 
-        // Mutate this record with functor F (see public API above) assuming the record we're
-        // pointing to has this type.
+        // Mutate this record with functor F (see public API above).
         template <typename R, typename F>
-        R mutate(Type8 type, F& f) {
-        #define CASE(T) case SkRecords::T##_Type: return f(this->ptr<SkRecords::T>());
-            switch(type) { SK_RECORD_TYPES(CASE) }
+        R mutate(F& f) {
+        #define CASE(T) case SkRecords::T##_Type: return f((SkRecords::T*)fPtr);
+            switch(fType) { SK_RECORD_TYPES(CASE) }
         #undef CASE
             SkDEBUGFAIL("Unreachable");
             return R();
         }
 
-    private:
-        void* fPtr;
+        // On 32-bit machines we store type in 4 bytes, followed by a pointer.  Simple.
+        // On 64-bit machines we store a pointer with the type slotted into two top (unused) bytes.
+        // FWIW, SkRecords::Type is tiny.  It can easily fit in one byte.
+        SkRecords::Type fType : sizeof(void*) == 4 ? 32 : 16;
+        uintptr_t       fPtr  : sizeof(void*) == 4 ? 32 : 48;
     };
+    static_assert(sizeof(Record) == 8, "Record not packed right.");
+
+    // fRecords needs to be a data structure that can append fixed length data, and need to
+    // support efficient random access and forward iteration.  (It doesn't need to be contiguous.)
+    unsigned fCount, fReserved;
+    SkAutoSTMalloc<kInlineRecords, Record> fRecords;
 
     // fAlloc needs to be a data structure which can append variable length data in contiguous
     // chunks, returning a stable handle to that data for later retrieval.
-    //
-    // fRecords and fTypes need to be data structures that can append fixed length data, and need to
-    // support efficient random access and forward iteration.  (They don't need to be contiguous.)
-
-    // fCount and fReserved measure both fRecords and fTypes, which always grow in lock step.
-    unsigned fCount;
-    unsigned fReserved;
-    SkAutoTMalloc<Record> fRecords;
-    SkAutoTMalloc<Type8> fTypes;
     SkVarAlloc fAlloc;
-    // Strangely the order of these fields matters.  If the unsigneds don't go first we're 56 bytes.
-    // tomhudson and mtklein have no idea why.
+    char fInlineAlloc[1 << kLgInlineAllocBytes];
 };
-SK_COMPILE_ASSERT(sizeof(SkRecord) <= 56, SkRecordSize);
 
 #endif//SkRecord_DEFINED