cc: Audit and static_assert the types in PaintOpBuffer can be moved.

cc/paint/paint_op_buffer.h

Index: cc/paint/paint_op_buffer.h
diff --git a/cc/paint/paint_op_buffer.h b/cc/paint/paint_op_buffer.h
index e8d31142123089221a75cfde8da6d15b60ccdfef..329b626bdc08d039e758a6a5c57fedbee5b57cc5 100644
--- a/cc/paint/paint_op_buffer.h
+++ b/cc/paint/paint_op_buffer.h
@@ -72,6 +72,117 @@ enum class PaintOpType : uint8_t {
   LastPaintOpType = Translate,
 };
 
+template <typename T>
+struct SafePaintOpData;
+
+// sk_sp<T> pointers behave if their memory address is changed because there
+// is never a pointer back to the sk_sp<T> itself from inside the T. This is
+// true because multiple sk_sp<> may point to the same T and they are
+// transient compared to the lifetime of any T. Additionally, the move
+// constructor doesn't change any state that needs to be reflected in the new
+// object.
+template <typename U>
+struct SafePaintOpData<sk_sp<U>> {
+  using type = sk_sp<U>;
+};
+// scoped_refptr<> is able to be moved in memory for the same reasons as
+// sk_sp<>.
+template <typename U>
+struct SafePaintOpData<scoped_refptr<U>> {
+  using type = scoped_refptr<U>;
+};
+
+// PaintFlags is a collection of trivial data types and sk_sp<> ref pointers.
+// The trivial data types satisfy std::is_trivially_copyable, and sk_sp<> is
+// valid to have its memory address changed for reasons decribed on
+// SafePaintOpData<sk_sp>.
+template <>
+struct SafePaintOpData<PaintFlags> {
+  using type = PaintFlags;
+};
+
+// PaintImage is a set of trivial types and an sk_sp<> which is able to be
+// moved in memory as described on SafePaintOpData<sk_sp>. PaintImage does not
+// hold any pointers back to itself directly or indirectly.
+template <>
+struct SafePaintOpData<PaintImage> {
+  using type = PaintImage;
+};
+
+// ThreadsafePath is an SkPath which contains trivial types, sk_sp<>, and
+// SkAtomic<uint8_t>. sk_sp<> is valid as described on SafePaintOpData<sk_sp>.
+// SkAtomic<uint8_t> stores a trivial type internally, and takes its address
+// but does not store it.
+template <>
+struct SafePaintOpData<ThreadsafePath> {
+  using type = ThreadsafePath;
+};
+
+// These types can be copied trivially.
+// TODO(danakj): These could use std::is_trivially_move_constructible once that
+// is available on all our toolchains.
+template <>
+struct SafePaintOpData<SkRect> {
+  using type = SkRect;
+  static_assert(base::is_trivially_copyable<type>::value, "");
+};
+template <>
+struct SafePaintOpData<SkIRect> {
+  using type = SkIRect;
+  static_assert(base::is_trivially_copyable<type>::value, "");
+};
+template <>
+struct SafePaintOpData<SkRRect> {
+  using type = SkRRect;
+  static_assert(base::is_trivially_copyable<type>::value, "");
+};
+template <>
+struct SafePaintOpData<SkClipOp> {
+  using type = SkClipOp;
+  static_assert(base::is_trivially_copyable<type>::value, "");
+};
+template <>
+struct SafePaintOpData<SkBlendMode> {
+  using type = SkBlendMode;
+  static_assert(base::is_trivially_copyable<type>::value, "");
+};
+template <>
+struct SafePaintOpData<SkColor> {
+  using type = SkColor;
+  static_assert(base::is_trivially_copyable<type>::value, "");
+};
+template <>
+struct SafePaintOpData<PaintCanvas::AnnotationType> {
+  using type = PaintCanvas::AnnotationType;
+  static_assert(base::is_trivially_copyable<type>::value, "");
+};
+template <>
+struct SafePaintOpData<PaintCanvas::SrcRectConstraint> {
+  using type = PaintCanvas::SrcRectConstraint;
+  static_assert(base::is_trivially_copyable<type>::value, "");
+};
+template <>
+struct SafePaintOpData<ThreadsafeMatrix> {
+  using type = ThreadsafeMatrix;
+  static_assert(base::is_trivially_copyable<type>::value, "");
+};
+
+// It is a requirement for every field in a PaintOp that we can memcpy it to
+// another place in memory and free the only one (without calling any
+// constructors or destructors). std::is_trivially_copyable<> would guarantee
+// this, but is a superset of the guarantees that we need. For instance sk_sp<>
+// works correctly when we memcpy+free it, but it has a non-trivial move
+// constructor and destructor, which would make it fail a check for
+// std::is_trivially_copyable. std::is_trivially_move_constructible<> also
+// provides enough as we are moving the objects from one address to another
+// conceptually, but also is not satisfied by sk_sp<> since it modifies the
+// moved-from type.
+// For non-primitive fields of type T in a PaintOp, declare them as a
+// SafePaintOpData<T>::type, which provides us with an explanation of safety
+// for each type.
+// NOTABLY std::vector does *not* work if its memory address is changed, as
+// implementations can (and do) sometimes have a pointer back to the vector from
+// its internals, so running its move-assignment operator is required.
 struct CC_PAINT_EXPORT PaintOp {
   uint32_t type : 8;
   uint32_t skip : 24;
@@ -124,9 +235,13 @@ struct CC_PAINT_EXPORT PaintOpWithFlags : PaintOp {
   // provide a modified PaintFlags. The RasterWithFlags() method is static with
   // a const PaintOpWithFlags* parameter so that it can be used as a function
   // pointer.
-  PaintFlags flags;
+  SafePaintOpData<PaintFlags>::type flags;
 };
 
+// A PaintOp with an additional block of memory with variable size. The
+// additional data held in the PaintOpWithData must be able to be moved in
+// memory without consequences. As such it must not (directly or indirectly)
+// contain any pointers back to itself or to the PaintOp.
 struct CC_PAINT_EXPORT PaintOpWithData : PaintOpWithFlags {
   // Having data is just a helper for ops that have a varying amount of data and
   // want a way to store that inline.  This is for ops that pass in a
@@ -168,8 +283,16 @@ struct CC_PAINT_EXPORT PaintOpWithArrayBase : PaintOpWithFlags {
       : PaintOpWithFlags(flags) {}
 };
 
+// A PaintOp with an additional block of memory with variable size that is
+// an array of a known type. The types held in the array must be able to be
+// moved in memory without consequences. As such it must not (directly or
+// indirectly) contain any pointers back to itself or to the PaintOp. We use
+// std::is_trivially_copyable() to verify this (it is more strict but satisfies
+// our needs).
 template <typename M>
 struct CC_PAINT_EXPORT PaintOpWithArray : PaintOpWithArrayBase {
+  static_assert(base::is_trivially_copyable<M>::value, "");
+
   // Paint op that has a M[count] and a char[bytes].
   // Array data is stored first so that it can be aligned with T's alignment
   // with the arbitrary unaligned char data after it.
@@ -239,9 +362,9 @@ struct CC_PAINT_EXPORT AnnotateOp final : PaintOp {
                      SkCanvas* canvas,
                      const SkMatrix& original_ctm);
 
-  PaintCanvas::AnnotationType annotation_type;
-  SkRect rect;
-  sk_sp<SkData> data;
+  SafePaintOpData<PaintCanvas::AnnotationType>::type annotation_type;
+  SafePaintOpData<SkRect>::type rect;
+  SafePaintOpData<sk_sp<SkData>>::type data;
 };
 
 struct CC_PAINT_EXPORT ClipPathOp final : PaintOp {
@@ -253,8 +376,8 @@ struct CC_PAINT_EXPORT ClipPathOp final : PaintOp {
                      const SkMatrix& original_ctm);
   int CountSlowPaths() const;
 
-  ThreadsafePath path;
-  SkClipOp op;
+  SafePaintOpData<ThreadsafePath>::type path;
+  SafePaintOpData<SkClipOp>::type op;
   bool antialias;
 };
 
@@ -266,8 +389,8 @@ struct CC_PAINT_EXPORT ClipRectOp final : PaintOp {
                      SkCanvas* canvas,
                      const SkMatrix& original_ctm);
 
-  SkRect rect;
-  SkClipOp op;
+  SafePaintOpData<SkRect>::type rect;
+  SafePaintOpData<SkClipOp>::type op;
   bool antialias;
 };
 
@@ -279,8 +402,8 @@ struct CC_PAINT_EXPORT ClipRRectOp final : PaintOp {
                      SkCanvas* canvas,
                      const SkMatrix& original_ctm);
 
-  SkRRect rrect;
-  SkClipOp op;
+  SafePaintOpData<SkRRect>::type rrect;
+  SafePaintOpData<SkClipOp>::type op;
   bool antialias;
 };
 
@@ -291,15 +414,15 @@ struct CC_PAINT_EXPORT ConcatOp final : PaintOp {
                      SkCanvas* canvas,
                      const SkMatrix& original_ctm);
 
-  ThreadsafeMatrix matrix;
+  SafePaintOpData<ThreadsafeMatrix>::type matrix;
 };
 
 struct CC_PAINT_EXPORT DrawArcOp final : PaintOpWithFlags {
   static constexpr PaintOpType kType = PaintOpType::DrawArc;
   static constexpr bool kIsDrawOp = true;
   DrawArcOp(const SkRect& oval,
-            SkScalar start_angle,
-            SkScalar sweep_angle,
+            float start_angle,
+            float sweep_angle,
             bool use_center,
             const PaintFlags& flags)
       : PaintOpWithFlags(flags),
@@ -318,19 +441,16 @@ struct CC_PAINT_EXPORT DrawArcOp final : PaintOpWithFlags {
                               SkCanvas* canvas,
                               const SkMatrix& original_ctm);
 
-  SkRect oval;
-  SkScalar start_angle;
-  SkScalar sweep_angle;
+  SafePaintOpData<SkRect>::type oval;
+  float start_angle;
+  float sweep_angle;
   bool use_center;
 };
 
 struct CC_PAINT_EXPORT DrawCircleOp final : PaintOpWithFlags {
   static constexpr PaintOpType kType = PaintOpType::DrawCircle;
   static constexpr bool kIsDrawOp = true;
-  DrawCircleOp(SkScalar cx,
-               SkScalar cy,
-               SkScalar radius,
-               const PaintFlags& flags)
+  DrawCircleOp(float cx, float cy, float radius, const PaintFlags& flags)
       : PaintOpWithFlags(flags), cx(cx), cy(cy), radius(radius) {}
   static void Raster(const PaintOp* op,
                      SkCanvas* canvas,
@@ -343,9 +463,9 @@ struct CC_PAINT_EXPORT DrawCircleOp final : PaintOpWithFlags {
                               SkCanvas* canvas,
                               const SkMatrix& original_ctm);
 
-  SkScalar cx;
-  SkScalar cy;
-  SkScalar radius;
+  float cx;
+  float cy;
+  float radius;
 };
 
 struct CC_PAINT_EXPORT DrawColorOp final : PaintOp {
@@ -356,8 +476,8 @@ struct CC_PAINT_EXPORT DrawColorOp final : PaintOp {
                      SkCanvas* canvas,
                      const SkMatrix& original_ctm);
 
-  SkColor color;
-  SkBlendMode mode;
+  SafePaintOpData<SkColor>::type color;
+  SafePaintOpData<SkBlendMode>::type mode;
 };
 
 struct CC_PAINT_EXPORT DrawDisplayItemListOp final : PaintOp {
@@ -377,7 +497,7 @@ struct CC_PAINT_EXPORT DrawDisplayItemListOp final : PaintOp {
   bool HasDiscardableImages() const;
   int CountSlowPaths() const;
 
-  scoped_refptr<DisplayItemList> list;
+  SafePaintOpData<scoped_refptr<DisplayItemList>>::type list;
 };
 
 struct CC_PAINT_EXPORT DrawDRRectOp final : PaintOpWithFlags {
@@ -398,16 +518,16 @@ struct CC_PAINT_EXPORT DrawDRRectOp final : PaintOpWithFlags {
                               SkCanvas* canvas,
                               const SkMatrix& original_ctm);
 
-  SkRRect outer;
-  SkRRect inner;
+  SafePaintOpData<SkRRect>::type outer;
+  SafePaintOpData<SkRRect>::type inner;
 };
 
 struct CC_PAINT_EXPORT DrawImageOp final : PaintOpWithFlags {
   static constexpr PaintOpType kType = PaintOpType::DrawImage;
   static constexpr bool kIsDrawOp = true;
   DrawImageOp(const PaintImage& image,
-              SkScalar left,
-              SkScalar top,
+              float left,
+              float top,
               const PaintFlags* flags);
   ~DrawImageOp();
   static void Raster(const PaintOp* op,
@@ -422,9 +542,9 @@ struct CC_PAINT_EXPORT DrawImageOp final : PaintOpWithFlags {
                               const SkMatrix& original_ctm);
   bool HasDiscardableImages() const;
 
-  PaintImage image;
-  SkScalar left;
-  SkScalar top;
+  SafePaintOpData<PaintImage>::type image;
+  float left;
+  float top;
 };
 
 struct CC_PAINT_EXPORT DrawImageRectOp final : PaintOpWithFlags {
@@ -448,10 +568,10 @@ struct CC_PAINT_EXPORT DrawImageRectOp final : PaintOpWithFlags {
                               const SkMatrix& original_ctm);
   bool HasDiscardableImages() const;
 
-  PaintImage image;
-  SkRect src;
-  SkRect dst;
-  PaintCanvas::SrcRectConstraint constraint;
+  SafePaintOpData<PaintImage>::type image;
+  SafePaintOpData<SkRect>::type src;
+  SafePaintOpData<SkRect>::type dst;
+  SafePaintOpData<PaintCanvas::SrcRectConstraint>::type constraint;
 };
 
 struct CC_PAINT_EXPORT DrawIRectOp final : PaintOpWithFlags {
@@ -470,17 +590,13 @@ struct CC_PAINT_EXPORT DrawIRectOp final : PaintOpWithFlags {
                               SkCanvas* canvas,
                               const SkMatrix& original_ctm);
 
-  SkIRect rect;
+  SafePaintOpData<SkIRect>::type rect;
 };
 
 struct CC_PAINT_EXPORT DrawLineOp final : PaintOpWithFlags {
   static constexpr PaintOpType kType = PaintOpType::DrawLine;
   static constexpr bool kIsDrawOp = true;
-  DrawLineOp(SkScalar x0,
-             SkScalar y0,
-             SkScalar x1,
-             SkScalar y1,
-             const PaintFlags& flags)
+  DrawLineOp(float x0, float y0, float x1, float y1, const PaintFlags& flags)
       : PaintOpWithFlags(flags), x0(x0), y0(y0), x1(x1), y1(y1) {}
   static void Raster(const PaintOp* op,
                      SkCanvas* canvas,
@@ -495,10 +611,10 @@ struct CC_PAINT_EXPORT DrawLineOp final : PaintOpWithFlags {
 
   int CountSlowPaths() const;
 
-  SkScalar x0;
-  SkScalar y0;
-  SkScalar x1;
-  SkScalar y1;
+  float x0;
+  float y0;
+  float x1;
+  float y1;
 };
 
 struct CC_PAINT_EXPORT DrawOvalOp final : PaintOpWithFlags {
@@ -517,7 +633,7 @@ struct CC_PAINT_EXPORT DrawOvalOp final : PaintOpWithFlags {
                               SkCanvas* canvas,
                               const SkMatrix& original_ctm);
 
-  SkRect oval;
+  SafePaintOpData<SkRect>::type oval;
 };
 
 struct CC_PAINT_EXPORT DrawPathOp final : PaintOpWithFlags {
@@ -537,7 +653,7 @@ struct CC_PAINT_EXPORT DrawPathOp final : PaintOpWithFlags {
                               const SkMatrix& original_ctm);
   int CountSlowPaths() const;
 
-  ThreadsafePath path;
+  SafePaintOpData<ThreadsafePath>::type path;
 };
 
 struct CC_PAINT_EXPORT DrawPosTextOp final : PaintOpWithArray<SkPoint> {
@@ -574,7 +690,7 @@ struct CC_PAINT_EXPORT DrawRecordOp final : PaintOp {
   bool HasDiscardableImages() const;
   int CountSlowPaths() const;
 
-  sk_sp<const PaintRecord> record;
+  SafePaintOpData<sk_sp<const PaintRecord>>::type record;
 };
 
 struct CC_PAINT_EXPORT DrawRectOp final : PaintOpWithFlags {
@@ -593,7 +709,7 @@ struct CC_PAINT_EXPORT DrawRectOp final : PaintOpWithFlags {
                               SkCanvas* canvas,
                               const SkMatrix& original_ctm);
 
-  SkRect rect;
+  SafePaintOpData<SkRect>::type rect;
 };
 
 struct CC_PAINT_EXPORT DrawRRectOp final : PaintOpWithFlags {
@@ -612,13 +728,13 @@ struct CC_PAINT_EXPORT DrawRRectOp final : PaintOpWithFlags {
                               SkCanvas* canvas,
                               const SkMatrix& original_ctm);
 
-  SkRRect rrect;
+  SafePaintOpData<SkRRect>::type rrect;
 };
 
 struct CC_PAINT_EXPORT DrawTextOp final : PaintOpWithData {
   static constexpr PaintOpType kType = PaintOpType::DrawText;
   static constexpr bool kIsDrawOp = true;
-  DrawTextOp(size_t bytes, SkScalar x, SkScalar y, const PaintFlags& flags)
+  DrawTextOp(size_t bytes, float x, float y, const PaintFlags& flags)
       : PaintOpWithData(flags, bytes), x(x), y(y) {}
   static void Raster(const PaintOp* op,
                      SkCanvas* canvas,
@@ -634,16 +750,16 @@ struct CC_PAINT_EXPORT DrawTextOp final : PaintOpWithData {
   void* GetData() { return GetDataForThis(this); }
   const void* GetData() const { return GetDataForThis(this); }
 
-  SkScalar x;
-  SkScalar y;
+  float x;
+  float y;
 };
 
 struct CC_PAINT_EXPORT DrawTextBlobOp final : PaintOpWithFlags {
   static constexpr PaintOpType kType = PaintOpType::DrawTextBlob;
   static constexpr bool kIsDrawOp = true;
   DrawTextBlobOp(sk_sp<SkTextBlob> blob,
-                 SkScalar x,
-                 SkScalar y,
+                 float x,
+                 float y,
                  const PaintFlags& flags);
   ~DrawTextBlobOp();
   static void Raster(const PaintOp* op,
@@ -657,9 +773,9 @@ struct CC_PAINT_EXPORT DrawTextBlobOp final : PaintOpWithFlags {
                               SkCanvas* canvas,
                               const SkMatrix& original_ctm);
 
-  sk_sp<SkTextBlob> blob;
-  SkScalar x;
-  SkScalar y;
+  SafePaintOpData<sk_sp<SkTextBlob>>::type blob;
+  float x;
+  float y;
 };
 
 struct CC_PAINT_EXPORT NoopOp final : PaintOp {
@@ -678,12 +794,12 @@ struct CC_PAINT_EXPORT RestoreOp final : PaintOp {
 
 struct CC_PAINT_EXPORT RotateOp final : PaintOp {
   static constexpr PaintOpType kType = PaintOpType::Rotate;
-  explicit RotateOp(SkScalar degrees) : degrees(degrees) {}
+  explicit RotateOp(float degrees) : degrees(degrees) {}
   static void Raster(const PaintOp* op,
                      SkCanvas* canvas,
                      const SkMatrix& original_ctm);
 
-  SkScalar degrees;
+  float degrees;

[enne (OOO), 2017/06/01 16:04:58]

If you think we should get rid of SkScalar types, I think there's a larger
change to be made elsewhere in other APIs too.  Changing it only here seems
inconsistent, because we go SkScalar -> float -> SkScalar again.  Sure, it's
"really" a float all the way through, but still.

I'd weakly like to leave this as-is in this patch, but could be convinced
otherwise.

[danakj, 2017/06/01 18:01:17]

I thought it made sense to use float here because it's a known primitive and it
is actually a float. I could use SafePaintOpData<SkScalar> but that seemed meh
to me. I'd like to make all our APIs use float instead, but this seemed like a
good start.

[enne (OOO), 2017/06/01 18:07:33]

I didn't see anything wrong with SafePaintOpData<SkScalar> (especially since
then it's clear that everything goes through SafePaintOpData).  Do you think we
shouldn't do SafePaintOpData<float> for consistency?

 };
 
 struct CC_PAINT_EXPORT SaveOp final : PaintOp {
@@ -709,7 +825,7 @@ struct CC_PAINT_EXPORT SaveLayerOp final : PaintOpWithFlags {
                               SkCanvas* canvas,
                               const SkMatrix& original_ctm);
 
-  SkRect bounds;
+  SafePaintOpData<SkRect>::type bounds;
 };
 
 struct CC_PAINT_EXPORT SaveLayerAlphaOp final : PaintOp {
@@ -720,19 +836,19 @@ struct CC_PAINT_EXPORT SaveLayerAlphaOp final : PaintOp {
                      SkCanvas* canvas,
                      const SkMatrix& original_ctm);
 
-  SkRect bounds;
+  SafePaintOpData<SkRect>::type bounds;
   uint8_t alpha;
 };
 
 struct CC_PAINT_EXPORT ScaleOp final : PaintOp {
   static constexpr PaintOpType kType = PaintOpType::Scale;
-  ScaleOp(SkScalar sx, SkScalar sy) : sx(sx), sy(sy) {}
+  ScaleOp(float sx, float sy) : sx(sx), sy(sy) {}
   static void Raster(const PaintOp* op,
                      SkCanvas* canvas,
                      const SkMatrix& original_ctm);
 
-  SkScalar sx;
-  SkScalar sy;
+  float sx;
+  float sy;
 };
 
 struct CC_PAINT_EXPORT SetMatrixOp final : PaintOp {
@@ -748,18 +864,18 @@ struct CC_PAINT_EXPORT SetMatrixOp final : PaintOp {
                      SkCanvas* canvas,
                      const SkMatrix& original_ctm);
 
-  ThreadsafeMatrix matrix;
+  SafePaintOpData<ThreadsafeMatrix>::type matrix;
 };
 
 struct CC_PAINT_EXPORT TranslateOp final : PaintOp {
   static constexpr PaintOpType kType = PaintOpType::Translate;
-  TranslateOp(SkScalar dx, SkScalar dy) : dx(dx), dy(dy) {}
+  TranslateOp(float dx, float dy) : dx(dx), dy(dy) {}
   static void Raster(const PaintOp* op,
                      SkCanvas* canvas,
                      const SkMatrix& original_ctm);
 
-  SkScalar dx;
-  SkScalar dy;
+  float dx;
+  float dy;
 };
 
 using LargestPaintOp = DrawDRRectOp;