1057093 - JS::ubi::Node should expose the buffers held by <img> tags.

Status: NEW

(Core :: DOM: Core & HTML, defect)

Description

[Jim Blandy :jimb]

Because images, compressed but especially uncompressed, are major contributors to pages' memory consumption, it would be especially valuable to teach JS::ubi::Node about img's buffer types, so it could report their sizes to our memory analysis tools.

This depends on bug 1057507, which would let JS::ubi::Node point to nsINode, the DOM node base type. Some mechanism similar to that described in bug 1056992 would allow us to specialize JS::ubi::Node for image elements.

I understand that image elements have a rather complex internal structure:

HTMLImageElement inherits from
  nsImageLoadingContent points to
    imgRequestProxy points to
      ProxyBehavior is implemented by
        RequestBehavior points to
          imgRequest points to either
            mozilla::image::Image, whose subclasses can get an imgStatusTracker
            or imgStatusTracker directly, which points to
              mozilla::image::Image, subclassed by
                mozilla::image::RasterImage which points to
                  mozilla::image::FrameBlender which points to
                    mozilla::image::FrameSequence which points to
                      mozilla::image::FrameDataPair which points to
                        mozilla::image::imgFrame which points to
                          mozilla::gfx::DataSourceSurface whose subclass
                            mozilla::gfx::DataSourceSurfaceCairo points to
                              cairo_surface_t

at which point I got a little bored.

*This entire structure does not need to be represented by distinct JS::ubi::Node specializations.*

In fact, it could all be a single ubi::Node, as long as the size reported includes all this internal structure. If there is sharing going on (it wasn't clear to me from skimming the types), that shared data should get its own ubi::Node specialization, which its users have edges to.