DrawTargetWebgl renders a path by uploading vertex data to the back of
a large VBO using glBufferSubData then issuing a draw call, orphaning
the buffer when it becomes full. This results in many glBufferSubData
calls being interleaved with draw calls. On Mali GPUs this causes
severe performance issues as the driver is unable to determine that
any pending draw calls do not reference the updated region of the
buffer, and therefore must create a copy of the buffer for each
update.
However, since *we* know that we never overwrite a region that is
referenced by a submitted draw call, we can force the driver to avoid
making these copies. We do so by adding a new function
UnsynchronizedBufferSubData(), which acts like BufferSubData so long
as this rule is followed. Internally, this uses glMapBufferRange with
GL_MAP_UNSYNCHRONIZED_BIT, allowing the driver to omit the extraneous
copies.
Differential Revision: https://phabricator.services.mozilla.com/D174685
942a3f5367