Previously, we would do a fine-grained visibility check for
prims against the dirty rect stack (after coarse grained
tile visibility), then prepare the primitive, then determine
which command buffer(s) the prim should be added to, based
on which tile(s) the prim affects.
The patch changes this so that the fine-grained visibility
check returns a list of command buffer(s) that the prim
should be added to. This is passed to the prim prepare
step, and then used to directly add prims to the buffers
rather than checking which tiles are affected by the prim.
The motivation for doing this will become apparent in
follow up patches. We want to be able to encode
multiple command buffer commands per-prim, whereas it
was previously only possible to encode primitive
commands. By allowing prim-prepare to write directly
to the command buffers, rather than return a list of
primitive commands, we can write whatever commands
are needed. Future patches will use this to write
segment rect streams, and other information.
A side effect of this is that the `tile_rect` field
in the `PrimitiveVisibility` struct is no longer
required. This reduces the size of `PrimitiveInstance`
from 104 bytes to 88 bytes, which is likely to be
a reasonable performance win on pages that have
high primitive counts.
Differential Revision: https://phabricator.services.mozilla.com/D172081
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https://firefox-source-docs.mozilla.org/contributing/contribution_quickref.html
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https://www.mozilla.org/firefox/channel/desktop/#nightly
Keep in mind that nightly builds, which are used by Firefox developers for
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