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The usual linear-scan register allocation algorithm can't handle
preallocated registers, since we might be forced to choose a color for
a non-preallocated variable that overlaps with a pre-allocated variable.
But in such cases we can simply split the live range of the offending
variable when we reach the beginning of the pre-allocated variable's
live range. This is still optimal in the sense that it always finds a
coloring whenever one is possible, but we may not insert the smallest
possible number of moves. However, since it's actually the scheduler
which splits live ranges afterwards, we can simply fold in the move
while keeping its fake dependencies, and then everything still works! In
other words, inserting a live range split for a value register during
register allocation is pretty much free.
This means that we can split register allocation in two. First globally
allocate the cross-block registers accessed through load_reg and
store_reg instructions, which is still done via graph coloring, and then
run a linear scan algorithm over each block, treating the load_reg and
store_reg nodes as referring to pre-allocated registers. This makes the
existing RA more complicated, but it has two benefits: first, using
round-robin with the linear scan allocator results in much fewer fake
dependencies, resulting in around 15 less instructions in the glmark2
jellyfish shader and fixing a regression in instruction count since
branching support went in. Second, it will simplify handling spilling.
With just graph coloring for everything, every time we spill a node, we
have to create new value registers which become new nodes in the graph
and re-run RA. This is worsened by the fact that when writing a value to
a temporary, we need to have an extra register available to load the
write address with a load_const node. With the new scheme, we can ignore
this entirely in the first part and then in the second part we can just
reserve an extra register in sections where we know we have to spill. So
no re-running RA many times, and we can get a good result quickly.
The current implementation does linear scan backwards, so that we can
insert the fake dependencies while allocating and avoid creating any
move nodes at all when we have to split a live range. However, it turns
out that this makes handling schedule_first nodes a bit more
complicated, so it's not clear if that was worth it.
Note:
The commit was originally authored by Connor Abbott <cwabbott@gmail.com>
and was cherry-picked from <mesa/mesa!2315>.
Rebasing was necessary due to changes to BITSET_FOREACH_SET,
see
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`Mesa <https://mesa3d.org>`_ - The 3D Graphics Library ====================================================== Source ------ This repository lives at https://gitlab.freedesktop.org/mesa/mesa. Other repositories are likely forks, and code found there is not supported. Build & install --------------- You can find more information in our documentation (`docs/install.rst <https://mesa3d.org/install.html>`_), but the recommended way is to use Meson (`docs/meson.rst <https://mesa3d.org/meson.html>`_): .. code-block:: sh $ mkdir build $ cd build $ meson .. $ sudo ninja install Support ------- Many Mesa devs hang on IRC; if you're not sure which channel is appropriate, you should ask your question on `OFTC's #dri-devel <irc://irc.oftc.net/dri-devel>`_, someone will redirect you if necessary. Remember that not everyone is in the same timezone as you, so it might take a while before someone qualified sees your question. To figure out who you're talking to, or which nick to ping for your question, check out `Who's Who on IRC <https://dri.freedesktop.org/wiki/WhosWho/>`_. The next best option is to ask your question in an email to the mailing lists: `mesa-dev\@lists.freedesktop.org <https://lists.freedesktop.org/mailman/listinfo/mesa-dev>`_ Bug reports ----------- If you think something isn't working properly, please file a bug report (`docs/bugs.rst <https://mesa3d.org/bugs.html>`_). Contributing ------------ Contributions are welcome, and step-by-step instructions can be found in our documentation (`docs/submittingpatches.rst <https://mesa3d.org/submittingpatches.html>`_). Note that Mesa uses gitlab for patches submission, review and discussions.