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This has a number of advantages compared to the pass I wrote years ago: - It can easily perform either Global CSE or block-local CSE, without needing to roll any dataflow analysis, thanks to SSA def analysis. This global CSE is able to detect and coalesce memory loads across blocks. Although it may increase spilling a little, the reduction in memory loads seems to more than compensate. - Because SSA guarantees that values are never written more than once, the new CSE pass can directly reuse an existing value. The old pass emitted copies at the point where it discovered a value because it had no idea whether it'd be mutated later. This led it to generate a ton of trash for copy propagation to clean up later, and also a nasty fragility where CSE, register coalescing, and copy propagation could all fight one another by generating and cleaning up copies, leading to infinite optimization loops unless we were really careful. Generating less trash improves our CPU efficiency. - It uses hash tables like nir_instr_set and nir_opt_cse, instead of linearly walking lists and comparing each element. This is much more CPU efficient. - It doesn't use liveness analysis, which is one of the most expensive analysis passes that we have. Def analysis is cheaper. In addition to CSE'ing SSA values, we continue to handle flag writes, as this is a huge source of CSE'able values. These remain block local. However, we can simply track the last flag write, rather than creating entire sets of instruction entries like the old pass. Much simpler. The only real downside to this pass is that, because the backend is currently only partially SSA, it has limited visibility and isn't able to see all values. However, the results appear to be good enough that the new pass can effectively replace the old pass in almost all cases. Reviewed-by: Caio Oliveira <caio.oliveira@intel.com> Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28666> |
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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://docs.mesa3d.org/install.html>`_), but the recommended way is to use Meson (`docs/meson.rst <https://docs.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://docs.mesa3d.org/bugs.html>`_). Contributing ------------ Contributions are welcome, and step-by-step instructions can be found in our documentation (`docs/submittingpatches.rst <https://docs.mesa3d.org/submittingpatches.html>`_). Note that Mesa uses gitlab for patches submission, review and discussions.