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This avoids some performance regressions on Gen12 platforms caused by SIMD32 fragment shaders reported in titles like Dota2, TF2, Xonotic, and GFXBench5 Car Chase and Aztec Ruins. The most obvious pattern in the regressing shaders I identified among these workloads is that they all had non-uniform discard statements, which are handled rather optimistically by the current IR analysis pass: No penalty is currently applied to the SIMD32 variant of the shader in the form of differing branching weights like we do for other control flow instructions in order to account for the greater likelihood of divergence of a SIMD32 shader. Simply changing that by giving the same treatment to discard statements as we give to other branching instructions seemed to hurt more than it helped on platforms earlier than Gen12, since it reversed most of the improvement obtained from SIMD32 fragment shaders in Manhattan for no measurable benefit in other workloads (Manhattan has a handful of shaders with statically non-uniform discard statements which actually perform better in SIMD32 mode due to their approximate dynamic uniformity). For that reason this change is applied to Gen12+ platforms only. I've been running a number of tests trying to understand the difference in behavior between Gen12 and earlier platforms, and most of the evidence I've gathered seems to point at EU fusion being the culprit: Unlike previous generations, on Gen12 EUs are arranged in pairs which execute instructions in lockstep, giving an effective warp size of 64 threads in SIMD32 mode, which seems to increase the likelihood for control flow divergence in some of the affected shaders significantly. Fixes: |
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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.html <https://mesa3d.org/install.html>`_), but the recommended way is to use Meson (`docs/meson.html <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 `Freenode's #dri-devel <irc://chat.freenode.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.html <https://mesa3d.org/bugs.html>`_). Contributing ------------ Contributions are welcome, and step-by-step instructions can be found in our documentation (`docs/submittingpatches.html <https://mesa3d.org/submittingpatches.html>`_). Note that Mesa uses gitlab for patches submission, review and discussions.