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Note the writemask handling is chosen for consistency with the rest of NIR. In every other instance, writemask=w requires a vec4 source. This is hardcoded into nir_validate and nir_print as what it means to have a writemask. More importantly, consistency with how register writemasks currently work. nir_print hides it, but r0.w = fneg ssa_1.x is actually a vec4 instruction with source ssa_1.xxxx. As a silly example nir_dest_num_components(that) = 4 in the old model. I realize this is quite strange coming from a scalar ISA, but it's perfectly natural for the class of vec4 hardware for which this was designed. In that hardware, conceptually all instructions are vec4`, so the sequence "fneg ssa_1 and write to channel w" is implemented as "fneg a vec4 with ssa_1.x in the last component and write that vec4 out but mask to write only the w channel". Isn't this inefficient? It can be. To save power, Midgard has scalar ALUs in addition to vec4 ALUs. Those details are confined to the backend VLIW scheduler; the instruction selection is still done as vec4. This mechanism has little in common with AMD's SALUs. Midgard has a wave size of 1, with special hacks for derivatives. As a result, all backends consuming register writemasks are expecting this pattern of code. Changing the store to take a vec1 instead of a vec4 would require changing every backend to reswizzle the sources to resurrect the vec4. I started typing a branch to do this yesterday, but it made a mess of both Midgard and nir-to-tgsi. Without any good reason to think it'd actually help performance, I abandoned the idea. Getting all 15 backends converted to the helpers is enough of a challenge without forcing 10 backends to reswizzle their sources too. Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io> Reviewed-by: Faith Ekstrand <faith.ekstrand@collabora.com> Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23089> |
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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.