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On Intel platforms before Gen6, there is no min or max instruction.
Instead, a comparison instruction (*more on this below) and a SEL
instruction are used. Per other IEEE rules, the regular comparison
instruction, CMP, will always return false if either source is NaN. A
sequence like
cmp.l.f0.0(16) null<1>F g30<8,8,1>F g22<8,8,1>F
(+f0.0) sel(16) g8<1>F g30<8,8,1>F g22<8,8,1>F
will generate the wrong result for min if g22 is NaN. The CMP will
return false, and the SEL will pick g22.
To account for this, the hardware has a special comparison instruction
CMPN. This instruction behaves just like CMP, except if the second
source is NaN, it will return true. The intention is to use it for min
and max. This sequence will always generate the correct result:
cmpn.l.f0.0(16) null<1>F g30<8,8,1>F g22<8,8,1>F
(+f0.0) sel(16) g8<1>F g30<8,8,1>F g22<8,8,1>F
The problem is... for whatever reason, we don't emit CMPN. There was
even a comment in lower_minmax that calls out this very issue! The bug
is actually older than the "Fixes" below even implies. That's just when
the comment was added. That we know of, we never observed a failure
until #4254.
If src1 is known to be a number, either because it's not float or it's
an immediate number, use CMP. This allows cmod propagation to still do
its thing. Without this slight optimization, about 8,300 shaders from
shader-db are hurt on Iron Lake.
Fixes the following piglit tests (from piglit!475):
tests/spec/glsl-1.20/execution/fs-nan-builtin-max.shader_test
tests/spec/glsl-1.20/execution/fs-nan-builtin-min.shader_test
tests/spec/glsl-1.20/execution/vs-nan-builtin-max.shader_test
tests/spec/glsl-1.20/execution/vs-nan-builtin-min.shader_test
Closes: #4254
Fixes:
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| .appveyor | ||
| .gitlab/issue_templates | ||
| .gitlab-ci | ||
| bin | ||
| build-support | ||
| ci-expects | ||
| docs | ||
| doxygen | ||
| include | ||
| scons | ||
| src | ||
| subprojects | ||
| .dir-locals.el | ||
| .editorconfig | ||
| .gitignore | ||
| .gitlab-ci.yml | ||
| .mailmap | ||
| .travis.yml | ||
| Android.common.mk | ||
| Android.mk | ||
| appveyor.yml | ||
| CleanSpec.mk | ||
| common.py | ||
| meson.build | ||
| meson_options.txt | ||
| README.rst | ||
| REVIEWERS | ||
| SConstruct | ||
| VERSION | ||
`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 `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.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.