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This implements the "tortoise and the hare" algorithm for detecting cycles in graphs. We use the caller's iterator as the hare and our own internal copy as the tortoise. Conveniently, VkBaseOutStructure (and VkBaseInStructure which is identical except the pointer type on pNext) have a pointer we can use for the tortoise and an sType which we can use for a counter to ensure we only increment the tortose every other loop iteration. There are more efficient algorithms than tortoise and hare but they require allocating memory for something like a hash set of seen nodes. Since this for debug purposes only, it's ok for it to be a bit inefficient in the case where it hits the assert. In the usual case of no loops, it's the same runtime efficiency as the unchecked version except that it does a tiny bit of math and 50% more pointer chases. Version 1 worked fine with clang and with GCC 12.1 with -O0 but not with optimizations. See https://gitlab.freedesktop.org/mesa/mesa/-/issues/6895. Unfortunately, the first version required modifying a temporary declared const inside the for loop and that seems to have been the problem. This version instead has an iterator struct which is managed by an outer for loop and the inner for loop exists to declare the user's requested iteration variable and manage the actual iteration. Because the outer for loop is effectively `for (bool done = false; !done; done = true)`, it will execute exactly once, regardless of the inner loop, so break and continue inside the inner loop should work the same as if it's a single for loop. The other major difference with the new version is that the code is the same for debug and release except the half_iter and loop check are gone. I've verified by hand that this produces virtually identical code to the old simple iterators on both GCC andl clang with an optimized build. Reviewed-by: Jesse Natalie <jenatali@microsoft.com> Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com> Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17630> |
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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.