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If there is a single use of fmul, and that single use is fadd, it makes sense to fuse ffma, as we already do. However, if there are multiple uses, fusing may impede code gen. Consider the source fragment: a = fmul(x, y) b = fadd(a, z) c = fmin(a, t) d = fmax(b, c) The fmul has two uses. The current ffma fusing is greedy and will produce the following "optimized" code. a = fmul(x, y) b = ffma(x, y, z) c = fmin(a, t) d = fmax(b, c) Actually, this code is worse! Instead of 1 fmul + 1 fadd, we now have 1 fmul + 1 ffma. In effect, two multiplies (and a fused add) instead of one multiply and an add. Depending on the ISA, that could impede scheduling or increase code size. It can also increase register pressure, extending the live range. It's tempting to gate on is_used_once, but that would hurt in cases where we really do fuse everything, e.g.: a = fmul(x, y) b = fadd(a, z) c = fadd(a, t) For ISAs that fuse ffma, we expect that 2 ffma is faster than 1 fmul + 2 fadd. So what we really want is to fuse ffma iff the fmul will get deleted. That occurs iff all uses of the fmul are fadd and will themselves get fused to ffma, leaving fmul to get dead code eliminated. That's easy to implement with a new NIR search helper, checking that all uses are fadd. shader-db results on Mali-G57 [open shader-db + subset of closed]: total instructions in shared programs: 179491 -> 178991 (-0.28%) instructions in affected programs: 36862 -> 36362 (-1.36%) helped: 190 HURT: 27 total cycles in shared programs: 10573.20 -> 10571.75 (-0.01%) cycles in affected programs: 72.02 -> 70.56 (-2.02%) helped: 28 HURT: 1 total fma in shared programs: 1590.47 -> 1582.61 (-0.49%) fma in affected programs: 319.95 -> 312.09 (-2.46%) helped: 194 HURT: 1 total cvt in shared programs: 812.98 -> 813.03 (<.01%) cvt in affected programs: 118.53 -> 118.58 (0.04%) helped: 65 HURT: 81 total quadwords in shared programs: 98968 -> 98840 (-0.13%) quadwords in affected programs: 2960 -> 2832 (-4.32%) helped: 20 HURT: 4 total threads in shared programs: 4693 -> 4697 (0.09%) threads in affected programs: 4 -> 8 (100.00%) helped: 4 HURT: 0 v2: Update trace checksums for virgl due to numerical differences. Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com> Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18814> |
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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.