Alyssa Rosenzweig
a90b0743f3
In 97a1bbeaf26 ("agx: Fix discards"), we made our discard lowering very simple,
since we had just discovered the underlying instruction behaviour and needed a
hotfix for misrendering in the wild. Now that we understand the behaviour, we
can do better. There are two potential performance issues with the lowering in
that commit:
1. It generates extra sample_mask instructions. For a shader that has a single
discard_if at root level, it would generate two instructions
sample_mask foo, 0
sample_mask ~0, ~0
rather than a single
sample_mask ~0, ~foo
2. It runs depth/stencil testing/updates at the end of the shader, even when it
could be run immediately after the discard. This might cause pipeline stalls.
The solution is to insert the "trigger testing" sample_mask instruction as soon
after the "discard" instruction as possible, fusing them if they would be next
to each other. There are two cases:
1. The last discard is executed unconditionally. In this case, we can test
immediately after, unconditionally, and fuse together.
2. The last discard is executed conditionally. In this case, we test in the
first unconditional block after the discard. Example shader:
...
loop {
if .. {
loop {
discard_if <-- discard here
...
}
..
}
...
}
<---- we test here
...
store_output
Together this covers all the usual patterns for single-sampled discard. We could
still do better with multisampling, but whatever.
Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23998>
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