b3da29ae58
In general, it is unsafe to speculatively hoist conditionally executed loads
into the preamble. For example, if the shader does:
if (ptr is valid) {
foo(*ptr)
}
we cannot dereference ptr in the preamble without knowing that the pointer is
valid (which may not be determinable, since it might not be uniform).
nir_opt_preamble needs to stop speculating in this case, or otherwise using
preambles can cause faults on legal shaders.
However, some platforms may be able to speculate loads safely. For example,
Apple hardware is able to suppress MMU faults, making speculation safe. This is
controlled global register to control this behaviour, set at boot-time by the
kernel. (macOS suppresses these faults unconditionally, this feature may be
used in their implementation of sparse textures. Currently Linux does not
suppress any faults but this may change later.)
Since nir_opt_preamble should work soundly and optimally on a variety of
platforms, we need to respect the ACCESS flag.
Thanks to the if-else hoisting implemented earlier in the series, this isn't too
terrible of a band-aid on Asahi:
total instructions in shared programs: 1499674 -> 1507699 (0.54%)
instructions in affected programs: 78865 -> 86890 (10.18%)
helped: 0
HURT: 337
Instructions are HURT.
total bytes in shared programs: 10238284 -> 10279308 (0.40%)
bytes in affected programs: 554504 -> 595528 (7.40%)
helped: 3
HURT: 334
Bytes are HURT.
total halfregs in shared programs: 452049 -> 454015 (0.43%)
halfregs in affected programs: 7569 -> 9535 (25.97%)
helped: 7
HURT: 150
Halfregs are HURT.
There are no shader-db changes on ir3 as expected, since ir3 can safely
speculate all instructions in my shader-db.
Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24011>
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