This caused a problem on Sandybridge where an open-coded
bitfieldReverse() function could be optimized to a
nir_op_bitfield_reverse that would generate an unsupported BFREV
instruction in the backend. This was encountered in some Unreal4 tech
demos in shader-db. The bug was not previously noticed because we don't
actually try to run those demos on Sandybridge.
The fixes tag is a bit a lie. The actual bug was introduced about
26,000 commits earlier in 371c4b3c48 ("nir: Recognize open-coded
bitfield_reverse."). Without the NIR lowering pass, the flag needed to
avoid the optimization does not exist. Hopefully nobody will care to
fix this on an earlier Mesa release.
Reviewed-by: Matt Turner <mattst88@gmail.com>
Fixes: 7afa26d4e3 ("nir: Add lowering for nir_op_bitfield_reverse.")
d3fd1c761a