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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 371c4b3c ("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: 7afa26d4 ("nir: Add lowering for nir_op_bitfield_reverse.") (cherry picked from commit d3fd1c76)
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