This prevents nir_lower_io_to_temporaries from emitting new writes to the
old globals that we meant to have disappear through
DCE/remove_unused_variables. If you don't do this, then unless you call
nir_opt_undef() and it successfully catches io_to_temps' new writes of
undefs to the scalar components, the scalar vars will stick around and
have stores that conflict with the real vector vars.
This hasn't been a problem for the end result of codegen because
nir_opt_undef() did succeed. However, things went south with vars_to_ssa
mediump lowering, which obscured the result from opt_undef. And, it's
really mind-bending to see undef writes to the outputs for a chunk of the
shader compiler pipeline anyway.
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18218>
Emma Anholt
019ca611fa