In a NIR generated using SPIR-V initializers to variables, copy
propagation can end up transforming
vec1 32 ssa_33 = deref_var &@1 (shared mat2x4)
vec1 32 ssa_35 = mov ssa_33
vec1 32 ssa_7 = deref_cast (mat2x4 *)ssa_35 (shared mat2x4) /* ptr_stride=0 */
into
vec1 32 ssa_33 = deref_var &@1 (shared mat2x4)
vec1 32 ssa_7 = deref_cast (mat2x4 *)ssa_33 (shared mat2x4) /* ptr_stride=0 */
Before the optimization, the "head" of a path of deref that uses ssa_7
will be the cast. After, it will be the variable in ssa_33. Since
the types are the same, this is a trivial cast that would be picked up
by nir_opt_deref.
If we need to compare such deref-chain after optimization with another
deref-chain for the same variable, the compare function will get
confused by the cast in the middle.
One alternative would be to add nir_opt_deref to places that compare
derefs, but that might not scale well, so skip the trivial casts when
generating the paths instead.
Motivated by the discussion in
https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3047#note_383660.
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3420>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3420>
Caio Marcelo de Oliveira Filho
8548fe19f0