Implicit uses of non-register value types places impossible to satisfy
constraints on the legalizer / artifact combiner. These prevent
writing sensible legalize rules for the artifacts without triggering
infinite loops in the legalizer.
The verifier really needs to enforce this, but I'm not sure what the
exact conditions would look like yet.
G_BITCAST can be lowered with a pair of G_UNMERGE_VALUES and
G_MERGE_VALUES with different types, but G_UNMERGE_VALUES of a vector
can also be implemented with a bitcast to a scalar, which introduces
the possibility for infinite loops. Try to eliminate an illegal source
register type in the artifact combiner to avoid this from happening.
Avoids infinite looping in the legalizer in a future patch which
allows lowering G_UNMERGE_VALUES of a vector source with a G_BITCAST.
Summary:
Legalization can introduce the trunc(trunc) pattern. This can cause
problems if one of these intermediate truncs is not legal.
Combine truncs of this pattern, if the resulting trunc is legal.
Reviewers: arsenm, aemerson, dsanders
Reviewed By: arsenm
Subscribers: jvesely, wdng, nhaehnle, rovka, kerbowa, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D76601
Matt Arsenault