[PATCH] D29454: [DAGCombine] RFC: Recognise any_extend_vector_inreg and truncation style shuffle masks
Simon Pilgrim via Phabricator via llvm-commits
llvm-commits at lists.llvm.org
Thu Feb 2 07:38:27 PST 2017
RKSimon created this revision.
During legalization we are often creating shuffles (via a build_vector scalarization stage) that are "any_extend_vector_inreg" style masks, and also other masks that are the equivalent of "truncate_vector_inreg" (if we had such a thing).
This RFC is an attempt to match these cases to help undo the effects of just leaving shuffle lowering to handle it - which typically means we lose track of the undefined elements of the shuffles resulting in an unnecessary extension+truncation stage for widened illegal types.
The 2011-10-21-widen-cmp.ll regression can be fixed by making SIGN_EXTEND_VECTOR_IN_REG legal in SSE instead of lowering them to X86ISD::VSEXT (PR31712). An alternative could be to improve target-specific DemandedElts handling at the DAG level (not a small amount of work....).
The regressions encountered in https://reviews.llvm.org/D28537 would be fixed by this.
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