[PATCH] D119075: [DAGCombine][ARM] Custom lower smaller-than-legal MULH/AVG/ABD

[Eli Friedman via Phabricator via llvm-commits]

efriedma added inline comments.


================
Comment at: llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp:8775-8777
   // Combine to mulh if mulh is legal/custom for the narrow type on the target.
-  if (!TLI.isOperationLegalOrCustom(MulhOpcode, NarrowVT))
+  if (!TLI.isOperationLegal(MulhOpcode, NarrowVT) &&
+      !TLI.isOperationCustom(MulhOpcode, NarrowVT))
----------------
I think the predicate you actually need here is something like `!TLI.isOperationLegalOrCustom(MulhOpcode, NarrowVT) && (LegalTypes || !TLI.isOperationCustom(MulhOpcode, NarrowVT))`.  i.e. before type legalization, allow custom lowering.  After type legalization, only allow custom lowering if the type is legal.

Constructing an operator with an illegal type after type legalization is likely to crash the compiler.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D119075/new/

https://reviews.llvm.org/D119075