[LegalizeVectorTypes] When scalarizing the operand of a unary op like TRUNC, use a SCALAR_TO_VECTOR rather than a single element BUILD_VECTOR to convert back to a vector type.

X86 considers v1i1 a legal type under AVX512 and as such a truncate from a v1iX type to v1i1 can be turned into a scalar truncate plus a conversion to v1i1. We would much prefer a v1i1 SCALAR_TO_VECTOR over a one element BUILD_VECTOR.

During lowering we were detecting the v1i1 BUILD_VECTOR as a splat BUILD_VECTOR like we try to do for v2i1/v4i1/etc. In this case we create (select i1 splat_elt, v1i1 all-ones, v1i1 all-zeroes). That goes through some more legalization and we end up with a CMOV choosing between 0 and 1 in scalar and a scalar_to_vector.

Arguably we could detect the v1i1 BUILD_VECTOR and do this better in X86 target code. But just using a SCALAR_TO_VECTOR in legalization is much easier.

llvm-svn: 326637

1 files changed, 1 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp b/llvm/lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp
index 8fb5c5e7ed7..5e7279c058c 100644
--- a/llvm/lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp
@@ -525,7 +525,7 @@ SDValue DAGTypeLegalizer::ScalarizeVecOp_UnaryOp(SDNode *N) {
                            N->getValueType(0).getScalarType(), Elt);
   // Revectorize the result so the types line up with what the uses of this
   // expression expect.
-  return DAG.getBuildVector(N->getValueType(0), SDLoc(N), Op);
+  return DAG.getNode(ISD::SCALAR_TO_VECTOR, SDLoc(N), N->getValueType(0), Op);
 }
 
 /// The vectors to concatenate have length one - use a BUILD_VECTOR instead.