Questions about type based "createBuildVecShuffle()" in DAG Combiner

Project Infrastructure LLVM Dev List Archives
1

In LLVM DAG Combiner, DAGCombiner::createBuildVecShuffle() is type based.

DAGCombiner.cpp,

│17184 // We can’t generate a shuffle node with mismatched input and output types.

│17185 // Try to make the types match the type of the output.

2

The ISD::VECTOR_SHUFFLE opcode is defined to have the same input/output types. That design is baked into all shuffle handling in SelectionDAG. I don’t know the history of why it was designed that way, but it would be hard to change at this point.