Hello.
This is less a question but more a solution to a problem I encountered yesterday. Still, if you have any comments please don't hesitate to give feedback.
In my research processor back end, in [Target]ISelDAGToDAG.cpp I give in method Select():
SDNode *vloadSpecial = CurDAG->getMachineNode(
Connex::VLOAD_D_WO_IMM,
TYPE_VECTOR, // problems with CSE
//CurDAG->getVTList(TYPE_VECTOR, MVT::Glue),
// NO problems with CSE
CurDAG->getEntryNode() // We add a chain edge
);
But this getMachineNode() call returns the same SDNode over and over again in the same basic block due to CSE in the instruction selection phase, since the machine node doesn't take any actual inputs.
Therefore, I add MVT::Glue to the return types to avoid that llc performs CSE on these nodes. See why this is so at http://llvm.org/docs/doxygen/html/SelectionDAG_8cpp_source.html#l06206 . More exactly:
06208 bool DoCSE = VTs.VTs[VTs.NumVTs-1] != MVT::Glue;
...
06214 if (DoCSE) {
06215 FoldingSetNodeID ID;
06216 AddNodeIDNode(ID, ~Opcode, VTs, OpsArray);
06217 IP = nullptr;
06218 if (SDNode *E = FindNodeOrInsertPos(ID, DL.getDebugLoc(), IP)) {
06219 return cast<MachineSDNode>(UpdadeSDLocOnMergedSDNode(E, DL));
06220 }
06221 }
I simply added MVT::Glue to the MachineNode, without bothering to use this edge with any successor node and me specifying such output in TableGen for VLOAD_D_WO_IMM - I used CurDAG->getVTList(TYPE_VECTOR, MVT::Glue) instead of TYPE_VECTOR in the code above.
Note that allowing the instruction-selection phase to do CSE actually results in incorrect machine code, with fewer VLOAD_D_WO_IMM instructions than required.
This code is correct for a few tests I ran, but I wonder if I am missing anything.
Best regards,
Alex