llvm dev - Jan 2017 - getMachineNode() returns the same SDNode over and over again due to CSE in the instruction selection phase

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

On 1/11/2017 11:50 AM, Alex Susu via llvm-dev wrote:
>   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.
If this node getting CSE'ed is a problem, CurDAG->getEntryNode() is 
probably not the right chain.  If you're replacing an existing node, you 
should use the same chain that node used.

-Eli

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