[llvm-dev] Pseudo-instruction that overwrites its input register

[Dr. ERDI Gergo via llvm-dev]

On Sun, 4 Jun 2017, Dr. ERDI Gergo wrote:

> the type difference I mentioned ((i16, i16, ch) vs (i16, ch) on the 
> output ports)

OK I managed to make some progress on this, by using a MergeValues node:

       SDNode* LDW = CurDAG->getMachineNode(
         AVR::LDWRdPtr, SDLoc(N), VT, PtrVT, MVT::Other,
         LD->getBasePtr(), LD->getChain());
       SDValue Unpack[] = { SDValue(LDW, 0), SDValue(LDW, 2) };
       SDNode* NN = CurDAG->getMergeValues(Unpack, SDLoc(N)).getNode();

       ReplaceNode(N, NN);


which gets me from

   t17: i16,ch = load<LD2[%v25](align=1)(dereferenceable)> t16:1, t2, undef:i16

to

   t24: i16,i16,ch = LDWRdPtr t2, t16:1

looking tood; but then it fails during scheduling with

llc: CodeGen/SelectionDAG/InstrEmitter.cpp:303: 
unsigned int llvm::InstrEmitter::getVR(
     llvm::SDValue,
     llvm::DenseMap<llvm::SDValue, unsigned int>&):
Assertion `I != VRBaseMap.end() && "Node emitted out of order - late"' failed.

For reference, the full DAG before and after ISEL:


SelectionDAG has 22 nodes:
   t0: ch = EntryToken
   t2: i16,ch = CopyFromReg t0, Register:i16 %vreg0
   t5: i16,ch = load<LD2[%v25](align=1)(dereferenceable)> t0, t2, undef:i16
     t9: ch,glue = callseq_start t5:1, TargetConstant:i16<0>
   t11: ch,glue = CopyToReg t9, Register:i16 %R25R24, t5
   t13: ch,glue = CALL t11, TargetGlobalAddress:i16<i8 (i16)* @read_ram> 0, Register:i16 %R25R24, RegisterMask:Untyped, t11:1
   t14: ch,glue = callseq_end t13, TargetConstant:i16<0>, TargetConstant:i16<0>, t13:1
     t16: i8,ch,glue = CopyFromReg t14, Register:i8 %R24, t14:1
   t17: i16,ch = load<LD2[%v25](align=1)(dereferenceable)> t16:1, t2, undef:i16
     t18: ch,glue = callseq_start t17:1, TargetConstant:i16<0>
   t19: ch,glue = CopyToReg t18, Register:i16 %R25R24, t17
   t20: ch,glue = CALL t19, TargetGlobalAddress:i16<i8 (i16)* @read_ram> 0, Register:i16 %R25R24, RegisterMask:Untyped, t19:1
   t21: ch,glue = callseq_end t20, TargetConstant:i16<0>, TargetConstant:i16<0>, t20:1
     t22: i8,ch,glue = CopyFromReg t21, Register:i8 %R24, t21:1
   t23: ch = RET_FLAG t22:1



SelectionDAG has 23 nodes:
   t0: ch = EntryToken
   t2: i16,ch = CopyFromReg t0, Register:i16 %vreg0
     t9: i16,ch,glue = ADJCALLSTACKDOWN TargetConstant:i16<0>, t27:1
   t11: ch,glue = CopyToReg t9:1, Register:i16 %R25R24, t27
   t13: ch,glue = CALLk TargetGlobalAddress:i16<i8 (i16)* @read_ram> 0, Register:i16 %R25R24, RegisterMask:Untyped, t11, t11:1
   t14: i16,ch,glue = ADJCALLSTACKUP TargetConstant:i16<0>, TargetConstant:i16<0>, t13, t13:1
     t18: i16,ch,glue = ADJCALLSTACKDOWN TargetConstant:i16<0>, t25:1
   t19: ch,glue = CopyToReg t18:1, Register:i16 %R25R24, t25
   t20: ch,glue = CALLk TargetGlobalAddress:i16<i8 (i16)* @read_ram> 0, Register:i16 %R25R24, RegisterMask:Untyped, t19, t19:1
   t21: i16,ch,glue = ADJCALLSTACKUP TargetConstant:i16<0>, TargetConstant:i16<0>, t20, t20:1
     t16: i8,ch,glue = CopyFromReg t14:1, Register:i8 %R24, t14:2
   t24: i16,i16,ch = LDWRdPtr t2, t16:1
   t25: i16,ch = merge_values t24, t24:2
   t26: i16,i16,ch = LDWRdPtr t2, t0
   t27: i16,ch = merge_values t26, t26:2
     t22: i8,ch,glue = CopyFromReg t21:1, Register:i8 %R24, t21:2
   t23: ch = RET t22:1