llvm dev - Feb 2016 - TableGen customized node with mayStore attribute is deleted if there is no use

Hi,

I define a customized node with customized type. The job of this customized
node is to move a value from one register class to another class. I find
that if there is no use of the destination register, this node will be
deleted from SDAG. For some reasons, I want to keep this node. So I attach
mayStore attribute to this node and I hope it will not be deleted. However,
it does not work like I assume. There must be something wrong. But I don't
know it's because this trick does not work in theory  or it's because my
implementation is wrong.

def MoveTy : SDTypeProfile<1, 1, []>;
def MoveFlag : SDNode<"FOOISD::MOVE_FLAG", MoveTy, [SDNPHasChain,
SDNPSideEffect, SDNPMayStore, SDNPMayLoad]>;

let hasSideEffects = 1, mayStore = 1, mayLoad = 1 in {
def MOVE : InstFOO<(outs ARegs:$dst), (ins BRegs:$src),
                    "move $dst, $src",
                     [(set i32:$dst, (MoveFlag i32:$src))]>;

For example, I add this node into SDAG when I want to move the formal
argument from the specific register class to other register class. I
implement it in LowerFormalArguments() like this:
  ....
  for (auto &VA : ArgLocs) {
    if (VA.isRegLoc()) {
      // Arguments passed in registers
      EVT RegVT = VA.getLocVT();
      VReg = RegInfo.createVirtualRegister(&FOO::BRegsRegClass);
      RegInfo.addLiveIn(VA.getLocReg(), VReg);
      SDValue ArgIn = DAG.getCopyFromReg(Chain, dl, VReg, RegVT);
      SDValue ArgIn_copy = DAG.getNode(FOOISD::MOVE_FLAG , dl, MVT::i32,
Chain, ArgIn); // this node is added in order to move the value from BRegs
class to ARegs class and I want it be kept even it is not used later on
      InVals.push_back(ArgIn_copy);
    }
  }

After checking the recursivelyDeleteUnusedNodes() in DAGCombinner.cpp, this
function will delete the unused node and it does not care about the other
attributes.

I must have something wrong. I appreciate if you can point it out. Any
suggestion is appreciable.

Regards,

Xiangyang
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> On Jan 31, 2016, at 20:27, Xiangyang Guo via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> For example, I add this node into SDAG when I want to move the formal
argument from the specific register class to other register class. I implement
it in LowerFormalArguments() like this:
>   ....
>   for (auto &VA : ArgLocs) {
>     if (VA.isRegLoc()) {
>       // Arguments passed in registers
>       EVT RegVT = VA.getLocVT();
>       VReg = RegInfo.createVirtualRegister(&FOO::BRegsRegClass);
>       RegInfo.addLiveIn(VA.getLocReg(), VReg);
>       SDValue ArgIn = DAG.getCopyFromReg(Chain, dl, VReg, RegVT);
>       SDValue ArgIn_copy = DAG.getNode(FOOISD::MOVE_FLAG , dl, MVT::i32,
Chain, ArgIn); // this node is added in order to move the value from BRegs class
to ARegs class and I want it be kept even it is not used later on
Here if you just want a register class copy, you should create a new virtual
register with the other class and use a CopyToReg node. No need for a custom
node here.

>       InVals.push_back(ArgIn_copy);
You are losing the chain. You need to add ArgIn_copy.getValue(1) to a vector,
and then the return value of LowerFormalArguments should be a TokenFactor over
all of the chains produced.

-Matt

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Thanks, Matt.

It works now.

Regards,

Xiangyang

2016-02-01 4:12 GMT-05:00 Matt Arsenault <arsenm2 at gmail.com>:
>
> On Jan 31, 2016, at 20:27, Xiangyang Guo via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> For example, I add this node into SDAG when I want to move the formal
> argument from the specific register class to other register class. I
> implement it in LowerFormalArguments() like this:
>   ....
>   for (auto &VA : ArgLocs) {
>     if (VA.isRegLoc()) {
>       // Arguments passed in registers
>       EVT RegVT = VA.getLocVT();
>       VReg = RegInfo.createVirtualRegister(&FOO::BRegsRegClass);
>       RegInfo.addLiveIn(VA.getLocReg(), VReg);
>       SDValue ArgIn = DAG.getCopyFromReg(Chain, dl, VReg, RegVT);
>       SDValue ArgIn_copy = DAG.getNode(FOOISD::MOVE_FLAG , dl, MVT::i32,
> Chain, ArgIn); // this node is added in order to move the value from BRegs
> class to ARegs class and I want it be kept even it is not used later on
>
>
> Here if you just want a register class copy, you should create a new
> virtual register with the other class and use a CopyToReg node. No need for
> a custom node here.
>
>
>       InVals.push_back(ArgIn_copy);
>
>
> You are losing the chain. You need to add ArgIn_copy.getValue(1) to a
> vector, and then the return value of LowerFormalArguments should be a
> TokenFactor over all of the chains produced.
>
> -Matt
>
>
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