[llvm] [BPF] improve error handling by custom lowering & fail() (PR #75088)

[via llvm-commits]

eddyz87 wrote:

Hi @inclyc, thank you for working on this.

I think these chages are fine.
The the default `Expand` action for sdiv/srem does not help us, because they rely on `ISD::SDIVREM` / `ISD::UDIVREM` that we don't support either.

<details>
<summary><code>Expand</code> implementation code</summary>

```c++
bool SelectionDAGLegalize::ExpandNode(SDNode *Node) {
  ...
  case ISD::UREM:
  case ISD::SREM:
    if (TLI.expandREM(Node, Tmp1, DAG))
      Results.push_back(Tmp1);
    break;
  case ISD::UDIV:
  case ISD::SDIV: {
    bool isSigned = Node->getOpcode() == ISD::SDIV;
    unsigned DivRemOpc = isSigned ? ISD::SDIVREM : ISD::UDIVREM;
    EVT VT = Node->getValueType(0);
    if (TLI.isOperationLegalOrCustom(DivRemOpc, VT)) {
      SDVTList VTs = DAG.getVTList(VT, VT);
      Tmp1 = DAG.getNode(DivRemOpc, dl, VTs, Node->getOperand(0),
                         Node->getOperand(1));
      Results.push_back(Tmp1);
    }
    break;
  }
  ...
}

bool TargetLowering::expandREM(SDNode *Node, SDValue &Result,
                               SelectionDAG &DAG) const {
  EVT VT = Node->getValueType(0);
  SDLoc dl(Node);
  bool isSigned = Node->getOpcode() == ISD::SREM;
  unsigned DivOpc = isSigned ? ISD::SDIV : ISD::UDIV;
  unsigned DivRemOpc = isSigned ? ISD::SDIVREM : ISD::UDIVREM;
  SDValue Dividend = Node->getOperand(0);
  SDValue Divisor = Node->getOperand(1);
  if (isOperationLegalOrCustom(DivRemOpc, VT)) {
    SDVTList VTs = DAG.getVTList(VT, VT);
    Result = DAG.getNode(DivRemOpc, dl, VTs, Dividend, Divisor).getValue(1);
    return true;
  }
  if (isOperationLegalOrCustom(DivOpc, VT)) {
    // X % Y -> X-X/Y*Y
    SDValue Divide = DAG.getNode(DivOpc, dl, VT, Dividend, Divisor);
    SDValue Mul = DAG.getNode(ISD::MUL, dl, VT, Divide, Divisor);
    Result = DAG.getNode(ISD::SUB, dl, VT, Dividend, Mul);
    return true;
  }
  return false;
}
```

</details>

As a nitpick, there is no test for error reported on `srem`.

https://github.com/llvm/llvm-project/pull/75088