[llvm] [BPF] Handle unreachable with a kfunc call (PR #131731)

[via llvm-commits]

================
@@ -320,6 +320,7 @@ struct BPFMIPreEmitPeephole : public MachineFunctionPass {
   bool adjustBranch();
   bool insertMissingCallerSavedSpills();
   bool removeMayGotoZero();
+  bool addExitAfterUnreachable();
----------------
eddyz87 wrote:

Unexpectedly, generating `exit` at selection DAG level is a head scratcher. The following seems to work:

```cpp
@@ -759,7 +774,10 @@ SDValue BPFTargetLowering::LowerTRAP(SDValue Op, SelectionDAG &DAG) const {
   CLI.DL = DL;
   CLI.NoMerge = false;
   CLI.DoesNotReturn = true;
-  return LowerCall(CLI, InVals);
+  SDValue Chain = LowerCall(CLI, InVals);
+  SDValue Glue = Chain.getValue(1);
+  return DAG.getNode(BPFISD::RET_GLUE, DL, MVT::Other,
+                     SmallVector<SDValue>{Chain, Glue});
 }
```

Relies on `BPFretglue` translation to `exit` as described in `BPFInstrInfo.td`.

https://github.com/llvm/llvm-project/pull/131731