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

[via llvm-commits]

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@@ -1622,6 +1627,25 @@ void BTFDebug::endModule() {
   // Collect global types/variables except MapDef globals.
   processGlobals(false);
 
+  // Create a BTF entry for func __unreachable_helper.
+  const Module *M = MMI->getModule();
+  Function *F = M->getFunction("__unreachable_helper");
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yonghong-song wrote:

>From llvm perspective, adding a new insn is easy, likes below:
```
diff --git a/llvm/lib/Target/BPF/BPFInstrInfo.td b/llvm/lib/Target/BPF/BPFInstrInfo.td
index 2dcf1eae086b..59bad209ea54 100644
--- a/llvm/lib/Target/BPF/BPFInstrInfo.td
+++ b/llvm/lib/Target/BPF/BPFInstrInfo.td
@@ -790,11 +790,25 @@ class RET<string OpcodeStr>
   let BPFClass = BPF_JMP;
 }
 
+class TRAP<string OpcodeStr>
+    : TYPE_ALU_JMP<BPF_EXIT.Value, BPF_K.Value,
+                   (outs),
+                   (ins),
+                   !strconcat(OpcodeStr, ""),
+                   []> {
+  let Inst{31-0} = 1;
+  let BPFClass = BPF_JMP;
+}
+
 let isReturn = 1, isTerminator = 1, hasDelaySlot=0, isBarrier = 1,
     isNotDuplicable = 1 in {
   def RET : RET<"exit">;
 }
 
+let isTrap = 1 in {
+  def UNREACHABLE : TRAP<"unreachable">;
+} // isTerminator = 1
+
 // ADJCALLSTACKDOWN/UP pseudo insns
 let Defs = [R11], Uses = [R11], isCodeGenOnly = 1 in {
 def ADJCALLSTACKDOWN : Pseudo<(outs), (ins i64imm:$amt1, i64imm:$amt2),
```
Different encoding is possible. I am chosing current kfunc approach since it minimizes the kernel change, spares one new insn, and can work on old kernel.

But I am open to the new insn approach or real bpf_unreachable() kfunc approach if we can reach a consensus.

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