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

[via llvm-commits]

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@@ -22,6 +22,8 @@ class BPFTargetMachine;
 class InstructionSelector;
 class PassRegistry;
 
+static const char *BPF_UNREACHABLE = "bpf_unreachable";
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eddyz87 wrote:

Q: given that reserved identifiers start from `__`, should we use `__bpf_unreachable` here (and on kernel side)?
Otherwise this is in conflict with section 7.1.3 of [here](https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1124.pdf).

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