[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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4ast wrote:

> While `.ksyms` is just a convention, I think that emitting kfunc calls from clang might be useful. E.g. currently `memcpy`, `memset`, `memmove`, `memcmp` intrinsics are unrolled, but there is no point in doing so.

This convention is pretty much a de-facto standard now. Several libraries support it.
It's not unusual for a compiler to use a particular section name like ".text" or ".rodata" for specific needs.
The name is target dependent. ".ksyms" is another special name.
So I think it's appropriate for a compiler to emit "call blah_unreachable" and "call blah_memcpy" with ".ksyms" section name. It's cleaner and more flexible than "call magic_const" or inventing a new trap-like insn.

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