[PATCH] D106000: [NFC] X86: Annotate sibcall test

[Matheus Izvekov via Phabricator via llvm-commits]

mizvekov added inline comments.


================
Comment at: llvm/test/CodeGen/X86/sibcall.ll:252
+; QOI: We don't sibcall this for unknown reasons (the unreachable
+; rather than a ret?)
 define dso_local void @t10() nounwind ssp {
----------------
I think this one has to do with this: https://github.com/llvm/llvm-project/blob/3bf101f34cd466f103af00c764dc1cddb6eb14a6/llvm/lib/CodeGen/Analysis.cpp#L506

```
The block must end in a return statement or unreachable.

FIXME: Decline tailcall if it's not guaranteed and if the block ends in
an unreachable, for now. The way tailcall optimization is currently
implemented means it will add an epilogue followed by a jump. That is
not profitable. Also, if the callee is a special function (e.g.
longjmp on x86), it can end up causing miscompilation that has not
been fully understood.
```

The tail call is not done just because it is not a musttail and the code generated might have worse runtime cost.

So when we say we have a QOI issue, there are two issues conflated here:
* Do we know how to do it profitably purely from a instruction cost perspective?
* Can we figure out the sequence of instructions we actually need to tail call this at all?


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D106000/new/

https://reviews.llvm.org/D106000