[PATCH] D102612: SwiftTailCC: teach verifier musttail rules applicable to this CC.

[Tim Northover via Phabricator via llvm-commits]

t.p.northover marked an inline comment as done.
t.p.northover added a comment.

> We might want to consider relaxing the rules for tailcc as well as swifttailcc, but I guess it doesn't need to happen in this patch.

I'm happy to do it here, I think it's definitely an improvement worth having.



================
Comment at: llvm/docs/LangRef.rst:11339
    - The ret instruction must return the (possibly bitcasted) value
-     produced by the call or void.
-   - The caller and callee prototypes must match. Pointer types of
-     parameters or return types may differ in pointee type, but not
-     in address space.
+     produced by the call, undef, or void.
+   - The caller and callee prototypes must match if the calling convention is
----------------
efriedma wrote:
> There's one source of complexity related to allowing undef here you might not have considered: return type promotion (converting a large return value to an indirect argument).  In particular, if we need to promote the return type of the musttail call.  I think you can make this work, but not sure if targets generate the right sequence.
Return type promotion seems really iffy with tail calls even now. For this module:

```declare [16 x i64] @foo()

define [16 x i64] @bar() {
  %res = musttail call [16 x i64] @foo()
  ret [16 x i64] %res
}```

AArch64 & ARM don't actually do a tail call but are otherwise correct (no diagnostic because of where the check is; this patch fixes that). X86 does the tail call but tells `@foo` to use a random bit of `@bar`'s caller's stack for the array.

I don't think the verifier can check that though, it's quite entwined in CodeGen and Verifier.cpp has nothing but a `TargetTriple` to work with.

Best I can think of is to document that functions with return type promotion probably won't work as expected here.


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  https://reviews.llvm.org/D102612/new/

https://reviews.llvm.org/D102612