remove LLVM va_end calls

[folkertdev]

tracking issue: #44930

The operation is a no-op, so we skip it.

[folkertdev]

r? RalfJung

The changes are trivial, but technically we're introducing LLVM UB here, so passing this to you for signoff.

We also don't use LLVM's va_copy, but I think that is meaningfully different because we never call LLVM's va_end or va_arg on that, hence LLVM has no way of observing this at all.

But LLVM can actually see that we emit a va_start without a corresponding va_end, and based on its documentation that is UB and it's allowed to do whatever.

[rustbot]

RalfJung is not on the review rotation at the moment.
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[rustbot]

rustc_codegen_cranelift is developed in its own repository. If possible, consider making this change to rust-lang/rustc_codegen_cranelift instead.

cc @bjorn3

rustc_codegen_gcc is developed in its own repository. If possible, consider making this change to rust-lang/rustc_codegen_gcc instead.

cc @antoyo, @GuillaumeGomez

[RalfJung]

LLVM can also see what we memcpy a VaList and then call va_arg on both the old and new value which is UB (because it's not documented to be allowed). So if we want to not have LLVM UB, we need to actually emit va_copy / va_end for copying / dropping all VaList.

@nikic what do you think?

[RalfJung]

What was the reason again for us not calling va_copy / va_end when cloning / dropping a VaList?

View changes since the review

[folkertdev]

Well another constraint on the LLVM va_end is that it must be called in the same function as the corresponding va_start or va_copy, but also va_list can be moved. So va_end does not actually correspond to rust Drop.

[folkertdev]

LLVM can also see what we memcpy a VaList and then call va_arg on both the old and new value which is UB (because it's not documented to be allowed).

It feels different, we're not using LLVM's va_arg but our own thing, but it does modify the VaList so I guess that counts.

View changes since this review

[folkertdev]

Well another constraint on the LLVM va_end is that it must be called in the same function as the corresponding va_start or va_copy, but also va_list can be moved. So va_end does not actually correspond to rust Drop.