[llvm-bugs] [Bug 38711] New: wasm32: Regression with trivial bitcasts

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=38711

            Bug ID: 38711
           Summary: wasm32: Regression with trivial bitcasts
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: WebAssembly
          Assignee: unassignedbugs at nondot.org
          Reporter: alex at crichton.co
                CC: dan433584 at gmail.com, llvm-bugs at lists.llvm.org,
                    sbc at chromium.org

I'm a bit late to this diff but we're attempting to update LLVM in the Rust
repo [1] but the update is causing some of our wasm tests to fail. A bisection
of the failure points to this revision in LLVM -
https://reviews.llvm.org/D49517.

I haven't put the time in yet to reduce this down to a standalone IR file as
I'm hoping those CC'd can help to see what's going on here. Taking one of our
failing test cases in rust-lang/rust I created a wasm file before and after the
change, yielding a diff that looks like --
https://gist.github.com/alexcrichton/9b8c71afa892cb89234a6b5ef2113028.

It looks like a few shims were inserted (perhaps erroneously?) and the shim
inserted for the Vec/Deref looking symbol is the one which is causing the trap
(the `unreachable` instruction at the end)

Would y'all be able to help take a look to see what's going on here?

[1]: https://github.com/rust-lang/rust/pull/53611

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