[llvm-bugs] [Bug 44784] New: LTO failure with mixed DWARF version metadata

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=44784

            Bug ID: 44784
           Summary: LTO failure with mixed DWARF version metadata
           Product: new-bugs
           Version: unspecified
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: dmajor at mozilla.com
                CC: dblaikie at gmail.com, froydnj at gmail.com,
                    htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org
            Blocks: 44555

While testing the 10.0 release branch, we ran into the following situation:

* We use rustc 1.39 (based on LLVM 9) to build (with LTO) some Rust code, and
the resulting object file has a piece of metadata saying the DWARF version is 2
and the behavior is `llvm::Module::Warning`.

* We use clang 10 to build (with LTO) some C++ code, and the resulting object
file has a piece of metadata saying the DWARF version is 4 and the behavior is
`llvm::Module::Max`, because of the change in [1].

(I don't know why we have mismatched DWARF versions.)

* When we link the objects together, previously we would just get a bunch of
warning spam, but now we fail with an LTO error because of the mismatch in
behaviors [2].

@dblaikie: Is this scenario expected to be supported? From my reading of the
changeset, it sounds like the desired behavior would be to upgrade to the
higher version, and perhaps warn, but not fail the build. So maybe the
mismatched-behaviors check is too aggressive in this case. But correct me if
I've misunderstood the intent. Or maybe it's our fault for using mixed LLVM
versions.

[1]
https://github.com/llvm/llvm-project/commit/e956952edec140d2475aa7c8cbe20fbdd3320634
[2]
https://github.com/llvm/llvm-project/blob/d8117542ac57f6051674ca70ea14c0e0d7d9b046/llvm/lib/Linker/IRMover.cpp#L1280


Referenced Bugs:

https://bugs.llvm.org/show_bug.cgi?id=44555
[Bug 44555] [meta] 10.0.0 Release Blockers
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