[llvm-bugs] [Bug 49675] New: Coroutine miscompile

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=49675

            Bug ID: 49675
           Summary: Coroutine miscompile
           Product: clang
           Version: 11.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++2a
          Assignee: unassignedclangbugs at nondot.org
          Reporter: michal.chojnowski at scylladb.com
                CC: blitzrakete at gmail.com, erik.pilkington at gmail.com,
                    llvm-bugs at lists.llvm.org, richard-llvm at metafoo.co.uk

clang miscompiles (misoptimizes?) coroutines.

Here's a minimal example for clang 11.0.1: https://godbolt.org/z/EYE16d
Here's a minimal example for both 11.0.1 and trunk:
https://godbolt.org/z/7nPG39

The bug only occurs with optimizations enabled. `opt-bisect-limit` shows that
the linked test cases break after `BISECT: running pass (64) Combine redundant
instructions on function (_Z3foo1A)`. I didn't confirm that this particular
pass is the culprit though; maybe it only makes an earlier miscompile apparent.

Perhaps this bug is not coroutine-specific. However, we have encountered this
miscompile (we aren't certain that the root cause is always the same, but they
all look related) in practice several unrelated times with coroutines, and
never without coroutines.

I also mentioned this case under https://bugs.llvm.org/show_bug.cgi?id=48857,
but I'm opening a separate bug because I'm not actually sure that it's related
to the original case in 48857.

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