[llvm-dev] BZ12772. Can't reproduce in tree. How could it be fixed?

[Jon Chesterfield via llvm-dev]

Hi,

What should I do if I cannot trigger a failure in any of the in-tree
targets?

Specifically, bug (https://bugs.llvm.org//show_bug.cgi?id=12772) was raised
back in 2012 with a proposed patch but no test case. I've put a couple of
hours into trying to demonstrate the bug in tree but have not been
successful. The bugzilla suggests the the patch works on three out of tree
targets and that others have similarly failed to produce an in tree repro.
Applying the patch causes no existing tests to fail.

This isn't a problem in that I'll just mark CONCAT_VECTORS as legal in our
back end.

What is the usual strategy for dealing with bugs in untested code where is
has proven uneconomic to trigger a bug in an upstream back end? For
example, would it be worth converting the patch from 2012 into a
phabricator diff and seeking feedback there?

Kind regards,

Jon
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