BZ12772. Can't reproduce in tree. How could it be fixed?


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

Specifically, bug ( 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,


There are in-tree targets with multiple legal vector types (off the top of my head, x86, ARM, Aarch64). None of those targets have CONCAT_VECTORS set to EXPAND, though; the vector registers overlap, so there's generally some cheaper way to implement CONCAT_VECTORS.

You could just delete the broken code, I guess? That wouldn't really solve the problem, but at least it would cause a selection failure rather than silently miscompiling.