[Bug tree-optimization/102542] [12 Regression] ICE Segmentation fault since r12-3876-g4a960d548b7d7d94

[aldyh at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102542

Aldy Hernandez <aldyh at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |jeffreyalaw at gmail dot com

--- Comment #5 from Aldy Hernandez <aldyh at gcc dot gnu.org> ---
(In reply to rguenther@suse.de from comment #4)
> On Thu, 30 Sep 2021, aldyh at gcc dot gnu.org wrote:

Thanks for the loop explanation.  It's quite helpful.

> So the threading at hand rotates the loop (which is only so-so OK),
> that might be problematic in case it creates a non-do-while loop
> after loop header copying.
> 
> In this process it might also lose track of the loop, causing it to
> be re-discovered as separate entity which has problems of its own.
> 
> IMHO we should avoid threadings through a loop header if the path
> ends in the loop itself, even if it doesn't introduce a new entry but
> just re-directs the existing one.

It looks like Jeff and you have stumbled on various corner cases we must
address in cancel_invalid_paths().

Could I inconvenience you to tweak this function with your insight?  It's a
tiny function, and it seems you're much more qualified to add the restriction
code.  If not, I'm sure I can stumble around it and send it for review.

Orthogonal to this, it does look like the extra thread is causing an ICE in slp
which must be addressed??

Thanks.