https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71403
--- Comment #7 from Jeffrey A. Law <law at redhat dot com> --- So, to follow-up on my own comment. We start with 3 nested loops. The first backward jump threading pass threads the backedge of the outermost loop to the head of the middle loop. That essentially combines the loops -- where parts of the original middle loop are not executed every iteration anymore. But we have more total iterations. ISTM we need a way to invalidate most of what we know about the loop when this happens. Whether or not this is a good thing I think is open to debate. It happens to be good for this test as so much stuff collapses away, but I don't know if it's typically a good thing in the real world or not. Obviously we can avoid these problems by rejecting these jump threads -- essentially if the thread path starts out at some loop nesting X, if we ever traverse into X+1, reject the path. It's not ideal, but it's doable.
