[Bug tree-optimization/71403] [7 Regression] wrong code (segfault) at -O3 on x86_64-linux-gnu

["law at redhat dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71403

Jeffrey A. Law <law at redhat dot com> changed:

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                 CC|                            |law at redhat dot com

--- Comment #2 from Jeffrey A. Law <law at redhat dot com> ---
So we start with 3 nested loops. Let's call them the A, D, and B loops.

Trivial examination shows the B loop never loops.  It's a total nop.

The A loop iterates twice.  On the first iteration of the A loop, the D loop
will iterate twice (ignore for now that the D loop just increases D and has no
other side effects).

The A & D loops get collapsed into a single loop via a series of
transformations.  That collapsed loop iterates twice incrementing the value of
D to 2.  There's another iteration which increments the value of A (but does
not modify D).  Then A increments again and we exit.  The latch edge is
traversed *3* times.  All is fine and good.

We have this in cunroll:

Loop 2 iterates at most 2 times.
Loop 2 likely iterates at most 2 times.

Umm, no, the loop actually iterates 3 times.  I don't know if it's the shape of
the CFG or something else that is confusing the iterations analysis code, but
the loop definitely iterates 3 times.