[Bug middle-end/66375] [4.8/4.9/5 Regression] wrong code at -O2 and -O3 on x86_64-linux-gnu

["rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66375

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |law at gcc dot gnu.org

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
PRE doesn't seem to do sth wrong here.  The testcase can be fixed by for
example disabling VRP2.  VRP2 needs either store-motion or complete unrolling
to
make it trigger the miscompile.  Nicest IL before VRP2 is with -fno-ivopts
-fno-tree-loop-im.  The transform VRP2 does is

Folding statement: if (a.4_17 <= 13)
Simplified relational if (a.4_17 <= 13)
 into if (a.4_17 != 14)

and

  Registering jump thread: (6, 7) incoming edge;  (7, 8) normal;
  Threaded jump 6 --> 7 to 12

  <bb 6>:
  # prephitmp_3 = PHI <prephitmp_22(5)>
  # prephitmp_39 = PHI <prephitmp_22(5)>

  <bb 7>:
  # prephitmp_26 = PHI <prephitmp_39(6), pretmp_23(3)>
  if (prephitmp_26 != 113)
    goto <bb 8>;
  else
    goto <bb 9>;

  <bb 8>:
  __builtin_abort ();

so it looks jump-threading related to me.  Somehow the equivalences I
see being recorded miss the backedge for prephitmp_22 but only record zero.