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From: "amacleod at redhat dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/99966] Bounds check not eliminated by assert
Date: Tue, 11 Jan 2022 18:59:33 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99966

Andrew Macleod <amacleod at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |amacleod at redhat dot com

--- Comment #2 from Andrew Macleod <amacleod at redhat dot com> ---
looking at VRP2:
the initial assert provides:

Relational : (start_4(D) < end_5(D))
Relational : (_16 > start_4(D))
Relational : (end_5(D) <=3D _16)


unfortunately, the loop back edge is:
    i_12 =3D i_17 + 1;
    if (end_5(D) !=3D i_12)
      goto <bb 5>; [89.00%]
    else
      goto <bb 8>; [11.00%]

so it only registers what it knows from the branch:
Relational : (i_12 !=3D end_5(D))

so when it gets to the bounds check at:
    if (i_12 >=3D _16)
      goto <bb 6>; [0.04%]
    else
      goto <bb 10>; [99.96%]

It doesn't really know the answer.  If the loop branch had remained
if (i_12 < end_5) instead of being changed, then we would instead get the
relation:
Relational : (i_12 < end_5(D))
which combined with the earlier Relational : (end_5(D) <=3D _16), should re=
gister
 the transitive relation (i_12 < _16), and enable removal of the check.

This is transformed in ivopts, which runs after vrp1.  A quick check shows =
at
VRP1 time, the bounds are still using the < format...

In fact...  running with --param=3Dvrp1-mode=3Dranger registers this exact
transitive relation, and produces:

Relational : (i_2 < _16)
Relational : (i_2 < end_5(D))
    <bb 6> [local count: 952547451]:
    if (0 !=3D 0)
      goto <bb 7>; [0.04%]
    else
      goto <bb 8>; [99.96%]

and eliminates the bounds check.

So we can close this PR when we turn ranger on by default for VRP1.=