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From: "iains at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/114663] New: Several contracts test cases fail with
 -fsanitize=undefined -fsanitize-trap
Date: Tue, 09 Apr 2024 18:42:05 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 14.0
X-Bugzilla-Keywords: testsuite-fail, wrong-code
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114663

            Bug ID: 114663
           Summary: Several contracts test cases fail with
                    -fsanitize=3Dundefined -fsanitize-trap
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Keywords: testsuite-fail, wrong-code
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: iains at gcc dot gnu.org
                CC: jason at gcc dot gnu.org
  Target Milestone: ---

I found this while working on -funreachable-traps (but the failure equally
occurs with -fsanitize=3Dundefined -fsanitize-trap)

FAIL: g++.dg/contracts/contracts10.C   execution test
FAIL: g++.dg/contracts/contracts18.C   execution test
FAIL: g++.dg/contracts/contracts19.C   execution test
FAIL: g++.dg/contracts/contracts2.C   execution test

Initial analysis is that somehow the lowering of the contracts code is
exploiting UB [which has a large measure of irony if true] to make these ca=
ses
pass, for example contracts2.C optimised tree dump contains:

;; Function main (main, funcdef_no=3D0, decl_uid=3D2531, cgraph_uid=3D1,
symbol_order=3D0)

int main ()
{
  int x;
  int D.2551;
  const struct  D.2542;
  int _2;

  <bb 2> :
  x_1 =3D 1;
  if (x_1 < 0)
    goto <bb 4>; [INV]
  else
    goto <bb 3>; [INV]

  <bb 3> :
  __builtin_unreachable ();

  <bb 4> :
  if (x_1 <=3D 0)
    goto <bb 5>; [INV]
  else
    goto <bb 6>; [INV]

  <bb 5> :

=3D=3D=3D=3D=3D

When (default) the __builtin_unreachable () is replaced with nothing (i.e. =
it
falls though) the test case passes.

When we replace the __builtin_unreachable () with a trap (either using the
ubsan or -funreachable-traps style) the test case fails with the trap.

This seems to be unlikely to be what was intended (or if it was intended it=
's
terribly fragile); I'm labelling it wrong code for now.

Similar code patterns exist in the other cases mentioned.=