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Subject: [Bug tree-optimization/55875] New: [4.8 Regression] IVopts caused miscompilation
Date: Fri, 04 Jan 2013 12:09:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55875

             Bug #: 55875
           Summary: [4.8 Regression] IVopts caused miscompilation
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: jakub@gcc.gnu.org


The following testcase is miscompiled at -O3 on x86_64-linux starting with
http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=192989
(ok, that revision doesn't compile, needs r192900 follow-up).

struct A
{
  short int a1;
  unsigned char a2;
  unsigned int a3;
};

struct B
{
  unsigned short b1;
  const A *b2;
};

B b;

__attribute__((noinline, noclone))
int foo (unsigned x)
{
  __asm volatile ("" : "+r" (x) : : "memory");
  return x;
}

inline void
bar (const int &)
{
}

__attribute__((noinline)) void
baz ()
{
  const A *a = b.b2;
  unsigned int i;
  unsigned short n = b.b1;
  for (i = 0; i < n; ++i)
    if (a[i].a1 == 11)
      {
    if (i > 0 && (a[i - 1].a2 & 1))
      continue;
    bar (foo (2));
    return;
      }
}

int
main ()
{
  A a[4] = { { 10, 0, 0 }, { 11, 1, 0 }, { 11, 1, 0 }, { 11, 1, 0 } };
  b.b1 = 4;
  b.b2 = a;
  baz ();
  return 0;
}

In *.slp we have:
  <bb 5>:
  if (i_21 != 0)
    goto <bb 6>;
  else
    goto <bb 7>;

  <bb 6>:
  _11 = i_21 + 4294967295;
  _12 = (long unsigned int) _11;
  _13 = _12 * 8;
  _14 = a_4 + _13;
  _15 = _14->a2;
but ivopts turns that into:
  <bb 5>:
  i_25 = (unsigned int) ivtmp.9_31;
  if (i_25 != 0)
    goto <bb 6>;
  else
    goto <bb 7>;

  <bb 6>:
  _28 = ivtmp.9_31 * 8;
  _27 = a_4 + _28;
  _26 = _27 + 34359738362;
  _15 = MEM[base: _26, offset: 0B];
which is wrong, i_21 + -1U wrapped around (and wasn't executed for i_21 being
0), while 34359738362 is 0xffffffffULL * 8 + 2, thus it ignores the wrapping
and does what the original code would do for
  _12 = (long unsigned int) i_21;
  _77 = _12 + 4294967295;
  _13 = _77 * 8;
i.e. as if the -1U addition was done in the wider precision.