[Bug tree-optimization/47365] New: [4.5 Regression] wrong code with -O -ftree-pre

[zsojka at seznam dot cz]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47365

           Summary: [4.5 Regression] wrong code with -O -ftree-pre
           Product: gcc
           Version: 4.5.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: zsojka@seznam.cz
              Host: x86_64-pc-linux-gnu
            Target: x86_64-pc-linux-gnu


Created attachment 23035
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=23035
reduced testcase

Reported by Vegard Nossum at IRC.

Output:
$ gcc-4.5 -O -ftree-pre testcase.c
$ ./a.out 
Aborted

In the testcase.c.092t.pre dump, there is:
...
<bb 2>:
  D.2739_4 = b.a[0].i;
  i.0_5 = i;
  if (D.2739_4 != i.0_5)
    goto <bb 3>;
  else
    goto <bb 4>;

<bb 3>:
  b.a[0] = b.a[1];
  b.a[1].i = D.2739_4;
  pretmp.3_6 = b.a[1].i;
  goto <bb 6>;

<bb 4>:
...
<bb 9>:

<bb 6>:
  # prephitmp.4_8 = PHI <D.2739_4(9), pretmp.3_6(3)>
  D.2739_9 = prephitmp.4_8;
  D.2739_9 = prephitmp.4_8;
  if (D.2739_9 == 0)
    goto <bb 7>;
...

Problematic line is
  pretmp.3_6 = b.a[1].i;
which reads the new value of "b.a[1].i". It should either happen one line
earlier, or it should read "b.a[0].i".

It seems that the fact "b.a[0] = b.a[1]" overwrites "b.a[0].i" is ignored.

Of couse I can be wrong.

Tested revisions:
r169006 - OK
4.5 r168785 - fail
4.4 r168785 - OK