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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/62030] New: wrong code due to aliasing issue
Date: Wed, 06 Aug 2014 05:08:00 -0000
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X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: rtl-optimization
X-Bugzilla-Version: 4.10.0
X-Bugzilla-Keywords: alias, wrong-code
X-Bugzilla-Severity: normal
X-Bugzilla-Who: pinskia at gcc dot gnu.org
X-Bugzilla-Status: UNCONFIRMED
X-Bugzilla-Priority: P3
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62030

            Bug ID: 62030
           Summary: wrong code due to aliasing issue
           Product: gcc
           Version: 4.10.0
            Status: UNCONFIRMED
          Keywords: alias, wrong-code
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
            Blocks: 61964
            Target: mipsisa64-elf

With a slightly modified version of the testcase for PR 61964 (I have an
optimization pass which does the optimization but I can confirm it with an
upstream GCC with the modified source), I get the failure on mipsisa64-elf.
Here is the testcase:
/* { dg-do run } */

extern void abort (void);

struct node { struct node *next, *prev; } node;
struct head { struct node *first; } heads[5];
int k = 2;
struct head *head = &heads[2];

static int __attribute__((noinline))
foo()
{
  node.prev = (void *)head;
  head->first = &node;

  struct node *n = head->first;
  struct head *h = &heads[k];
  struct node *next = n->next;

  if (n->prev == (void *)h)
    h->first = next;
  else
    n->prev->next = next;

  n->next = h->first;
  return n->next == &node;
}

int main()
{
  if (foo ())
    abort ();
  return 0;
}

Compile with -O2 -march=octeon to see the failure.

CE1 is where the combining of the two stores happen.