[Bug c/48793] New: Optimization -O2 Incorrectly Removes Logic

["chris at durso dot org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48793

           Summary: Optimization -O2 Incorrectly Removes Logic
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: chris@durso.org


Created attachment 24118
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=24118
main.ii

When dealing with min_int (0x80000000, a peculiar value as min_int < 0 &&
min_int == -1*min_int) it appears that -O2 and above on on some versions of gcc
will under some circumstances optimize away particular logic for dealing with
this edge case.

Of the versions I've tested:

gcc version 4.1.2 20080704 (Red Hat 4.1.2-48) SUCCESSFUL
gcc version 4.2.1 (Apple Inc. build 5666) (dot 3) FAILS
gcc version 4.4.3 (GCC) Target: i386-pc-solaris2.10 FAILS


FAILURE EXAMPLE:

chdurso@searchdev12:m_index_query_hash_core> uname -a ;
/ebay/vendor/pkg/gcc-4.4.3/bin/g++ -v; /ebay/vendor/pkg/gcc-4.4.3/bin/g++ -Wall
-Wextra -save-temps -O2 -o hello main.cpp ; echo START_OUTPUT; ./hello
SunOS searchdev12.arch.ebay.com 5.10 Generic_120012-14 i86pc i386 i86pc Solaris
Using built-in specs.
Target: i386-pc-solaris2.10
Configured with: /ebay/vendor/Archive/gcc-4.4.3/configure
--prefix=/ebay/vendor/pkg/gcc-4.4.3 --with-gnu-as --with-as=/usr/sfw/bin/gas
--without-gnu-ld --with-ld=/usr/ccs/bin/ld --enable-shared
--enable-languages=c,c++
Thread model: posix
gcc version 4.4.3 (GCC) 
START_OUTPUT
The following number is tricky as it is the only negative number unpaired with
a positive analog in 2's compliment arithemetic.  It behaves unintuitively and
apparently breaks an optimization.

tricky: -2147483648

abs(tricky): -2147483648
 1 * tricky = -2147483648
-1 * tricky = -2147483648
tricky + 1 = -2147483647
tricky - 1 = 2147483647
test 0: tricky: -2147483648
test 1: tricky: -2147483648
Assertion failed: tricky == 0x7fffffff && "on -O2 and above this asserts
apparently drops last if statement", file main.cpp, line 35
Abort (core dumped)

// main.cpp
//

#include <iostream>
#include <stdlib.h>
#include <cassert>
using namespace std;


int main(int , const char**){

  cout <<  "The following number is tricky as it is the only negative number "
    "unpaired with a positive analog in 2's compliment arithemetic.  "  
       "It behaves unintuitively and apparently breaks an optimization."<< endl
<< endl;

  int tricky = 0x80000000;
  cout << "tricky: " << tricky << endl << endl;
  cout << "abs(tricky): "  << abs(tricky) << endl 
       << " 1 * tricky = " << 1* tricky   << endl
       << "-1 * tricky = " << -1* tricky  << endl
       << "tricky + 1 = "  <<  tricky + 1  << endl
       << "tricky - 1 = "  <<  tricky - 1  << endl;

  tricky = 0x1;
  for(int i(0); i < 31; ++i) // can't simply assign, but a loop here will
result allow failure
    tricky <<=  1;

  assert(tricky == int(0x80000000));
  // tricky = 0x80000000; this unnecessary assignment will cause it work
correctly
  cout << "test 0: tricky: " << tricky << endl;
  if (tricky<0) tricky = -tricky;       // should always get here
  cout << "test 1: tricky: " << tricky << endl;
  if (tricky < 0) tricky = 0x7fffffff;  // should always get here

  assert(tricky == 0x7fffffff && "on -O2 and above this asserts apparently
drops last if statement");

  return 0;
}

ATTACHED main.ii