[Bug c++/28777] New: Zerodetection (i != 0) compiled with -O2 don't work

[Bug c++/28777] New: Zerodetection (i != 0) compiled with -O2 don't work

[hgsawicki at web dot de]

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Compiling the following code with -O2 produces an buggy excecutable:
(Using no optimization or -O1 produces an valid excecutable)
~~~ snip ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
#include <iostream>

int main( void )
{
    for ( int i = 1; i != 0; i = i + 1 )
    {
        if ( ! (i % 100000000) )
        {
            std::cout << i << std::endl;

            if ( i == 0 )
            {
                std::cout << i << std::endl;
            }
        }
    }

    return 0;
}
~~~ snip ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Commands to build and execute:

~/c++work/multtest> g++ -O2 -o multtest main.cpp
~/c++work/multtest> ./multtest
100000000
200000000
:
-200000000
-100000000
0
100000000
200000000
:                       // and continues

== Environment =======================================

~/c++work/multtest> gcc --version
gcc (GCC) 4.1.1 (Gentoo 4.1.1)
Copyright (C) 2006 Free Software Foundation, Inc.
Dies ist freie Software; die Kopierbedingungen stehen in den Quellen. Es
gibt KEINE Garantie; auch nicht für MARKTGÄNGIGKEIT oder FÜR SPEZIELLE ZWECKE.

~/c++work/multtest> uname --all
Linux atlantis 2.6.17-gentoo-r5 #5 SMP PREEMPT Mon Aug 14 19:48:18 CEST 2006
i686 AMD Athlon(tm) XP 2500+ AuthenticAMD GNU/Linux

~/c++work/multtest> cat /proc/cpuinfo
processor       : 0
vendor_id       : AuthenticAMD
cpu family      : 6
model           : 10
model name      : AMD Athlon(tm) XP 2500+
stepping        : 0
cpu MHz         : 1837.783
cache size      : 512 KB
fdiv_bug        : no
hlt_bug         : no
f00f_bug        : no
coma_bug        : no
fpu             : yes
fpu_exception   : yes
cpuid level     : 1
wp              : yes
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov
pat pse36 mmx fxsr sse syscall mmxext 3dnowext 3dnow up ts
bogomips        : 3679.34


-- 
           Summary: Zerodetection (i != 0) compiled with -O2 don't work
           Product: gcc
           Version: 4.1.1
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: hgsawicki at web dot de


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28777

[Bug tree-optimization/28777] Zerodetection (i != 0) compiled with -O2 don't work

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2006-08-18 21:08 -------
This is most likely a dup of another bug which was fixed in 4.2.0.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|major                       |normal
          Component|c++                         |tree-optimization
           Keywords|                            |wrong-code


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28777

[Bug tree-optimization/28777] Zerodetection (i != 0) compiled with -O2 don't work

[rguenth at gcc dot gnu dot org]

------- Comment #2 from rguenth at gcc dot gnu dot org  2006-08-19 15:26 -------
It is signed overflow being undefined and so the i == 0 test being optimized
away.
Use -fwrapv or an unsigned loop counter.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28777

[Bug tree-optimization/28777] Zerodetection (i != 0) compiled with -O2 don't work

[pinskia at gcc dot gnu dot org]

------- Comment #3 from pinskia at gcc dot gnu dot org  2006-08-21 05:03 -------
(In reply to comment #2)
> It is signed overflow being undefined and so the i == 0 test being optimized
> away.
> Use -fwrapv or an unsigned loop counter.

And the "!= 0" is being optimized to 1 because signed overflow is undefined.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28777