[Bug c++/51885] New: g++ compiler options -O2 and -O3 modifies program results

[laurentlouis.maurin at wanadoo dot fr]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51885

             Bug #: 51885
           Summary: g++ compiler options -O2 and -O3 modifies program
                    results
    Classification: Unclassified
           Product: gcc
           Version: 4.6.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: laurentlouis.mau...@wanadoo.fr


Created attachment 26356
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=26356
example program with comments

platform : Fedora 16 x86_64
compiler : gcc-c++-4.6.2-1.fc16.x86_64

see attaches source file ; it is an example of the quicksort algorithm written
with pointers and templates

one template is specialised and uses a reinterpret_cast (it is part of a bigger
file)

it sorts this array :  {12, -1, 2}

a) first test (compilation without any optimisation)

compilation : g++ main.cpp && ./a.out

result OK : {-1, 2, 12}

b) second test (-O2 optimisation)

compilation : g++ -O2 main.cpp && ./a.out

wrong result : {2, 12, -1}

3) third test (-O3 optimisation)

compilation : g++ -O3 main.cpp && ./a.out

wrong result : {2, 12, -1}



Strange behaviour : if i include between line #65 and line #67 (see attached
file)

std::cout<<*ptr_start<<std::endl;

then, the results are good in any case


i believe this is a bug ... the optimisation option should not change the
program result

best regards

Laurent