Question

I have 3 different machines with the following configurations:

1. OpenSuSe 12.1, linux kernel 3.1.10, gcc 4.6.2
2. Debian 6, linux kernel 2.6.32, gcc 4.4.5
3. CentOS 5.6, linux kernel 2.6.18, gcc 4.1.2

All their architectures are x86_64. Note that CentOS’ software versions are older than Debian’s, but versions of OpenSuSe software are newer than Debian’s versions.

And I have the following example code:

#include <cstdio>
#include <cstdlib>

unsigned int cols=5;
unsigned int rows=6;

int main()
{
        //allocating...
        double **mat=new double*[rows];
        double *col=new double[rows];
        for(unsigned int i=0;i<rows;++i)
        {
                mat[i]=new double[cols];
        }

        //filling with something...
        for(unsigned int i=0;i<rows;++i)
        {
                for(unsigned int j=0;j<cols;++j)
                {
                        mat[i][j]=i+j;
                }
        }

        //testing...
        unsigned long long sum,add;
        for(unsigned int i=0;i<cols;++i)
        {
                sum=0;
                for(unsigned int j=0;j<rows;++j)
                {
                        col[j]=mat[j][i];
                        add=*((unsigned long long*) (&(col[j])));
                        sum+=add;
                }

                printf("%llu\n",sum);
        }

        return 0;
}

If I compile this code without any options:

g++ code.cpp

it runs in the same way on all machines.

But if I compile it with -O2, it behaves in the same way on the 1st and the 3rd machines, but on the 2nd machine (Debian) the -O2 optimization breaks it.

This is the output of this code on the 1st machine with -O2:

4619567317775286272
9238008735643729920
9250393634618998784
9259400833873739776
9266719183268216832

And this is on the 2nd machine with -O2:

0
4619567317775286272
9238008735643729920
9250393634618998784
9259400833873739776

The output of the second machine looks like the 1st’ output, shifted down by one line.

If I replace (&(col[j])) with (&(mat[j][i])), the code begins to work normaly.

Here is the compiler’s output on the 2nd machine with -v and -O2 options:

Using built-in specs.
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 4.4.5-8' --with-bugurl=file:///usr/share/doc/gcc-4.4/README.Bugs --enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-4.4 --enable-shared --enable-multiarch --enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.4 --libdir=/usr/lib --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug --enable-objc-gc --with-arch-32=i586 --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.4.5 (Debian 4.4.5-8) 
COLLECT_GCC_OPTIONS='-v' '-O2' '-shared-libgcc' '-mtune=generic'
 /usr/lib/gcc/x86_64-linux-gnu/4.4.5/cc1plus -quiet -v -D_GNU_SOURCE dmtest.cpp -quiet -dumpbase dmtest.cpp -mtune=generic -auxbase dmtest -O2 -version -o /tmp/cc6v7CNY.s
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/4.4.5/../../../../x86_64-linux-gnu/include"
ignoring nonexistent directory "/usr/include/x86_64-linux-gnu"
#include "..." search starts here:
#include <...> search starts here:
 /usr/include/c++/4.4
 /usr/include/c++/4.4/x86_64-linux-gnu
 /usr/include/c++/4.4/backward
 /usr/local/include
 /usr/lib/gcc/x86_64-linux-gnu/4.4.5/include
 /usr/lib/gcc/x86_64-linux-gnu/4.4.5/include-fixed
 /usr/include
End of search list.
GNU C++ (Debian 4.4.5-8) version 4.4.5 (x86_64-linux-gnu)
        compiled by GNU C version 4.4.5, GMP version 4.3.2, MPFR version 3.0.0-p3.
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 5a2e15051eaa06a84cf6320b754ba993
COLLECT_GCC_OPTIONS='-v' '-O2' '-shared-libgcc' '-mtune=generic'
 as -V -Qy -o /tmp/ccL37GHG.o /tmp/cc6v7CNY.s
GNU assembler version 2.20.1 (x86_64-linux-gnu) using BFD version (GNU Binutils for Debian) 2.20.1-system.20100303
COMPILER_PATH=/usr/lib/gcc/x86_64-linux-gnu/4.4.5/:/usr/lib/gcc/x86_64-linux-gnu/4.4.5/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/4.4.5/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/4.4.5/:/usr/lib/gcc/x86_64-linux-gnu/
LIBRARY_PATH=/usr/lib/gcc/x86_64-linux-gnu/4.4.5/:/usr/lib/gcc/x86_64-linux-gnu/4.4.5/:/usr/lib/gcc/x86_64-linux-gnu/4.4.5/../../../../lib/:/lib/../lib/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-linux-gnu/4.4.5/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-O2' '-shared-libgcc' '-mtune=generic'
 /usr/lib/gcc/x86_64-linux-gnu/4.4.5/collect2 --build-id --eh-frame-hdr -m elf_x86_64 --hash-style=both -dynamic-linker /lib64/ld-linux-x86-64.so.2 /usr/lib/gcc/x86_64-linux-gnu/4.4.5/../../../../lib/crt1.o /usr/lib/gcc/x86_64-linux-gnu/4.4.5/../../../../lib/crti.o /usr/lib/gcc/x86_64-linux-gnu/4.4.5/crtbegin.o -L/usr/lib/gcc/x86_64-linux-gnu/4.4.5 -L/usr/lib/gcc/x86_64-linux-gnu/4.4.5 -L/usr/lib/gcc/x86_64-linux-gnu/4.4.5/../../../../lib -L/lib/../lib -L/usr/lib/../lib -L/usr/lib/gcc/x86_64-linux-gnu/4.4.5/../../.. /tmp/ccL37GHG.o -lstdc++ -lm -lgcc_s -lgcc -lc -lgcc_s -lgcc /usr/lib/gcc/x86_64-linux-gnu/4.4.5/crtend.o /usr/lib/gcc/x86_64-linux-gnu/4.4.5/../../../../lib/crtn.o

What’s the reason of such behavior? Is my code incorrect or it’s a bug in gcc 4.4.5?

Answer 1

According to this blog entry, GCC 4.4 enforces strict aliasing rules for -O2. The type-punning access therefore results in undefined behaviour. Unfortunately, in order to get warnings for breaking strict aliasing rules, you have to compile with -Wstrict-aliasing.

Maybe the newer versions of GCC have reverted to the old behaviour and leave the decision whether to enable strict aliasing to the user.