[Bug c++/17507] New: Problems with shift and increment operator precedence

[work at paul dot dubuc dot org]

The following code is derived from an effecient string hashing function and
seems to indicate a GCC compiler bug.  This code has been compiled with g++
2.95.3, 3.3.4 and 3.4.2 and behaves the same way in each case.  But it behaves
differently (and the way we would expect) when compiled with the Sun Forte C++
compiler.

#include <iostream>

int
main(int argc, char** argv)
{
    // Round address up to integral multiple
    const char* pc = argv[1];
    pc += 3;
    pc = reinterpret_cast<const char*>(reinterpret_cast<unsigned>(pc) & ~3);

    // Treat as pointer to unsigned, display address, and value
    const unsigned int*  pui = reinterpret_cast<const unsigned int*>(pc);
    std::cout << "         pui=" << (void*)pui << "  *pui=" << std::hex << *pui
<< std::endl;

    // Treat as pointer to unsigned char, display address
    const unsigned char* puc = reinterpret_cast<const unsigned char*>(pc);
    std::cout << "puc (before)=" << (void*)puc << std::endl;

    // Obtain value by shift and OR
    unsigned int uint = *puc++ << 24 | *puc++ << 16 | *puc++ << 8 | *puc++;

    // Display address and value
    std::cout << "puc  (after)=" << (void*)puc << "  uint=" << uint << std::endl;

    return 0;
}

Output when compiled with Forte C++:

 ./testf abcdef
         pui=ffbeea14  *pui=61626364
puc (before)=ffbeea14
puc  (after)=ffbeea18  uint=61626364

Output when compiled with GCC:

 ./testg abcdef
         pui=0xffbeea14  *pui=61626364
puc (before)=0xffbeea14
puc  (after)=0xffbeea15  uint=61616161

The pointer value, puc, is only incremented once (instead of 4 times).

We are trying to work around this problem, but other implementations produce
slower code and speed is important here.

g++ -v       
Reading specs from
/ldatae/gnu/gcc-3.3.4/bin/../lib/gcc-lib/sparc-sun-solaris2.8/3.3.4/specs
Configured with: ../gcc-3.3.4/configure
--prefix=/volws/pmd25/ldatae/gnu/gcc-3.3.4 --enable-shared --enable-threads
--enable-cpp --enable-languages=c++ --with-gnu-as
--with-as=/volws/pmd25/ldatae/gnu/gcc-3.3.4/bin/as --with-gnu-ld
--with-ld=/volws/pmd25/ldatae/gnu/gcc-3.3.4/bin/ld --host=sparc-sun-solaris2.8
Thread model: posix
gcc version 3.3.4

-- 
           Summary: Problems with shift and increment operator precedence
           Product: gcc
           Version: 3.3.4
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: work at paul dot dubuc dot org
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: sparc-sun-solaris2.8
  GCC host triplet: sparc-sun-solaris2.8
GCC target triplet: sparc-sun-solaris2.8


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17507