Re: bug in gcc (GCC) 4.0.1 20050727 (Red Hat 4.0.1-5)

[Richard Guenther <richard dot guenther at gmail dot com>]

On 8/2/05, Mark Frazer <mark@mjfrazer.org> wrote:
> Hello.  I'm not on the list, so please CC me with any replies.
> 
> I have come across a bug found during some code which serializes
> doubles.  The bug is only encountered when the optimization level is set
> to -O2 or greater.
> 
> The bug is not encountered when compiled under
>         gcc (GCC) 3.3.3 20040412 (Red Hat Linux 3.3.3-7)
> at any optimization level.
> 
> The de-serialization code is:
> 
> typedef unsigned char uint8;
> static uint8 next_byte(uint &offset, std::vector<uint8> const &bytecode)
>    throw (std::invalid_argument)
> {
>    if (offset >= bytecode.size())
>       throw (std::invalid_argument("Unexpected end of bytecode"));
>    return bytecode[offset++];
> }
> 
> double parse_double(uint &offset, std::vector<unsigned char> const &bytecode)
>    throw (std::invalid_argument)
> {
>    typedef unsigned long long uint64;
>    uint64 rtn = uint64(next_byte(offset, bytecode)) << 56;
>    rtn |= uint64(next_byte(offset, bytecode)) << 48;
>    rtn |= uint64(next_byte(offset, bytecode)) << 40;
>    rtn |= uint64(next_byte(offset, bytecode)) << 32;
>    rtn |= uint64(next_byte(offset, bytecode)) << 24;
>    rtn |= uint64(next_byte(offset, bytecode)) << 16;
>    rtn |= uint64(next_byte(offset, bytecode)) << 8;
>    rtn |= uint64(next_byte(offset, bytecode));
>    return *reinterpret_cast<double*>(&rtn);
> }
> 
> Full source code to a demonstration of the bug, and a Makefile is at
>         http://mjfrazer.org/~mjfrazer/tmp/pack-test/
> The tar file in the directory contains all the other files, so you just
> need to grab that.

Try -fno-strict-aliasing.  This may be related to PR23192.

Richard.