Need help with BOOST_FOREACH/compiler bug
- by Jacek Lawrynowicz
I know that boost or compiler should be last to blame, but I can't see another explanation here.
I'm using msvc 2008 SP1 and boost 1.43.
In the following code snippet execution never leaves third BOOST_FOREACH loop
typedef Graph<unsigned, unsigned>::VertexIterator Iter;
Graph<unsigned, unsigned> g;
g.createVertex(0x66);
// works fine
Iter it = g.getVertices().first, end = g.getVertices().second;
for(; it != end; ++it)
;
// fine
std::pair<Iter, Iter> p = g.getVertices();
BOOST_FOREACH(unsigned handle, p)
;
// fine
unsigned vertex_count = 0;
BOOST_FOREACH(unsigned handle, g.getVertices())
vertex_count++;
// oops, infinite loop
vertex_count = 0;
BOOST_FOREACH(unsigned handle, g.getVertices())
vertex_count++;
vertex_count = 0;
BOOST_FOREACH(unsigned handle, g.getVertices())
vertex_count++;
// ... last block repeated 7 times
Iterator code:
class Iterator
: public boost::iterator_facade<Iterator, unsigned const,
boost::bidirectional_traversal_tag>
{
public:
Iterator()
: list(NULL), handle(INVALID_ELEMENT_HANDLE)
{}
explicit Iterator(const VectorElementsList &list, unsigned handle = INVALID_ELEMENT_HANDLE)
: list(&list), handle(handle)
{}
friend std::ostream&
operator<<(std::ostream &s, const Iterator &it)
{
s << "[list: " << it.list <<", handle: " << it.handle << "]";
return s;
}
private:
friend class boost::iterator_core_access;
void increment()
{
handle = list->getNext(handle);
}
void decrement()
{
handle = list->getPrev(handle);
}
unsigned const& dereference() const
{
return handle;
}
bool equal(Iterator const& other) const
{
return handle == other.handle && list == other.list;
}
const VectorElementsList<T> *list;
unsigned handle;
};
Some ASM fun:
vertex_count = 0;
BOOST_FOREACH(unsigned handle, g.getVertices())
// initialization
013E1369 mov edi,dword ptr [___defaultmatherr+8 (13E5034h)] // end iterator handle: 0xFFFFFFFF
013E136F mov ebp,dword ptr [esp+0ACh] // begin iterator handle: 0x0
013E1376 lea esi,[esp+0A8h] // begin iterator list pointer
013E137D mov ebx,esi
013E137F nop
// forever loop begin
013E1380 cmp ebp,edi
013E1382 jne main+238h (13E1388h)
013E1384 cmp ebx,esi
013E1386 je main+244h (13E1394h)
013E1388 lea eax,[esp+18h]
013E138C push eax
// here iterator is incremented in ram
013E138D call boost::iterator_facade<detail::VectorElementsList<Graph<unsigned int,unsigned int>::VertexWrapper>::Iterator,unsigned int const ,boost::bidirectional_traversal_tag,unsigned int const &,int>::operator++ (13E18E0h)
013E1392 jmp main+230h (13E1380h)
vertex_count++;
// forever loop end
It's easy to see that iterator handle is cached in EBP and it never gets incremented despite of a call to iterator operator++() function.
I've replaced Itarator implmentation with one deriving from std::iterator and the issue persisted, so this is not iterator_facade fault.
This problem exists only on msvc 2008 SP1 x86 and amd64 release builds. Debug builds on msvc 2008 and debug/release builds on msvc 2010 and gcc 4.4 (linux) works fine.
Furthermore the BOOST_FOREACH block must be repeaded exacly 10 times. If it's repeaded 9 times, it's all OK.
I guess that due to BOOST_FOREACH use of template trickery (const auto_any), compiler assumes that iterator handle is constant and never reads its real value again.
I would be very happy to hear that my code is wrong, correct it and move on with BOOST_FOREACH, which I'm very found of (as opposed to BOOST_FOREVER :).
May be related to: http://stackoverflow.com/questions/1275852/why-does-boost-foreach-not-work-sometimes-with-c-strings
