Re: [Boost-users] Bug in boost::phoenix ?

Subject: Re: [Boost-users] Bug in boost::phoenix ?
From: Joel de Guzman (joel_at_[hidden])
Date: 2011-02-24 19:12:51

On 2/24/2011 11:05 PM, Nicolas Savoire wrote:
> I am using boost::phoenix lambda functions and I am observing some weird behaviours
> when compiling with optimizations (-O3).
>
> The first issue is that compiling the following piece of code
>
> Compiling:
>    const float x = 1.0f;
>    const float y = 1.0f;
>    std::cout<<  (arg1+arg2)(x,y);
>
> with g++ -Wall gives the warning:
> boost_1_46_0/boost/spirit/home/phoenix/operator/arithmetic.hpp:74:5: warning: returning reference to temporary
>
> And I suspect a bug in the project I am working on is a consequence of this.
> It is as if a boost::for_each( rng, lambda_fun ) was optimized out.
> I have opened a ticket on this issue:
> https://svn.boost.org/trac/boost/ticket/5190
> I suspect the issue comes from the type deduction inside phoenix.
> I first discovered the issue with boost 1.45.0, but it still here in 1.46.0.
>
> The other issue I am facing is similar. Consider the following code:
>
> namespace bp = boost::phoenix;
> using boost::phoenix::arg_names::arg1;
>
> int i=0;
> int a[] = {0};
>
> template<class UnaryFunction>
> inline void foo( UnaryFunction fun )
> {
>    fun(i);
> }
>
> int main()
> {
>    foo( bp::ref(a)[arg1]++ );
>    std::cout<<  a[0]<<  " ";
>
>    (bp::ref(a)[arg1]++)(i);
>    std::cout<<  a[0]<<  "\n";
> }
>
> Compiled without optimizations, it corectly prints: 1 2
> Whereas compiled with g++-4.5.1 -O3 with boost 1.45.0, it prints: 0 1,
>   as if the first lambda invocation was removed.
>
> The issue disappears if I use g++-4.4 or boost 1.46.0. I tracked the changes
> between boost 1.45.0 and 1.46.0, and it seems related to
> https://svn.boost.org/trac/boost/changeset/66411
> applying the part of the changeset concerning:
> fusion/support/detail/is_mpl_sequence.hpp
> fusion/support/is_sequence.hpp
> fusion/support/sequence_base.hpp
> to boost 1.45.0 fixes the issue, but I can't see how this is related to the
> bug I observed.
> Is it a compiler bug, a phoenix bug or a mistake in my code ?

Could you try with Phoenix3 (on going mini-review) and see if this
is an issue? I can try and investigate with Phoenix2 if you can
provide a minimal cpp test case that I can run.

Regards,
-- 
Joel de Guzman
http://www.boostpro.com
http://boost-spirit.com