Re: [boost] [1.35][functional] gcc-4.2:dereferencing type-punnedpointer will break strict-aliasing rules

From: Matt Doyle (mdoyle_at_[hidden])
Date: 2008-04-18 11:34:32

> [mailto:boost-bounces_at_[hidden]] On Behalf Of Giovanni 
> Piero Deretta
> Sent: Friday, April 18, 2008 8:26 AM
> To: boost_at_[hidden]
> Subject: Re: [boost] [1.35][functional] gcc-4.2:dereferencing 
> type-punnedpointer will break strict-aliasing rules
> 
> On Thu, Apr 17, 2008 at 4:22 AM, Mat Marcus 
> <mat-lists_at_[hidden]> wrote:
> > Hi,
> >
> >  In boost 1.35.0 I'm getting many of the following warnings from  
> > gcc-4.2.0 under cygwin when using boost/functional/hash:
> >
> >  ../boost_libraries/boost/functional/detail/hash_float.hpp: In 
> > function  'size_t boost::hash_detail::float_hash_impl(float)':
> >  
> ../boost_libraries/boost/functional/detail/hash_float.hpp:75: warning:
> >  dereferencing type-punned pointer will break strict-aliasing rules
> >  ../boost_libraries/boost/functional/detail/hash_float.hpp: In 
> > function  'size_t boost::hash_detail::float_hash_impl(double)':
> >  
> ../boost_libraries/boost/functional/detail/hash_float.hpp:82: warning:
> >  dereferencing type-punned pointer will break strict-aliasing rules
> >  ../boost_libraries/boost/functional/detail/hash_float.hpp: In 
> > function  'size_t boost::hash_detail::float_hash_impl(long double)':
> >  
> ../boost_libraries/boost/functional/detail/hash_float.hpp:90: warning:
> >  dereferencing type-punned pointer will break strict-aliasing rules
> >
> >  Is this a boost bug, or a spurious warning? Is there a fix 
> or a sound  
> > workaround?
> 
> The warning is correct. GCC can produce wrong code with 
> -fstrict-aliasing (the default).
> Casting a float* to to uint32_t* and dereferencing the result 
> is UB (if the pointer didn't point to an integer in the first 
> place of course).
> 
> Changing:
> 
>        inline std::size_t float_hash_impl(float v)
>        {
>             boost::uint32_t* ptr = (boost::uint32_t*)&v;
>             std::size_t seed = *ptr;
>             return seed;
>        }
> 
> to
> 
>      inline std::size_t float_hash_impl(float v)
>      {
>             std::size_t seed;
>             std::memcpy(&x, &v, sizeof(x));
>             return seed;
>      }
> 

/x/seed/ ??