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

From: Giovanni Piero Deretta (gpderetta_at_[hidden])
Date: 2008-04-18 11:51:03

On Fri, Apr 18, 2008 at 5:34 PM, Matt Doyle <mdoyle_at_[hidden]> wrote:
> > [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/ ??
>

Yes, of course :)

-- 
gpd