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

From: Giovanni Piero Deretta (gpderetta_at_[hidden])
Date: 2008-04-19 11:34:03

On Sat, Apr 19, 2008 at 5:30 PM, Bo Persson <bop_at_[hidden]> wrote:
>
> Giovanni Piero Deretta wrote:
>  > 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 :)
>
>
>  And also assuming that sizeof(seed) ==sizeof(float)  ?
>
Yes, but, as the code is only compiled in when under X86, you can
safely assume that.

-- 
gpd