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

From: Bo Persson (bop_at_[hidden])
Date: 2008-04-19 11:30:50

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)  ?

Bo Persson