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

From: Mat Marcus (mat-lists_at_[hidden])
Date: 2008-04-18 14:24:30

On Fri, Apr 18, 2008 at 8:25 AM, Giovanni Piero Deretta
<gpderetta_at_[hidden]> wrote:
>
> 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;
>     }
>
> should work and should produce optimal code, at least with a recent
> gcc. Similar transforms could be applied to float_has_impl([long]
> double).
> BTW, is BOOST_HASH_USE_x86_BINARY_HASH only defined for __CYGWIN__ or
> I'm reading the #ifdefs  incorrectly?
>
> --
> gpd

Thanks for the explanation and the patch.

 - Mat