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

From: Giovanni Piero Deretta (gpderetta_at_[hidden])
Date: 2008-04-18 11:25:53

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