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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

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