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

From: Giovanni Piero Deretta (gpderetta_at_[hidden])
Date: 2008-04-19 22:15:39

On Sat, Apr 19, 2008 at 8:38 PM, Bo Persson <bop_at_[hidden]> wrote:
> Giovanni Piero Deretta wrote:
>  > 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
>  >>  >>  >
>
> >>  >>  > 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.
>
>  Ok, I though it was used for x86_64 as well, but I might be wrong.
>

If I have read the preprocessor conditionals correctly, oddly enough
it will enabled only on cygwin/x86. In theory the same code should
work for all gccs (it should work for all x86 compilers in fact).

And yes, it will break for x86_64 cpus, where size_t is 64 bits, but
that is easily fixed.

-- 
gpd