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: 2008-04-19 11:31:20

On Fri, Apr 18, 2008 at 11:45 PM, Teemu Torma <teemu_at_[hidden]> wrote:
> On Friday 18 April 2008, Giovanni Piero Deretta wrote:
>  >      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.
>
>  Wouldn't casting using union be safe and efficient in all cases?
>  Something like:
>
>
>        inline std::size_t float_hash_impl(float v)
>        {
>               union { float v; std::size_t seed; } u;
>               u.v = v;
>               return u.seed;
>        }
>

This is UB according to the standard, but it happens to works in most
compilers  (gcc explicitly states that this is guaranteed to work),
but:

- memcpy produces already optimal code at least with gcc.
- gcc sometimes is more conservative with optimizations when it sees
an union (in fact I've seen gcc produce worse code with the union than
with the memcpy).

HTH,

-- 
gpd