On Fri, Apr 18, 2008 at 11:45 PM, Teemu Torma <teemu@torma.org> 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