Commits on Nov 23, 2017

1. Make float::from_bits transmute (and update the documentation to refl… …

   …ect this).
   
   The current implementation/documentation was made to avoid sNaN because of
   potential safety issues implied by old/bad LLVM documentation. These issues
   aren't real, so we can just make the implementation transmute (as permitted
   by the existing documentation of this method).
   
   Also the documentation didn't actually match the behaviour: it said we may
   change sNaNs, but in fact we canonicalized *all* NaNs.
   
   Also an example in the documentation was wrong: it said we *always* change
   sNaNs, when the documentation was explicitly written to indicate it was
   implementation-defined.
   
   This makes to_bits and from_bits perfectly roundtrip cross-platform, except
   for one caveat: although the 2008 edition of IEEE-754 specifies how to
   interpet the signaling bit, earlier editions didn't. This lead to some platforms
   picking the opposite interpretation, so all signaling NaNs on x86/ARM are quiet
   on MIPS, and vice-versa.
   
   NaN-boxing is a fairly important optimization, while we don't even guarantee
   that float operations properly preserve signalingness. As such, this seems like
   the more natural strategy to take (as opposed to trying to mangle the signaling
   bit on a per-platform basis).
   
   This implementation is also, of course, faster.

   

   Gankra committed Nov 23, 2017
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