about "Switch default hash to SIPHASH on 64 bit builds and ONE_AT_A_TIME on 32 bit builds"

http://perl5.git.perl.org/perl.git/commit/3db6cbfca39da94d152d3e860e2aa79b9c6bb161 
why? what are the disadvantages? What is the difference between a 64 
build and a 32 bit build? What about compilers/platforms that have 
compiler emulated 64 bit integers, like 32 bit x86?

There was an IRC conversation I read, but I didn't see any answers to 
the above.