http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53353
--- Comment #4 from Steven Fuerst <svfuerst at gmail dot com> 2012-05-15 02:09:05 UTC --- Actually, it is the rax:rdx pair that is most likely. ax:dx only has 32 bits. rax:rdx is specified by the 'A' constraint, the only gpr option that is 128 bits. (Yes, it would be nice to pass a 128 bit integer in some other pair of registers as an enhancement, but currently only the 'A' constraint works.)
