https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105023
--- Comment #6 from Segher Boessenkool <segher at gcc dot gnu.org> --- BLKmode is *not* valid for registers. reg:BLK at one time was a special marker for invalid asm operands, apparently. :BLK is for mem, and for parallel as well in some cases, but not for reg.
