https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78705
--- Comment #5 from David Malcolm <dmalcolm at gcc dot gnu.org> --- i386.md's define_expand "memory_blockage" has: operands[0] = gen_rtx_MEM (BLKmode, gen_rtx_SCRATCH (Pmode)); so it's due to the difference in Pmode between i386 and x86_64.
