https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80881
--- Comment #84 from LIU Hao <lh_mouse at 126 dot com> --- (In reply to Julian Waters from comment #83) > Liu Hao: The registers it's using seem to be all over the place. Prior it > was using rdx for the gs:[88] load and rax for everything else, now it's > either using any register it can find, or using rdx to store the result of > rdx+rax*8. I have no idea why the resulting assembly is so different, but > this could mean the resulting program runs less efficiently For EAX, ECX, EDX, EBX, ESI, EDI there's usually no difference, except that callee-saved ones (EBX, EBP, ESI and EDI) have to be pushed and popped. Accessing a 64-bit register, as well as R8D ~ R15D, requires an REX prefix, so such an instruction is one byte longer.
