On Thu, Aug 27, 2020 at 08:09:04PM +0200, Uros Bizjak wrote: > xorl %0,%0 is equivalent to xorq %0,%0 as both will zero the > entire register. Use xorl %0,%0 for all operand sizes to avoid > REX prefix byte when legacy registers are used and to avoid size > prefix byte when 16bit registers are used. > > Zeroing the full register is OK in this use case. xorl %0,%0 also > breaks register dependency chains, avoiding potential partial > register stalls with 8 and 16bit operands.
No objections, but talking about stalls is more than slightly ridiculous - we'd just taken a #PF, failed there, flipped pt_regs %rip to fixup section, returned from fault and are about to fail whatever syscall that had been; a stall here is really not an issue...
