>>> On 25.01.18 at 15:46, <andrew.coop...@citrix.com> wrote: > On 25/01/18 14:36, Jan Beulich wrote: >>>>> On 25.01.18 at 15:12, <andrew.coop...@citrix.com> wrote: >>> On 25/01/18 13:08, Jan Beulich wrote: >>>> It may also be worthwhile again to pull up the zeroing of %edx, >>>> using %dl instead of $0 in the movb (and maybe then also >>>> similarly in DO_SPEC_CTRL_EXIT_TO_XEN, but there I'm less >>>> certain it couldn't have a negative effect). >>> What negative effects are you worried about? These macros are self >>> contained. >> The result of the xor then is an input to the cmp, which may be >> one cycle more latency than with the immediate zero. > > Why? Cmp writes all flags and reads none of them, so doesn't have any > flags dependency on earlier instructions. > > It is only instructions which partially preserve older flag bits (and > where undefined behaviour is complicated) which suffer flags-merge > penalties.
I'm not talking about an eflags dependency, but that on the register (%edx being written by xor, %dl used by cmp). Jan _______________________________________________ Xen-devel mailing list Xen-devel@lists.xenproject.org https://lists.xenproject.org/mailman/listinfo/xen-devel
