2018年2月14日 20:21于 Andrew Cooper <andrew.coop...@citrix.com>写道:
>
> On 14/02/18 12:08, zhenzhong.duan wrote: 
> > 
> > > @@ -286,13 +286,13 @@ 
> > >      setz %dl 
> > >      and %dl, STACK_CPUINFO_FIELD(use_shadow_spec_ctrl)(%r14) 
> > Is it safe to remove the 'xor %edx, %edx' above? setz set whole byte 1 
> > or 0. 
> > 
>
> It is safe, but it is not a good idea. 
>
> Using setz is an 8bit operation, which will suffer a register merge 
> stall in the pipeline as we know for certain at this point that the 
> upper bits of %edx are nonzero at this point.  (An encoding which 
> allowed setz %eax would have been far more useful in 64bit code.) 
>
Thanks for explain. But just curious does processor has ideas the whole 
register is zeroed and avoid a merge?

thanks
zduan
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