On 14/02/18 13:04, zhenzhong.duan wrote: > > 2018年2月14日 20:21于 Andrew Cooper <andrew.coop...@citrix.com > <mailto: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? >
Yes. See the Zeroing Idioms section in the optimisation manual. Tracking which registers are zero helps breaks read-after-write data dependences in the pipeline. ~Andrew
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