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VZEROUPPER

Author: Agner Date: 2016-12-28 03:57

zboson wrote:

it appears that VZEROUPPER is even more important in some situations with Skylake then e.g. with Haswell.

I just made some experiments on a Haswell. The dirty state makes every non-VEX vector operation dependent on the previous value of the destination register. For example
vpor ymm1,ymm2,ymm3 ... pxor xmm4,xmm4 ; Has false dependence on ymm4

I have no access to a Skylake at the moment, but it may behave the same. This means that VZEROUPPER is still needed on all Intel processors, except Knights Landing.

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