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Re: [Xen-devel] [PATCH v2 03/23] x86: zero BSS using stosl instead of stosb

>>> On 22.07.15 at 10:42, <andrew.cooper3@xxxxxxxxxx> wrote:
> In the case of having aligned source and destination on a 16-byte
> boundary (which we can trivially arrange), then ERMSB (to give it its
> Intel name) and rep stosl differ only in the setup cost; they still
> scale at the same rate for changes in length.
> Therefore, assuming we arrange for 16-byte alignment, using rep stosl
> would appear to be a single 60ish cycle hit over using ERMSB, but would
> be substantially more efficient than using rep stosb on a non-ERMSB system.
> Overall, I think 16 byte alignment and rep stosl is the best compromise.

Or leaving such code alone, with the assumption that over time the
setup cost (on a growing number of systems) outweighs the benefits
(on a shrinking set).


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