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rep stos has a high startup time even on modern microarchitectures like Skylake. Intel optimization manuals discuss how for small sizes it is beneficial to go for streaming stores. Since those cannot be used without extra penalty in the kernel I investigated performance impact of just regular movs. The patch below implements a very simple scheme: a 32-byte loop followed by filling in the remainder of at most 31 bytes. It has a 256 breaking point on which it falls back to rep stos. It provides a significant win over the current primitive on several machines I tested (both Intel and AMD). A 64-byte loop did not provide any benefit even for multiple of 64 sizes. See the review for benchmark data. Reviewed by: kib Approved by: re (gjb) Sponsored by: The FreeBSD Foundation Differential Revision: https://reviews.freebsd.org/D17398
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