Re: [PATCH v6] eal/x86: optimize memcpy of small sizes

Stephen Hemminger
2026-01-13T23:19:53Z

On Mon, 12 Jan 2026 12:03:37 +0000
Morten Brørup <mb@smartsharesystems.com> wrote:

> The implementation for copying up to 64 bytes does not depend on address
> alignment with the size of the CPU's vector registers, so the code
> handling this was moved from the various implementations to the common
> function.
>
> Furthermore, the function for copying less than 16 bytes was replaced with
> a smarter implementation using fewer branches and potentially fewer
> load/store operations.
> This function was also extended to handle copying of up to 16 bytes,
> instead of up to 15 bytes. This small extension reduces the code path for
> copying two pointers.
>
> These changes provide two benefits:
> 1. The memory footprint of the copy function is reduced.
> Previously there were two instances of the compiled code to copy up to 64
> bytes, one in the "aligned" code path, and one in the "generic" code path.
> Now there is only one instance, in the "common" code path.
> 2. The performance for copying up to 64 bytes is improved.
> The memcpy performance test shows cache-to-cache copying of up to 32 bytes
> now only takes 2 cycles (5 cycles for 64 bytes) versus ca. 6.5 cycles
> before this patch.
>
> And finally, the missing implementation of rte_mov48() was added.
>
> Signed-off-by: Morten Brørup <mb@smartsharesystems.com>
> ---

Not sure all this is really needed.
Would rather get the patch series in that used cocci script to
replace all the fixed size rte_memcpy() with memcpy().
The compiler inlining is safer and as fast or faster.

Be careful, there have been and still are bugs in DPDK rte_memcpy()
where it reads past the size of the source bytes.