RE: [PATCH v3] eal/x86: optimize memcpy of small sizes

Morten Brørup
2025-11-28T15:55:27Z

> From: Konstantin Ananyev [mailto:konstantin.ananyev@huawei.com]
> Sent: Friday, 28 November 2025 15.03
>
> > +/**
> > + * Copy bytes from one location to another,
> > + * locations should not overlap.
> > + * Use with n <= 16.
> > + *
> > + * Note: Copying uninitialized memory is perfectly acceptable.
> > + * Using e.g. memcpy(dst, src, 8) instead of
> > + * *(unaligned_uint64_t*) = *(const unaligned_uint64_t *)src
> > + * avoids compiler warnings about source data may be uninitialized
> > + * [-Wmaybe-uninitialized].
> > + *
> > + * Note: Using "n & X" generates 3-byte "test" instructions,
> > + * instead of "n >= X", which would generate 4-byte "cmp"
> instructions.
> > + */
> > +static __rte_always_inline void *
> > +rte_mov16_or_less(void *dst, const void *src, size_t n)
> > +{
> > + /* Faster way when size is known at build time. */
> > + if (__rte_constant(n)) {
> > + if (n == 2)
> > + return memcpy(dst, src, 2);
> > + if (n == 4)
> > + return memcpy(dst, src, 4);
> > + if (n == 6) /* 4 + 2 */
> > + return memcpy(dst, src, 6);
> > + if (n == 8)
> > + return memcpy(dst, src, 8);
> > + if (n == 10) /* 8 + 2 */
> > + return memcpy(dst, src, 10);
> > + if (n == 12) /* 8 + 4 */
> > + return memcpy(dst, src, 12);
> > + if (n == 16) {
> > + rte_mov16((uint8_t *)dst, (const uint8_t *)src);
> > + return dst;
> > + }
> > + }
> > +
> > + if (n & 0x18) { /* n >= 8 */
>
> Probably 'n & 0x8'?

It's intentional, to catch n == 0x10 too.
It seems the associated comment should be more verbose. How about:
if (n & 0x18) { /* n >= 8, including n == 0x10, hence n & 0x18 */

>
> > + /* copy 8 ~ 16 bytes */
> > + memcpy(dst, src, 8);
> > + memcpy((uint8_t *)dst - 8 + n, (const uint8_t *)src - 8 +
> n, 8);
> > + } else if (n & 0x4) {
> > + /* copy 4 ~ 7 bytes */
> > + memcpy(dst, src, 4);
> > + memcpy((uint8_t *)dst - 4 + n, (const uint8_t *)src - 4 +
> n, 4);
> > + } else if (n & 0x2) {
> > + /* copy 2 ~ 3 bytes */
> > + memcpy(dst, src, 2);
> > + memcpy((uint8_t *)dst - 2 + n, (const uint8_t *)src - 2 +
> n, 2);
> > + } else if (n & 0x1) {
> > + /* copy 1 byte */
> > + memcpy(dst, src, 1);
> > + }
> > + return dst;
> > +}