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

Konstantin Ananyev
2025-11-28T18:10:36Z

> > 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 */

Ok, why just not simply : if (n >= 8) then?

> >
> > > + /* 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;
> > > +}