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

Konstantin Ananyev
2025-11-28T14:02:39Z

> +/**
> + * 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'?

> + /* 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;
> +}