RE: [PATCH v5] eal/x86: optimize memcpy of small sizes

Morten Brørup
2026-01-03T17:53:14Z

PING for review.

Regardless if you - for good and valid reasons - generally prefer memcpy() over rte_memcpy(), rte_memcpy() is not deprecated or being phased out, so improvements should be allowed.

> From: Morten Brørup [mailto:mb@smartsharesystems.com]
> Sent: Monday, 1 December 2025 16.55
>
> eal/x86: optimize memcpy of small sizes
>
> 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 typically only takes 2 cycles (4 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>
> ---
> v5:
> * Reverted v4: Replace SSE2 _mm_loadu_si128() with SSE3
> _mm_lddqu_si128().
> It was slower.
> * Improved some comments. (Konstantin Ananyev)
> * Moved the size range 17..32 inside the size <= 64 branch, so when
> building for SSE, the generated code can start copying the first
> 16 bytes before comparing if the size is greater than 32 or not.
> * Just require RTE_MEMCPY_AVX for using rte_mov32() in
> rte_mov33_to_64().
> v4:
> * Replace SSE2 _mm_loadu_si128() with SSE3 _mm_lddqu_si128().
> v3:
> * Fixed typo in comment.
> v2:
> * Updated patch title to reflect that the performance is improved.
> * Use the design pattern of two overlapping stores for small copies
> too.
> * Expanded first branch from size < 16 to size <= 16.
> * Handle more build time constant copy sizes.
> ---
> lib/eal/x86/include/rte_memcpy.h | 346 +++++++++++++++++--------------
> 1 file changed, 186 insertions(+), 160 deletions(-)
>
> diff --git a/lib/eal/x86/include/rte_memcpy.h
> b/lib/eal/x86/include/rte_memcpy.h
> index 46d34b8081..04d1a474d1 100644
> --- a/lib/eal/x86/include/rte_memcpy.h
> +++ b/lib/eal/x86/include/rte_memcpy.h
> @@ -55,52 +55,6 @@ extern "C" {
> static __rte_always_inline void *
> rte_memcpy(void *dst, const void *src, size_t n);
>
> -/**
> - * Copy bytes from one location to another,
> - * locations should not overlap.
> - * Use with n <= 15.
> - */
> -static __rte_always_inline void *
> -rte_mov15_or_less(void *dst, const void *src, size_t n)
> -{
> - /**
> - * Use the following structs to avoid violating C standard
> - * alignment requirements and to avoid strict aliasing bugs
> - */
> - struct __rte_packed_begin rte_uint64_alias {
> - uint64_t val;
> - } __rte_packed_end __rte_may_alias;
> - struct __rte_packed_begin rte_uint32_alias {
> - uint32_t val;
> - } __rte_packed_end __rte_may_alias;
> - struct __rte_packed_begin rte_uint16_alias {
> - uint16_t val;
> - } __rte_packed_end __rte_may_alias;
> -
> - void *ret = dst;
> - if (n & 8) {
> - ((struct rte_uint64_alias *)dst)->val =
> - ((const struct rte_uint64_alias *)src)->val;
> - src = (const uint64_t *)src + 1;
> - dst = (uint64_t *)dst + 1;
> - }
> - if (n & 4) {
> - ((struct rte_uint32_alias *)dst)->val =
> - ((const struct rte_uint32_alias *)src)->val;
> - src = (const uint32_t *)src + 1;
> - dst = (uint32_t *)dst + 1;
> - }
> - if (n & 2) {
> - ((struct rte_uint16_alias *)dst)->val =
> - ((const struct rte_uint16_alias *)src)->val;
> - src = (const uint16_t *)src + 1;
> - dst = (uint16_t *)dst + 1;
> - }
> - if (n & 1)
> - *(uint8_t *)dst = *(const uint8_t *)src;
> - return ret;
> -}
> -
> /**
> * Copy 16 bytes from one location to another,
> * locations should not overlap.
> @@ -132,6 +86,23 @@ rte_mov32(uint8_t *dst, const uint8_t *src)
> #endif
> }
>
> +/**
> + * Copy 48 bytes from one location to another,
> + * locations should not overlap.
> + */
> +static __rte_always_inline void
> +rte_mov48(uint8_t *dst, const uint8_t *src)
> +{
> +#if defined RTE_MEMCPY_AVX
> + rte_mov32((uint8_t *)dst, (const uint8_t *)src);
> + rte_mov32((uint8_t *)dst - 32 + 48, (const uint8_t *)src - 32 +
> 48);
> +#else /* SSE implementation */
> + rte_mov16((uint8_t *)dst + 0 * 16, (const uint8_t *)src + 0 *
> 16);
> + rte_mov16((uint8_t *)dst + 1 * 16, (const uint8_t *)src + 1 *
> 16);
> + rte_mov16((uint8_t *)dst + 2 * 16, (const uint8_t *)src + 2 *
> 16);
> +#endif
> +}
> +
> /**
> * Copy 64 bytes from one location to another,
> * locations should not overlap.
> @@ -172,6 +143,137 @@ rte_mov256(uint8_t *dst, const uint8_t *src)
> rte_mov128(dst + 1 * 128, src + 1 * 128);
> }
>
> +/**
> + * 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].
> + */
> +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;
> + }
> + }
> +
> + /*
> + * Note: Using "n & X" generates 3-byte "test" instructions,
> + * instead of "n >= X", which would generate 4-byte "cmp"
> instructions.
> + */
> + 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;
> +}
> +
> +/**
> + * Copy bytes from one location to another,
> + * locations should not overlap.
> + * Use with 17 (or 16) < n <= 32.
> + */
> +static __rte_always_inline void *
> +rte_mov17_to_32(void *dst, const void *src, size_t n)
> +{
> + /* Faster way when size is known at build time. */
> + if (__rte_constant(n)) {
> + if (n == 16) {
> + rte_mov16((uint8_t *)dst, (const uint8_t *)src);
> + return dst;
> + }
> + if (n == 18) /* 16 + 2 */
> + return memcpy(dst, src, 18);
> + if (n == 20) /* 16 + 4 */
> + return memcpy(dst, src, 20);
> + if (n == 24) /* 16 + 8 */
> + return memcpy(dst, src, 24);
> + if (n == 32) {
> + rte_mov32((uint8_t *)dst, (const uint8_t *)src);
> + return dst;
> + }
> + }
> +
> + /* Copy 17 (or 16) ~ 32 bytes. */
> + rte_mov16((uint8_t *)dst, (const uint8_t *)src);
> + rte_mov16((uint8_t *)dst - 16 + n, (const uint8_t *)src - 16 +
> n);
> + return dst;
> +}
> +
> +/**
> + * Copy bytes from one location to another,
> + * locations should not overlap.
> + * Use with 33 (or 32) < n <= 64.
> + */
> +static __rte_always_inline void *
> +rte_mov33_to_64(void *dst, const void *src, size_t n)
> +{
> + /* Faster way when size is known at build time. */
> + if (__rte_constant(n)) {
> + if (n == 32) {
> + rte_mov32((uint8_t *)dst, (const uint8_t *)src);
> + return dst;
> + }
> + if (n == 36) /* 32 + 4 */
> + return memcpy(dst, src, 36);
> + if (n == 40) /* 32 + 8 */
> + return memcpy(dst, src, 40);
> + if (n == 48) {
> + rte_mov48((uint8_t *)dst, (const uint8_t *)src);
> + return dst;
> + }
> + if (n == 64) {
> + rte_mov64((uint8_t *)dst, (const uint8_t *)src);
> + return dst;
> + }
> + }
> +
> + /* Copy 33 (or 32) ~ 64 bytes. */
> +#if defined RTE_MEMCPY_AVX
> + rte_mov32((uint8_t *)dst, (const uint8_t *)src);
> + rte_mov32((uint8_t *)dst - 32 + n, (const uint8_t *)src - 32 +
> n);
> +#else /* SSE implementation */
> + rte_mov16((uint8_t *)dst + 0 * 16, (const uint8_t *)src + 0 *
> 16);
> + rte_mov16((uint8_t *)dst + 1 * 16, (const uint8_t *)src + 1 *
> 16);
> + if (n > 48)
> + rte_mov16((uint8_t *)dst + 2 * 16, (const uint8_t *)src + 2
> * 16);
> + rte_mov16((uint8_t *)dst - 16 + n, (const uint8_t *)src - 16 +
> n);
> +#endif
> + return dst;
> +}
> +
> #if defined __AVX512F__ && defined RTE_MEMCPY_AVX512
>
> /**
> @@ -232,45 +334,21 @@ rte_mov512blocks(uint8_t *dst, const uint8_t
> *src, size_t n)
> }
> }
>
> +/**
> + * Copy bytes from one location to another,
> + * locations should not overlap.
> + * Use with n > 64.
> + */
> static __rte_always_inline void *
> -rte_memcpy_generic(void *dst, const void *src, size_t n)
> +rte_memcpy_generic_more_than_64(void *dst, const void *src, size_t n)
> {
> void *ret = dst;
> size_t dstofss;
> size_t bits;
>
> - /**
> - * Copy less than 16 bytes
> - */
> - if (n < 16) {
> - return rte_mov15_or_less(dst, src, n);
> - }
> -
> /**
> * Fast way when copy size doesn't exceed 512 bytes
> */
> - if (__rte_constant(n) && n == 32) {
> - rte_mov32((uint8_t *)dst, (const uint8_t *)src);
> - return ret;
> - }
> - if (n <= 32) {
> - rte_mov16((uint8_t *)dst, (const uint8_t *)src);
> - if (__rte_constant(n) && n == 16)
> - return ret; /* avoid (harmless) duplicate copy */
> - rte_mov16((uint8_t *)dst - 16 + n,
> - (const uint8_t *)src - 16 + n);
> - return ret;
> - }
> - if (__rte_constant(n) && n == 64) {
> - rte_mov64((uint8_t *)dst, (const uint8_t *)src);
> - return ret;
> - }
> - if (n <= 64) {
> - rte_mov32((uint8_t *)dst, (const uint8_t *)src);
> - rte_mov32((uint8_t *)dst - 32 + n,
> - (const uint8_t *)src - 32 + n);
> - return ret;
> - }
> if (n <= 512) {
> if (n >= 256) {
> n -= 256;
> @@ -381,41 +459,21 @@ rte_mov128blocks(uint8_t *dst, const uint8_t
> *src, size_t n)
> }
> }
>
> +/**
> + * Copy bytes from one location to another,
> + * locations should not overlap.
> + * Use with n > 64.
> + */
> static __rte_always_inline void *
> -rte_memcpy_generic(void *dst, const void *src, size_t n)
> +rte_memcpy_generic_more_than_64(void *dst, const void *src, size_t n)
> {
> void *ret = dst;
> size_t dstofss;
> size_t bits;
>
> - /**
> - * Copy less than 16 bytes
> - */
> - if (n < 16) {
> - return rte_mov15_or_less(dst, src, n);
> - }
> -
> /**
> * Fast way when copy size doesn't exceed 256 bytes
> */
> - if (__rte_constant(n) && n == 32) {
> - rte_mov32((uint8_t *)dst, (const uint8_t *)src);
> - return ret;
> - }
> - if (n <= 32) {
> - rte_mov16((uint8_t *)dst, (const uint8_t *)src);
> - if (__rte_constant(n) && n == 16)
> - return ret; /* avoid (harmless) duplicate copy */
> - rte_mov16((uint8_t *)dst - 16 + n,
> - (const uint8_t *)src - 16 + n);
> - return ret;
> - }
> - if (n <= 64) {
> - rte_mov32((uint8_t *)dst, (const uint8_t *)src);
> - rte_mov32((uint8_t *)dst - 32 + n,
> - (const uint8_t *)src - 32 + n);
> - return ret;
> - }
> if (n <= 256) {
> if (n >= 128) {
> n -= 128;
> @@ -573,38 +631,22 @@ rte_memcpy_generic(void *dst, const void *src,
> size_t n)
> }
> \
> }
>
> +/**
> + * Copy bytes from one location to another,
> + * locations should not overlap.
> + * Use with n > 64.
> + */
> static __rte_always_inline void *
> -rte_memcpy_generic(void *dst, const void *src, size_t n)
> +rte_memcpy_generic_more_than_64(void *dst, const void *src, size_t n)
> {
> __m128i xmm0, xmm1, xmm2, xmm3, xmm4, xmm5, xmm6, xmm7, xmm8;
> void *ret = dst;
> size_t dstofss;
> size_t srcofs;
>
> - /**
> - * Copy less than 16 bytes
> - */
> - if (n < 16) {
> - return rte_mov15_or_less(dst, src, n);
> - }
> -
> /**
> * Fast way when copy size doesn't exceed 512 bytes
> */
> - if (n <= 32) {
> - rte_mov16((uint8_t *)dst, (const uint8_t *)src);
> - if (__rte_constant(n) && n == 16)
> - return ret; /* avoid (harmless) duplicate copy */
> - rte_mov16((uint8_t *)dst - 16 + n, (const uint8_t *)src -
> 16 + n);
> - return ret;
> - }
> - if (n <= 64) {
> - rte_mov32((uint8_t *)dst, (const uint8_t *)src);
> - if (n > 48)
> - rte_mov16((uint8_t *)dst + 32, (const uint8_t *)src +
> 32);
> - rte_mov16((uint8_t *)dst - 16 + n, (const uint8_t *)src -
> 16 + n);
> - return ret;
> - }
> if (n <= 128) {
> goto COPY_BLOCK_128_BACK15;
> }
> @@ -696,44 +738,16 @@ rte_memcpy_generic(void *dst, const void *src,
> size_t n)
>
> #endif /* __AVX512F__ */
>
> +/**
> + * Copy bytes from one vector register size aligned location to
> another,
> + * locations should not overlap.
> + * Use with n > 64.
> + */
> static __rte_always_inline void *
> -rte_memcpy_aligned(void *dst, const void *src, size_t n)
> +rte_memcpy_aligned_more_than_64(void *dst, const void *src, size_t n)
> {
> void *ret = dst;
>
> - /* Copy size < 16 bytes */
> - if (n < 16) {
> - return rte_mov15_or_less(dst, src, n);
> - }
> -
> - /* Copy 16 <= size <= 32 bytes */
> - if (__rte_constant(n) && n == 32) {
> - rte_mov32((uint8_t *)dst, (const uint8_t *)src);
> - return ret;
> - }
> - if (n <= 32) {
> - rte_mov16((uint8_t *)dst, (const uint8_t *)src);
> - if (__rte_constant(n) && n == 16)
> - return ret; /* avoid (harmless) duplicate copy */
> - rte_mov16((uint8_t *)dst - 16 + n,
> - (const uint8_t *)src - 16 + n);
> -
> - return ret;
> - }
> -
> - /* Copy 32 < size <= 64 bytes */
> - if (__rte_constant(n) && n == 64) {
> - rte_mov64((uint8_t *)dst, (const uint8_t *)src);
> - return ret;
> - }
> - if (n <= 64) {
> - rte_mov32((uint8_t *)dst, (const uint8_t *)src);
> - rte_mov32((uint8_t *)dst - 32 + n,
> - (const uint8_t *)src - 32 + n);
> -
> - return ret;
> - }
> -
> /* Copy 64 bytes blocks */
> for (; n > 64; n -= 64) {
> rte_mov64((uint8_t *)dst, (const uint8_t *)src);
> @@ -751,10 +765,22 @@ rte_memcpy_aligned(void *dst, const void *src,
> size_t n)
> static __rte_always_inline void *
> rte_memcpy(void *dst, const void *src, size_t n)
> {
> + /* Common implementation for size <= 64 bytes. */
> + if (n <= 16)
> + return rte_mov16_or_less(dst, src, n);
> + if (n <= 64) {
> + /* Copy 17 ~ 64 bytes using vector instructions. */
> + if (n <= 32)
> + return rte_mov17_to_32(dst, src, n);
> + else
> + return rte_mov33_to_64(dst, src, n);
> + }
> +
> + /* Implementation for size > 64 bytes depends on alignment with
> vector register size. */
> if (!(((uintptr_t)dst | (uintptr_t)src) & ALIGNMENT_MASK))
> - return rte_memcpy_aligned(dst, src, n);
> + return rte_memcpy_aligned_more_than_64(dst, src, n);
> else
> - return rte_memcpy_generic(dst, src, n);
> + return rte_memcpy_generic_more_than_64(dst, src, n);
> }
>
> #undef ALIGNMENT_MASK
> --
> 2.43.0