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

Konstantin Ananyev
2026-01-11T15:52:32Z

>
> 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);

Just a s thought: would compiler and CPU be smart enough to realize
that there is no dependency between these 2 ops, and they can be executed in any
order?
Might be do mov32(); mov16() instead?
Again' didn't test anything, just a thought.

> > +#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;
> > + }

If n is constant; wouldn't compiler unroll such memcpy itself?
Specially for such small (<=16) values?
I mean. can't we just:
If (n < 16) memcpy(dst, src, n); else rte_mov16(dst, src);

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