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

["Morten Brørup" <mb@smartsharesystems.com>]

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