[PATCH 1/1] riscv: Add Zbkb optimized repeat_bytes helper

[Adhemerval Zanella Netto]

On 11/09/25 12:48, Pincheng Wang wrote:
> Introduce a RISC-V specific string-misc.h to provide an optimized
> repeat_bytes implementation when the Zbkb extension is available.
> The new version uses packh/packw/pack instructions instead of the
> generic multiply-based approach, reducing instruction count and avoiding
> a high-latency mul. This helper is used by memchr and memrchr, and falls
> back to the generic implementation when Zbkb is not present.
> 
> Signed-off-by: Pincheng Wang <pincheng.plct@isrc.iscas.ac.cn>
> ---
>  sysdeps/riscv/string-misc.h | 64 +++++++++++++++++++++++++++++++++++++
>  1 file changed, 64 insertions(+)
>  create mode 100644 sysdeps/riscv/string-misc.h
> 
> diff --git a/sysdeps/riscv/string-misc.h b/sysdeps/riscv/string-misc.h
> new file mode 100644
> index 0000000000..8d7af1ffd6
> --- /dev/null
> +++ b/sysdeps/riscv/string-misc.h
> @@ -0,0 +1,64 @@
> +/* Miscellaneous functions used in string implementations.  Generic C version.
> +   Copyright (C) 2023-2025 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <http://www.gnu.org/licenses/>.  */
> +
> +#ifndef _RISCV_STRING_MISC_H
> +#define _RISCV_STRING_MISC_H 1
> +
> +#include <limits.h>
> +#include <endian.h>
> +#include <string-optype.h>
> +
> +#if defined __riscv_zbkb
> +/* Extract the byte at index IDX from word X, with index 0 being the
> +   least significant byte.  */
> +static __always_inline unsigned char
> +extractbyte (op_t x, unsigned int idx)
> +{
> +  if (__BYTE_ORDER == __LITTLE_ENDIAN)
> +    return x >> (idx * CHAR_BIT);
> +  else
> +    return x >> (sizeof (x) - 1 - idx) * CHAR_BIT;
> +}
> +
> +/* Setup an word with each byte being c_in.  For instance, on a 64 bits
> +   machine with input as 0xce the functions returns 0xcececececececece.  */
> +static __always_inline op_t
> +repeat_bytes (unsigned char c_in)
> +{
> +  op_t c = c_in;
> +#if defined __riscv_zbkb && __riscv_xlen == 64
> +  /* 8bit -> 16bit -> 32bit -> 64bit pattern replication */
> +  __asm__ ("packh %0, %1, %1\n\t"
> +           "packw %0, %0, %0\n\t"
> +           "pack %0, %0, %0"
> +           : "=r"(c) : "r"(c));
> +#elif defined __riscv_zbkb && __riscv_xlen == 32
> +  /* 8bit -> 16bit -> 32bit pattern replication */
> +  __asm__ ("packh %0, %1, %1\n\t"
> +           "pack %0, %0, %0"
> +           : "=r"(c) : "r"(c));
> +#else
> +  c = ((op_t)-1 / 0xff) * c_in;
> +#endif

This is already within 'defined __riscv_zbkb', so there is no need for the
fallback nor check if __riscv_zbkb is defined. Just add a variant for
__riscv_xlen == 64 and another for 32.

> +  return c;
> +}
> +#else
> +#include <sysdeps/generic/string-misc.h>
> +#endif
> +
> +#endif /* _RISCV_STRING_MISC_H */