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

[Adhemerval Zanella Netto]

On 12/09/25 07:53, Pincheng Wang wrote:
> 在 2025/9/12 03:45, Adhemerval Zanella Netto 写道:
>>
>>
>> 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.
>>
> 
> Thank you for your feedback. You're right, and I'll fix this in the next version. However, would it be appropriate to add an `#error` in the fallback branch, similar to how it's done in sysdeps/riscv/sys/asm.h?

Sure, although I think we already such tests somewhere else.