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

[Pincheng Wang]

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?

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