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

[Pincheng Wang]

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
+  return c;
+}
+#else
+#include <sysdeps/generic/string-misc.h>
+#endif
+
+#endif /* _RISCV_STRING_MISC_H */
-- 
2.39.5