riscv: Add Zbkb optimized repeat_bytes helper

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 instruction count and avoiding
high latency instructions. This helper is used by several mem and string
functions, and falls back to the generic implementation when Zbkb is not
present.

Signed-off-by: Pincheng Wang <pincheng.plct@isrc.iscas.ac.cn>
Reviewed-by: Peter Bergner <bergner@tenstorrent.com>

diff --git a/sysdeps/riscv/string-misc.h b/sysdeps/riscv/string-misc.h
new file mode 100644 (file)
index 0000000..02c5d52
--- /dev/null
+++ b/sysdeps/riscv/string-misc.h
@@ -0,0 +1,64 @@
+/* Miscellaneous functions used in string implementations.  RISC-V version.
+   Copyright (C) 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 __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 __riscv_xlen == 32
+  /* 8bit -> 16bit -> 32bit pattern replication */
+  __asm__ ("packh %0, %1, %1\n\t"
+           "pack %0, %0, %0"
+           : "=r"(c) : "r"(c));
+#else
+  #error unsupported xlen
+#endif
+  return c;
+}
+#else
+#include <sysdeps/generic/string-misc.h>
+#endif
+
+#endif /* _RISCV_STRING_MISC_H */