[PATCH 03/17] Add string-maskoff.h generic header

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

Macros to operate on unaligned access for string operations:

  - create_mask: create a mask based on pointer alignment to sets up
    non-zero bytes before the beginning of the word so a following
    operation (such as find zero) might ignore these bytes.

  - highbit_mask: create a mask with high bit of each byte being 1,
    and the low 7 bits being all the opposite of the input.

These macros are meant to be used on optimized vectorized string
implementations.
---
 sysdeps/generic/string-maskoff.h | 73 ++++++++++++++++++++++++++++++++
 1 file changed, 73 insertions(+)
 create mode 100644 sysdeps/generic/string-maskoff.h

diff --git a/sysdeps/generic/string-maskoff.h b/sysdeps/generic/string-maskoff.h
new file mode 100644
index 0000000000..831647bda6
--- /dev/null
+++ b/sysdeps/generic/string-maskoff.h
@@ -0,0 +1,73 @@
+/* Mask off bits.  Generic C version.
+   Copyright (C) 2022 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 _STRING_MASKOFF_H
+#define _STRING_MASKOFF_H 1
+
+#include <endian.h>
+#include <limits.h>
+#include <stdint.h>
+#include <string-optype.h>
+
+/* Provide a mask based on the pointer alignment that sets up non-zero
+   bytes before the beginning of the word.  It is used to mask off
+   undesirable bits from an aligned read from an unaligned pointer.
+   For instance, on a 64 bits machine with a pointer alignment of
+   3 the function returns 0x0000000000ffffff for LE and 0xffffff0000000000
+   (meaning to mask off the initial 3 bytes).  */
+static inline op_t
+create_mask (uintptr_t i)
+{
+  i = i % sizeof (op_t);
+  if (__BYTE_ORDER == __LITTLE_ENDIAN)
+    return ~(((op_t)-1) << (i * CHAR_BIT));
+  else
+    return ~(((op_t)-1) >> (i * 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 inline op_t
+repeat_bytes (unsigned char c_in)
+{
+  return ((op_t)-1 / 0xff) * c_in;
+}
+
+/* Based on mask created by 'create_mask', mask off the high bit of each
+   byte in the mask.  It is used to mask off undesirable bits from an
+   aligned read from an unaligned pointer, and also taking care to avoid
+   match possible bytes meant to be matched.  For instance, on a 64 bits
+   machine with a mask created from a pointer with an alignment of 3
+   (0x0000000000ffffff) the function returns 0x7f7f7f0000000000 for BE
+   and 0x00000000007f7f7f for LE.  */
+static inline op_t
+highbit_mask (op_t m)
+{
+  return m & repeat_bytes (0x7f);
+}
+
+/* Return the address of the op_t word containing the address P.  For
+   instance on address 0x0011223344556677 and op_t with size of 8,
+   it returns 0x0011223344556670.  */
+static inline op_t *
+word_containing (char const *p)
+{
+  return (op_t *) (p - (uintptr_t) p % sizeof (op_t));
+}
+
+#endif /* _STRING_MASKOFF_H  */
-- 
2.34.1