Re: [PATCH v5 03/17] Add string-maskoff.h generic header

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

On 05/01/23 19:49, Noah Goldstein wrote:
> On Mon, Sep 19, 2022 at 12:59 PM Adhemerval Zanella via Libc-alpha
> <libc-alpha@sourceware.org> wrote:
>>
>> 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));
> 
> This can just be (p & (-sizeof(p)) I think.

Indeed it is simpler.

> Other than that look goods.

Thanks.