Re: [PATCH v6 14/17] alpha: Add string-fzb.h and string-fzi.h

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

On 10/01/23 23:45, Richard Henderson wrote:
> On 1/10/23 13:01, Adhemerval Zanella wrote:
>> From: Richard Henderson <richard.henderson@linaro.org>
>>
>> While alpha has the more important string functions in assembly,
>> there are still a few for find the generic routines are used.
>>
>> Use the CMPBGE insn, via the builtin, for testing of zeros.  Use a
>> simplified expansion of __builtin_ctz when the insn isn't available.
>>
>> Checked on alpha-linux-gnu.
>> ---
>>   sysdeps/alpha/string-fzb.h |  52 +++++++++++++++++
>>   sysdeps/alpha/string-fzi.h | 113 +++++++++++++++++++++++++++++++++++++
>>   2 files changed, 165 insertions(+)
>>   create mode 100644 sysdeps/alpha/string-fzb.h
>>   create mode 100644 sysdeps/alpha/string-fzi.h
>>
>> diff --git a/sysdeps/alpha/string-fzb.h b/sysdeps/alpha/string-fzb.h
>> new file mode 100644
>> index 0000000000..e3934ba413
>> --- /dev/null
>> +++ b/sysdeps/alpha/string-fzb.h
>> @@ -0,0 +1,52 @@
>> +/* Zero byte detection; boolean.  Alpha version.
>> +   Copyright (C) 2023 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_FZB_H
>> +#define _STRING_FZB_H 1
>> +
>> +#include <sys/cdefs.h>
>> +#include <string-optype.h>
>> +
>> +/* Note that since CMPBGE creates a bit mask rather than a byte mask,
>> +   we cannot simply provide a target-specific string-fza.h.  */
>> +
>> +/* Determine if any byte within X is zero.  This is a pure boolean test.  */
>> +
>> +static __always_inline _Bool
>> +has_zero (op_t x)
>> +{
>> +  return __builtin_alpha_cmpbge (0, x) != 0;
>> +}
>> +
>> +/* Likewise, but for byte equality between X1 and X2.  */
>> +
>> +static __always_inline _Bool
>> +has_eq (op_t x1, op_t x2)
>> +{
>> +  return has_zero (x1 ^ x2);
>> +}
>> +
>> +/* Likewise, but for zeros in X1 and equal bytes between X1 and X2.  */
>> +
>> +static __always_inline _Bool
>> +has_zero_eq (op_t x1, op_t x2)
>> +{
>> +  return has_zero (x1) | has_eq (x1, x2);
>> +}
>> +
>> +#endif /* _STRING_FZB_H */
>> diff --git a/sysdeps/alpha/string-fzi.h b/sysdeps/alpha/string-fzi.h
>> new file mode 100644
>> index 0000000000..bc2f0bdc91
>> --- /dev/null
>> +++ b/sysdeps/alpha/string-fzi.h
>> @@ -0,0 +1,113 @@
>> +/* string-fzi.h -- zero byte detection; indices.  Alpha 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_FZI_H
>> +#define _STRING_FZI_H
>> +
>> +#include <limits.h>
>> +#include <string-optype.h>
>> +
>> +/* Note that since CMPBGE creates a bit mask rather than a byte mask,
>> +   we cannot simply provide a target-specific string-fza.h.  */
>> +
>> +/* A subroutine for the index_zero functions.  Given a bitmask C,
>> +   return the index of the first bit set in memory order.  */
>> +
>> +static __always_inline unsigned int
>> +index_first (unsigned long int c)
>> +{
>> +#ifdef __alpha_cix__
>> +  return __builtin_ctzl (c);
>> +#else
>> +  c = c & -c;
>> +  return (c & 0xf0 ? 4 : 0) + (c & 0xcc ? 2 : 0) + (c & 0xaa ? 1 : 0);
>> +#endif
>> +}
>> +
>> +/* Similarly, but return the (memory order) index of the last bit
>> +   that is non-zero.  Note that only the least 8 bits may be nonzero.  */
>> +
>> +static __always_inline unsigned int
>> +index_last (unsigned long int x)
>> +{
>> +#ifdef __alpha_cix__
>> +  return __builtin_clzl (x) ^ 63;
>> +#else
>> +  unsigned r = 0;
>> +  if (x & 0xf0)
>> +    r += 4;
>> +  if (x & (0xc << r))
>> +    r += 2;
>> +  if (x & (0x2 << r))
>> +    r += 1;
>> +  return r;
>> +#endif
>> +}
>> +
>> +/* Given a word X that is known to contain a zero byte, return the
>> +   index of the first such within the word in memory order.  */
>> +
>> +static __always_inline unsigned int
>> +index_first_zero (op_t x)
>> +{
>> +  return index_first (__builtin_alpha_cmpbge (0, x));
> 
> The header split has drifted somewhat since the original: string-fza.h is missing and check_mask() seems misplaced.
> 
> In particular, from strchrnul,
> 
> 
> +  op_t mask = check_mask (find_zero_eq_all (word, repeated_c), s_int);
> +  if (mask != 0)
> +    return (char *) str + index_first (mask);
> 
> 
> We're using the generic find_zero_eq_all, then (correctly, for the moment) the generic check_mask, and then incorrectly the alpha-specific index_first.
> 
> We want to use an alpha-specific find_zero_eq_all (using cmpbge), an alpha-specific check_mask (shifting by bits, not bytes), and an alpha-specific index_first (searching only 8 low bits, not a full ctz).

Right, I had to consult the alpha manual to remind that cmpbge sets the lower
8 bits of the target register. And indeed running on qemu-users triggers a lot
of issues.

> 
> I wonder if it makes sense to introduce a target-specific find_t, making it clear that it's not the same type as op_t (for alpha, int instead of unsigned long (uint8_t having it's own issues on ancient alpha ev5)).

I think it makes sense, I will add this change.

> 
> I think check_mask (possibly renamed shift_find?) should belong in string-fza.h to match the implementation therein, or within a new header (possibly along with find_t?).

It also make sense.

> 
> Anyway, even the current set of headers would work, so long as Alpha implements all of them.

A alpha-specific string-fza.h is in fact quite simple, since we do not need to implement
all the function.

I will update the patch with your suggestions.