Patchwork [v3,03/18] Add string-maskoff.h generic header

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Submitter Adhemerval Zanella Netto
Date Jan. 10, 2018, 12:47 p.m.
Message ID <1515588482-15744-4-git-send-email-adhemerval.zanella@linaro.org>
Download mbox | patch
Permalink /patch/25310/
State New
Headers show

Comments

Adhemerval Zanella Netto - Jan. 10, 2018, 12:47 p.m.
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.

	Richard Henderson  <rth@twiddle.net>
	Adhemerval Zanella  <adhemerval.zanella@linaro.org>

	* sysdeps/generic/string-maskoff.h: New file.
---
 sysdeps/generic/string-maskoff.h | 64 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 64 insertions(+)
 create mode 100644 sysdeps/generic/string-maskoff.h
Paul Eggert - Jan. 10, 2018, 11:24 p.m.
On 01/10/2018 04:47 AM, Adhemerval Zanella wrote:
> +/* Create a mask with high bit of each byte being 1, and the low 7 bits
> +   being all the opposite of the input 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 pointer alignment
> +   of 3 the function returns 0x7f7f7f0000000000 (input meant to
> +   be 0xffffff0000000000) for BE and 0x00000000007f7f7f for LE (input
> +   meant to be 0x0000000000ffffff).  */
> +static inline op_t
> +highbit_mask (op_t m)
> +{
> +  return m & ~repeat_bytes (0x80);
> +}

The first line of the comment does not match the implementation, which 
would have to be something like "return (m ^ repeat_bytes (0x7f)) | 
repeat_bytes (0x80);" to match the comment. Also, I suggest replacing 
"~repeat_bytes (0x80)" with "repeat_bytes (0x7f)" as the latter is a bit 
simpler.
Adhemerval Zanella Netto - Jan. 11, 2018, 10:54 a.m.
On 10/01/2018 21:24, Paul Eggert wrote:
> On 01/10/2018 04:47 AM, Adhemerval Zanella wrote:
>> +/* Create a mask with high bit of each byte being 1, and the low 7 bits
>> +   being all the opposite of the input 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 pointer alignment
>> +   of 3 the function returns 0x7f7f7f0000000000 (input meant to
>> +   be 0xffffff0000000000) for BE and 0x00000000007f7f7f for LE (input
>> +   meant to be 0x0000000000ffffff).  */
>> +static inline op_t
>> +highbit_mask (op_t m)
>> +{
>> +  return m & ~repeat_bytes (0x80);
>> +}
> 
> The first line of the comment does not match the implementation, which would have to be something like "return (m ^ repeat_bytes (0x7f)) | repeat_bytes (0x80);" to match the comment. Also, I suggest replacing "~repeat_bytes (0x80)" with "repeat_bytes (0x7f)" as the latter is a bit simpler.

Thanks, I will change it locally.
Joseph Myers - Jan. 11, 2018, 1:29 p.m.
On Wed, 10 Jan 2018, Adhemerval Zanella wrote:

> +/* 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

Is there a missing "for BE" at the end of this line?

Patch

diff --git a/sysdeps/generic/string-maskoff.h b/sysdeps/generic/string-maskoff.h
new file mode 100644
index 0000000..6231798
--- /dev/null
+++ b/sysdeps/generic/string-maskoff.h
@@ -0,0 +1,64 @@ 
+/* Mask off bits.  Generic C version.
+   Copyright (C) 2018 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 <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;
+}
+
+/* Create a mask with high bit of each byte being 1, and the low 7 bits
+   being all the opposite of the input 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 pointer alignment
+   of 3 the function returns 0x7f7f7f0000000000 (input meant to
+   be 0xffffff0000000000) for BE and 0x00000000007f7f7f for LE (input
+   meant to be 0x0000000000ffffff).  */
+static inline op_t
+highbit_mask (op_t m)
+{
+  return m & ~repeat_bytes (0x80);
+}
+
+#endif /* STRING_MASKOFF_H  */