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

Message ID 1515588482-15744-4-git-send-email-adhemerval.zanella@linaro.org
State New
Headers show
Series
  • Improve generic string routines
Related show

Commit Message

Adhemerval Zanella 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

-- 
2.7.4

Comments

Paul Eggert Jan. 10, 2018, 11:24 p.m. | #1
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 Jan. 11, 2018, 10:54 a.m. | #2
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. | #3
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?

-- 
Joseph S. Myers
joseph@codesourcery.com
Adhemerval Zanella Jan. 11, 2018, 5:57 p.m. | #4
On 11/01/2018 11:29, Joseph Myers wrote:
> 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?

> 


I applied the following patch based on Paul's suggestion:

diff --git a/sysdeps/generic/string-maskoff.h b/sysdeps/generic/string-maskoff.h
index 6231798..98e7852 100644
--- a/sysdeps/generic/string-maskoff.h
+++ b/sysdeps/generic/string-maskoff.h
@@ -47,18 +47,17 @@ 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).  */
+/* 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 (0x80);
+  return m & repeat_bytes (0x7f);
 }
 
 #endif /* STRING_MASKOFF_H  */

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  */