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Make the "word-at-a-time" helper functions more commonly usable

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I have a new optimized x86 "strncpy_from_user()" that will use these
same helper functions for all the same reasons the name lookup code uses
them.  This is preparation for that.

This moves them into an architecture-specific header file.  It's
architecture-specific for two reasons:

 - some of the functions are likely to want architecture-specific
   implementations.  Even if the current code happens to be "generic" in
   the sense that it should work on any little-endian machine, it's
   likely that the "multiply by a big constant and shift" implementation
   is less than optimal for an architecture that has a guaranteed fast
   bit count instruction, for example.

 - I expect that if architectures like sparc want to start playing
   around with this, we'll need to abstract out a few more details (in
   particular the actual unaligned accesses).  So we're likely to have
   more architecture-specific stuff if non-x86 architectures start using
   this.

   (and if it turns out that non-x86 architectures don't start using
   this, then having it in an architecture-specific header is still the
   right thing to do, of course)

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Linus Torvalds 2012-04-06 13:54:56 -07:00
parent 23f347ef63
commit f68e556e23
2 changed files with 49 additions and 32 deletions
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46
arch/x86/include/asm/word-at-a-time.h Normal file
View file

@ -0,0 +1,46 @@
#ifndef _ASM_WORD_AT_A_TIME_H
#define _ASM_WORD_AT_A_TIME_H
/*
* This is largely generic for little-endian machines, but the
* optimal byte mask counting is probably going to be something
* that is architecture-specific. If you have a reliably fast
* bit count instruction, that might be better than the multiply
* and shift, for example.
*/
#ifdef CONFIG_64BIT
/*
* Jan Achrenius on G+: microoptimized version of
* the simpler "(mask & ONEBYTES) * ONEBYTES >> 56"
* that works for the bytemasks without having to
* mask them first.
*/
static inline long count_masked_bytes(unsigned long mask)
{
return mask*0x0001020304050608ul >> 56;
}
#else /* 32-bit case */
/* Carl Chatfield / Jan Achrenius G+ version for 32-bit */
static inline long count_masked_bytes(long mask)
{
/* (000000 0000ff 00ffff ffffff) -> ( 1 1 2 3 ) */
long a = (0x0ff0001+mask) >> 23;
/* Fix the 1 for 00 case */
return a & mask;
}
#endif
#define REPEAT_BYTE(x) ((~0ul / 0xff) * (x))
/* Return the high bit set in the first byte that is a zero */
static inline unsigned long has_zero(unsigned long a)
{
return ((a - REPEAT_BYTE(0x01)) & ~a) & REPEAT_BYTE(0x80);
}
#endif /* _ASM_WORD_AT_A_TIME_H */

35
fs/namei.c
View file

@ -1407,18 +1407,9 @@ static inline int can_lookup(struct inode *inode)
*/
#ifdef CONFIG_DCACHE_WORD_ACCESS
#ifdef CONFIG_64BIT
#include <asm/word-at-a-time.h>
/*
* Jan Achrenius on G+: microoptimized version of
* the simpler "(mask & ONEBYTES) * ONEBYTES >> 56"
* that works for the bytemasks without having to
* mask them first.
*/
static inline long count_masked_bytes(unsigned long mask)
{
return mask*0x0001020304050608ul >> 56;
}
#ifdef CONFIG_64BIT
static inline unsigned int fold_hash(unsigned long hash)
{
@ -1428,15 +1419,6 @@ static inline unsigned int fold_hash(unsigned long hash)
#else /* 32-bit case */
/* Carl Chatfield / Jan Achrenius G+ version for 32-bit */
static inline long count_masked_bytes(long mask)
{
/* (000000 0000ff 00ffff ffffff) -> ( 1 1 2 3 ) */
long a = (0x0ff0001+mask) >> 23;
/* Fix the 1 for 00 case */
return a & mask;
}
#define fold_hash(x) (x)
#endif
@ -1464,17 +1446,6 @@ unsigned int full_name_hash(const unsigned char *name, unsigned int len)
}
EXPORT_SYMBOL(full_name_hash);
#define REPEAT_BYTE(x) ((~0ul / 0xff) * (x))
#define ONEBYTES REPEAT_BYTE(0x01)
#define SLASHBYTES REPEAT_BYTE('/')
#define HIGHBITS REPEAT_BYTE(0x80)
/* Return the high bit set in the first byte that is a zero */
static inline unsigned long has_zero(unsigned long a)
{
return ((a - ONEBYTES) & ~a) & HIGHBITS;
}
/*
* Calculate the length and hash of the path component, and
* return the length of the component;
@ -1490,7 +1461,7 @@ static inline unsigned long hash_name(const char *name, unsigned int *hashp)
len += sizeof(unsigned long);
a = *(unsigned long *)(name+len);
/* Do we have any NUL or '/' bytes in this word? */
mask = has_zero(a) | has_zero(a ^ SLASHBYTES);
mask = has_zero(a) | has_zero(a ^ REPEAT_BYTE('/'));
} while (!mask);
/* The mask *below* the first high bit set */