From 36126f8f2ed8168eb13aa0662b9b9585cba100a9 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Sat, 26 May 2012 10:43:17 -0700 Subject: [PATCH] word-at-a-time: make the interfaces truly generic This changes the interfaces in to be a bit more complicated, but a lot more generic. In particular, it allows us to really do the operations efficiently on both little-endian and big-endian machines, pretty much regardless of machine details. For example, if you can rely on a fast population count instruction on your architecture, this will allow you to make your optimized file with that. NOTE! The "generic" version in include/asm-generic/word-at-a-time.h is not truly generic, it actually only works on big-endian. Why? Because on little-endian the generic algorithms are wasteful, since you can inevitably do better. The x86 implementation is an example of that. (The only truly non-generic part of the asm-generic implementation is the "find_zero()" function, and you could make a little-endian version of it. And if the Kbuild infrastructure allowed us to pick a particular header file, that would be lovely) The functions are as follows: - WORD_AT_A_TIME_CONSTANTS: specific constants that the algorithm uses. - has_zero(): take a word, and determine if it has a zero byte in it. It gets the word, the pointer to the constant pool, and a pointer to an intermediate "data" field it can set. This is the "quick-and-dirty" zero tester: it's what is run inside the hot loops. - "prep_zero_mask()": take the word, the data that has_zero() produced, and the constant pool, and generate an *exact* mask of which byte had the first zero. This is run directly *outside* the loop, and allows the "has_zero()" function to answer the "is there a zero byte" question without necessarily getting exactly *which* byte is the first one to contain a zero. If you do multiple byte lookups concurrently (eg "hash_name()", which looks for both NUL and '/' bytes), after you've done the prep_zero_mask() phase, the result of those can be or'ed together to get the "either or" case. - The result from "prep_zero_mask()" can then be fed into "find_zero()" (to find the byte offset of the first byte that was zero) or into "zero_bytemask()" (to find the bytemask of the bytes preceding the zero byte). The existence of zero_bytemask() is optional, and is not necessary for the normal string routines. But dentry name hashing needs it, so if you enable DENTRY_WORD_AT_A_TIME you need to expose it. This changes the generic strncpy_from_user() function and the dentry hashing functions to use these modified word-at-a-time interfaces. This gets us back to the optimized state of the x86 strncpy that we lost in the previous commit when moving over to the generic version. Signed-off-by: Linus Torvalds --- arch/openrisc/include/asm/Kbuild | 1 + arch/sparc/include/asm/Kbuild | 1 + arch/x86/include/asm/word-at-a-time.h | 32 +++++++++++++++-- fs/namei.c | 20 ++++++----- include/asm-generic/word-at-a-time.h | 52 +++++++++++++++++++++++++++ lib/strncpy_from_user.c | 47 ++++-------------------- 6 files changed, 101 insertions(+), 52 deletions(-) create mode 100644 include/asm-generic/word-at-a-time.h diff --git a/arch/openrisc/include/asm/Kbuild b/arch/openrisc/include/asm/Kbuild index c936483bc8e2..3f35c38d7b64 100644 --- a/arch/openrisc/include/asm/Kbuild +++ b/arch/openrisc/include/asm/Kbuild @@ -66,3 +66,4 @@ generic-y += topology.h generic-y += types.h generic-y += ucontext.h generic-y += user.h +generic-y += word-at-a-time.h diff --git a/arch/sparc/include/asm/Kbuild b/arch/sparc/include/asm/Kbuild index 2c2e38821f60..67f83e0a0d68 100644 --- a/arch/sparc/include/asm/Kbuild +++ b/arch/sparc/include/asm/Kbuild @@ -21,3 +21,4 @@ generic-y += div64.h generic-y += local64.h generic-y += irq_regs.h generic-y += local.h +generic-y += word-at-a-time.h diff --git a/arch/x86/include/asm/word-at-a-time.h b/arch/x86/include/asm/word-at-a-time.h index ae03facfadd6..5b238981542a 100644 --- a/arch/x86/include/asm/word-at-a-time.h +++ b/arch/x86/include/asm/word-at-a-time.h @@ -10,6 +10,11 @@ * bit count instruction, that might be better than the multiply * and shift, for example. */ +struct word_at_a_time { + const unsigned long one_bits, high_bits; +}; + +#define WORD_AT_A_TIME_CONSTANTS { REPEAT_BYTE(0x01), REPEAT_BYTE(0x80) } #ifdef CONFIG_64BIT @@ -37,10 +42,31 @@ static inline long count_masked_bytes(long mask) #endif -/* Return the high bit set in the first byte that is a zero */ -static inline unsigned long has_zero(unsigned long a) +/* Return nonzero if it has a zero */ +static inline unsigned long has_zero(unsigned long a, unsigned long *bits, const struct word_at_a_time *c) { - return ((a - REPEAT_BYTE(0x01)) & ~a) & REPEAT_BYTE(0x80); + unsigned long mask = ((a - c->one_bits) & ~a) & c->high_bits; + *bits = mask; + return mask; +} + +static inline unsigned long prep_zero_mask(unsigned long a, unsigned long bits, const struct word_at_a_time *c) +{ + return bits; +} + +static inline unsigned long create_zero_mask(unsigned long bits) +{ + bits = (bits - 1) & ~bits; + return bits >> 7; +} + +/* The mask we created is directly usable as a bytemask */ +#define zero_bytemask(mask) (mask) + +static inline unsigned long find_zero(unsigned long mask) +{ + return count_masked_bytes(mask); } /* diff --git a/fs/namei.c b/fs/namei.c index 93ff12b1a1de..c651f02c9fec 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -1452,7 +1452,8 @@ EXPORT_SYMBOL(full_name_hash); */ static inline unsigned long hash_name(const char *name, unsigned int *hashp) { - unsigned long a, mask, hash, len; + unsigned long a, b, adata, bdata, mask, hash, len; + const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS; hash = a = 0; len = -sizeof(unsigned long); @@ -1460,17 +1461,18 @@ static inline unsigned long hash_name(const char *name, unsigned int *hashp) hash = (hash + a) * 9; len += sizeof(unsigned long); a = load_unaligned_zeropad(name+len); - /* Do we have any NUL or '/' bytes in this word? */ - mask = has_zero(a) | has_zero(a ^ REPEAT_BYTE('/')); - } while (!mask); + b = a ^ REPEAT_BYTE('/'); + } while (!(has_zero(a, &adata, &constants) | has_zero(b, &bdata, &constants))); - /* The mask *below* the first high bit set */ - mask = (mask - 1) & ~mask; - mask >>= 7; - hash += a & mask; + adata = prep_zero_mask(a, adata, &constants); + bdata = prep_zero_mask(b, bdata, &constants); + + mask = create_zero_mask(adata | bdata); + + hash += a & zero_bytemask(mask); *hashp = fold_hash(hash); - return len + count_masked_bytes(mask); + return len + find_zero(mask); } #else diff --git a/include/asm-generic/word-at-a-time.h b/include/asm-generic/word-at-a-time.h new file mode 100644 index 000000000000..3f21f1b72e45 --- /dev/null +++ b/include/asm-generic/word-at-a-time.h @@ -0,0 +1,52 @@ +#ifndef _ASM_WORD_AT_A_TIME_H +#define _ASM_WORD_AT_A_TIME_H + +/* + * This says "generic", but it's actually big-endian only. + * Little-endian can use more efficient versions of these + * interfaces, see for example + * arch/x86/include/asm/word-at-a-time.h + * for those. + */ + +#include + +struct word_at_a_time { + const unsigned long high_bits, low_bits; +}; + +#define WORD_AT_A_TIME_CONSTANTS { REPEAT_BYTE(0xfe) + 1, REPEAT_BYTE(0x7f) } + +/* Bit set in the bytes that have a zero */ +static inline long prep_zero_mask(unsigned long val, unsigned long rhs, const struct word_at_a_time *c) +{ + unsigned long mask = (val & c->low_bits) + c->low_bits; + return ~(mask | rhs); +} + +#define create_zero_mask(mask) (mask) + +static inline long find_zero(unsigned long mask) +{ + long byte = 0; +#ifdef CONFIG_64BIT + if (mask >> 32) + mask >>= 32; + else + byte = 4; +#endif + if (mask >> 16) + mask >>= 16; + else + byte += 2; + return (mask >> 8) ? byte : byte + 1; +} + +static inline bool has_zero(unsigned long val, unsigned long *data, const struct word_at_a_time *c) +{ + unsigned long rhs = val | c->low_bits; + *data = rhs; + return (val + c->high_bits) & ~rhs; +} + +#endif /* _ASM_WORD_AT_A_TIME_H */ diff --git a/lib/strncpy_from_user.c b/lib/strncpy_from_user.c index c4c09b0e96ba..bb2b201d6ad0 100644 --- a/lib/strncpy_from_user.c +++ b/lib/strncpy_from_user.c @@ -4,37 +4,7 @@ #include #include - -static inline long find_zero(unsigned long mask) -{ - long byte = 0; - -#ifdef __BIG_ENDIAN -#ifdef CONFIG_64BIT - if (mask >> 32) - mask >>= 32; - else - byte = 4; -#endif - if (mask >> 16) - mask >>= 16; - else - byte += 2; - return (mask >> 8) ? byte : byte + 1; -#else -#ifdef CONFIG_64BIT - if (!((unsigned int) mask)) { - mask >>= 32; - byte = 4; - } -#endif - if (!(mask & 0xffff)) { - mask >>= 16; - byte += 2; - } - return (mask & 0xff) ? byte : byte + 1; -#endif -} +#include #ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS #define IS_UNALIGNED(src, dst) 0 @@ -51,8 +21,7 @@ static inline long find_zero(unsigned long mask) */ static inline long do_strncpy_from_user(char *dst, const char __user *src, long count, unsigned long max) { - const unsigned long high_bits = REPEAT_BYTE(0xfe) + 1; - const unsigned long low_bits = REPEAT_BYTE(0x7f); + const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS; long res = 0; /* @@ -66,18 +35,16 @@ static inline long do_strncpy_from_user(char *dst, const char __user *src, long goto byte_at_a_time; while (max >= sizeof(unsigned long)) { - unsigned long c, v, rhs; + unsigned long c, data; /* Fall back to byte-at-a-time if we get a page fault */ if (unlikely(__get_user(c,(unsigned long __user *)(src+res)))) break; - rhs = c | low_bits; - v = (c + high_bits) & ~rhs; *(unsigned long *)(dst+res) = c; - if (v) { - v = (c & low_bits) + low_bits; - v = ~(v | rhs); - return res + find_zero(v); + if (has_zero(c, &data, &constants)) { + data = prep_zero_mask(c, data, &constants); + data = create_zero_mask(data); + return res + find_zero(data); } res += sizeof(unsigned long); max -= sizeof(unsigned long);