lkml.org 
[lkml]   [2006]   [Nov]   [20]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
Subject[PATCH] Allow 32-bit and 64-bit hashes

The sym2 driver would like to hash a u32 value, and it could just
call hash_long() and rely on integer promotion on 64-bit machines, but
that seems a little wasteful.

Following Arjan's suggestion, I split the existing hash_long into
hash_u32 and hash_u64, and made hash_long an alias to the appropriate
function.

Signed-off-by: Matthew Wilcox <matthew@wil.cx>

diff --git a/include/linux/hash.h b/include/linux/hash.h
index acf17bb..0e6e5a9 100644
--- a/include/linux/hash.h
+++ b/include/linux/hash.h
@@ -13,23 +13,36 @@
* them can use shifts and additions instead of multiplications for
* machines where multiplications are slow.
*/
-#if BITS_PER_LONG == 32
/* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */
-#define GOLDEN_RATIO_PRIME 0x9e370001UL
-#elif BITS_PER_LONG == 64
+#define GOLDEN_RATIO_PRIME_32 0x9e370001UL
/* 2^63 + 2^61 - 2^57 + 2^54 - 2^51 - 2^18 + 1 */
-#define GOLDEN_RATIO_PRIME 0x9e37fffffffc0001UL
+#define GOLDEN_RATIO_PRIME_64 0x9e37fffffffc0001ULL
+
+#if BITS_PER_LONG == 32
+#define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_32
+#define hash_long(val, bits) hash_u32(val, bits)
+#elif BITS_PER_LONG == 64
+#define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_64
+#define hash_long(val, bits) hash_u64(val, bits)
#else
#error Define GOLDEN_RATIO_PRIME for your wordsize.
#endif

-static inline unsigned long hash_long(unsigned long val, unsigned int bits)
+static inline unsigned long hash_u32(u32 val, unsigned int bits)
+{
+ /* On some cpus multiply is faster, on others gcc will do shifts */
+ unsigned int hash = val * GOLDEN_RATIO_PRIME_32;
+
+ /* High bits are more random, so use them. */
+ return hash >> (32 - bits);
+}
+
+static inline unsigned long hash_u64(u64 val, unsigned int bits)
{
- unsigned long hash = val;
+ u64 hash = val;

-#if BITS_PER_LONG == 64
/* Sigh, gcc can't optimise this alone like it does for 32 bits. */
- unsigned long n = hash;
+ u64 n = hash;
n <<= 18;
hash -= n;
n <<= 33;
@@ -42,13 +55,9 @@ static inline unsigned long hash_long(un
hash += n;
n <<= 2;
hash += n;
-#else
- /* On some cpus multiply is faster, on others gcc will do shifts */
- hash *= GOLDEN_RATIO_PRIME;
-#endif

/* High bits are more random, so use them. */
- return hash >> (BITS_PER_LONG - bits);
+ return hash >> (64 - bits);
}

static inline unsigned long hash_ptr(void *ptr, unsigned int bits)
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
\
 
 \ /
  Last update: 2006-11-21 05:07    [W:0.024 / U:32.844 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site