summaryrefslogtreecommitdiff
path: root/include
diff options
context:
space:
mode:
authorLinus Torvalds <torvalds@linux-foundation.org>2016-05-02 12:46:42 -0700
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2016-05-11 11:21:13 +0200
commitfe21a25e8c0cc97a080cc73c135e92ddce61a660 (patch)
treeca37e58e5f65345885ec4a330576284088d88dfc /include
parentf3b51a03bea6dc4cda740481d48b2ff49abdced5 (diff)
Minimal fix-up of bad hashing behavior of hash_64()
commit 689de1d6ca95b3b5bd8ee446863bf81a4883ea25 upstream. This is a fairly minimal fixup to the horribly bad behavior of hash_64() with certain input patterns. In particular, because the multiplicative value used for the 64-bit hash was intentionally bit-sparse (so that the multiply could be done with shifts and adds on architectures without hardware multipliers), some bits did not get spread out very much. In particular, certain fairly common bit ranges in the input (roughly bits 12-20: commonly with the most information in them when you hash things like byte offsets in files or memory that have block factors that mean that the low bits are often zero) would not necessarily show up much in the result. There's a bigger patch-series brewing to fix up things more completely, but this is the fairly minimal fix for the 64-bit hashing problem. It simply picks a much better constant multiplier, spreading the bits out a lot better. NOTE! For 32-bit architectures, the bad old hash_64() remains the same for now, since 64-bit multiplies are expensive. The bigger hashing cleanup will replace the 32-bit case with something better. The new constants were picked by George Spelvin who wrote that bigger cleanup series. I just picked out the constants and part of the comment from that series. Cc: George Spelvin <linux@horizon.com> Cc: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Diffstat (limited to 'include')
-rw-r--r--include/linux/hash.h20
1 files changed, 18 insertions, 2 deletions
diff --git a/include/linux/hash.h b/include/linux/hash.h
index 1afde47e1528..79c52fa81cac 100644
--- a/include/linux/hash.h
+++ b/include/linux/hash.h
@@ -32,12 +32,28 @@
#error Wordsize not 32 or 64
#endif
+/*
+ * The above primes are actively bad for hashing, since they are
+ * too sparse. The 32-bit one is mostly ok, the 64-bit one causes
+ * real problems. Besides, the "prime" part is pointless for the
+ * multiplicative hash.
+ *
+ * Although a random odd number will do, it turns out that the golden
+ * ratio phi = (sqrt(5)-1)/2, or its negative, has particularly nice
+ * properties.
+ *
+ * These are the negative, (1 - phi) = (phi^2) = (3 - sqrt(5))/2.
+ * (See Knuth vol 3, section 6.4, exercise 9.)
+ */
+#define GOLDEN_RATIO_32 0x61C88647
+#define GOLDEN_RATIO_64 0x61C8864680B583EBull
+
static __always_inline u64 hash_64(u64 val, unsigned int bits)
{
u64 hash = val;
-#if defined(CONFIG_ARCH_HAS_FAST_MULTIPLIER) && BITS_PER_LONG == 64
- hash = hash * GOLDEN_RATIO_PRIME_64;
+#if BITS_PER_LONG == 64
+ hash = hash * GOLDEN_RATIO_64;
#else
/* Sigh, gcc can't optimise this alone like it does for 32 bits. */
u64 n = hash;