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authorLinus Torvalds <torvalds@linux-foundation.org>2014-09-13 11:24:03 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2014-09-13 11:24:03 -0700
commit23d0db76ffa13ffb95229946e4648568c3c29db5 (patch)
treea468c5bb0428be35da5200af5c3aa44dde163b81
parent72d931046030beb2d13dad6d205be0e228618432 (diff)
Make hash_64() use a 64-bit multiply when appropriate
The hash_64() function historically does the multiply by the GOLDEN_RATIO_PRIME_64 number with explicit shifts and adds, because unlike the 32-bit case, gcc seems unable to turn the constant multiply into the more appropriate shift and adds when required. However, that means that we generate those shifts and adds even when the architecture has a fast multiplier, and could just do it better in hardware. Use the now-cleaned-up CONFIG_ARCH_HAS_FAST_MULTIPLIER (together with "is it a 64-bit architecture") to decide whether to use an integer multiply or the explicit sequence of shift/add instructions. Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r--include/linux/hash.h4
1 files changed, 4 insertions, 0 deletions
diff --git a/include/linux/hash.h b/include/linux/hash.h
index bd1754c7ecef..d0494c399392 100644
--- a/include/linux/hash.h
+++ b/include/linux/hash.h
@@ -37,6 +37,9 @@ 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;
+#else
/* Sigh, gcc can't optimise this alone like it does for 32 bits. */
u64 n = hash;
n <<= 18;
@@ -51,6 +54,7 @@ static __always_inline u64 hash_64(u64 val, unsigned int bits)
hash += n;
n <<= 2;
hash += n;
+#endif
/* High bits are more random, so use them. */
return hash >> (64 - bits);