linux-kernel - [PATCH 3.2 35/46] Make hash_64() use a 64-bit multiply when appropriate

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Message-ID: <lsq.1465767282.20011651@decadent.org.uk>
Date:	Sun, 12 Jun 2016 22:34:42 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org,
	"Linus Torvalds" <torvalds@...ux-foundation.org>
Subject: [PATCH 3.2 35/46] Make hash_64() use a 64-bit multiply when
 appropriate

3.2.81-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Linus Torvalds <torvalds@...ux-foundation.org>

commit 23d0db76ffa13ffb95229946e4648568c3c29db5 upstream.

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@...ux-foundation.org>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
[bwh: This has no immediate effect in 3.2 because nothing defines
 CONFIG_ARCH_HAS_FAST_MULTIPLIER. However the following fix removes
 that condition.]
---
 include/linux/hash.h | 4 ++++
 1 file changed, 4 insertions(+)

--- a/include/linux/hash.h
+++ b/include/linux/hash.h
@@ -35,6 +35,9 @@ static inline u64 hash_64(u64 val, unsig
 {
 	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;
@@ -49,6 +52,7 @@ static inline u64 hash_64(u64 val, unsig
 	hash += n;
 	n <<= 2;
 	hash += n;
+#endif

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