linux-kernel - [PATCH v3 04/10] Change hash_64() return value to 32 bits

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Date:	Sat, 28 May 2016 15:57:17 -0400
From:	George Spelvin <linux@...encehorizons.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	lkml <linux-kernel@...r.kernel.org>
Cc:	"J . Bruce Fields" <bfields@...hat.com>,
	George Spelvin <linux@...encehorizons.net>
Subject: [PATCH v3 04/10] Change hash_64() return value to 32 bits

That's all that's ever asked for, and it makes the return
type of hash_long() consistent.

It also allows (upcoming patch) an optimized implementation
of hash_64 on 32-bit machines.

I tried adding a BUILD_BUG_ON to ensure the number of bits requested
was never more than 32 (most callers use a compile-time constant), but
adding <linux/bug.h> to <linux/hash.h> breaks the tools/perf compiler
unless tools/perf/MANIFEST is updated, and understanding that code base
well enough to update it is too much trouble.  I did the rest of an
allyesconfig build with such a check, and nothing tripped.

Signed-off-by: George Spelvin <linux@...encehorizons.net>
---
 include/linux/hash.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/linux/hash.h b/include/linux/hash.h
index 79c52fa8..f967dedb 100644
--- a/include/linux/hash.h
+++ b/include/linux/hash.h
@@ -48,7 +48,7 @@
 #define GOLDEN_RATIO_32 0x61C88647
 #define GOLDEN_RATIO_64 0x61C8864680B583EBull
 
-static __always_inline u64 hash_64(u64 val, unsigned int bits)
+static __always_inline u32 hash_64(u64 val, unsigned int bits)
 {
 	u64 hash = val;
 
@@ -72,7 +72,7 @@ static __always_inline u64 hash_64(u64 val, unsigned int bits)
 #endif
 
 	/* High bits are more random, so use them. */
-	return hash >> (64 - bits);
+	return (u32)(hash >> (64 - bits));
 }
 
 static inline u32 hash_32(u32 val, unsigned int bits)
@@ -84,7 +84,7 @@ static inline u32 hash_32(u32 val, unsigned int bits)
 	return hash >> (32 - bits);
 }
 
-static inline unsigned long hash_ptr(const void *ptr, unsigned int bits)
+static inline u32 hash_ptr(const void *ptr, unsigned int bits)
 {
 	return hash_long((unsigned long)ptr, bits);
 }
-- 
2.8.1