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

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Date:	25 May 2016 03:28:21 -0400
From:	"George Spelvin" <linux@...encehorizons.net>
To:	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org
Cc:	linux@...encehorizons.net, tglx@...utronix.de
Subject: [PATCH 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.

There's a WARN_ON in there in case I missed anything.  Most callers pass
a compile-time constant bits and will have no run-time overhead.

Signed-off-by: George Spelvin <linux@...encehorizons.net>
---
 include/linux/hash.h | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/include/linux/hash.h b/include/linux/hash.h
index 79c52fa8..b9201c33 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;
 
@@ -71,8 +71,14 @@ static __always_inline u64 hash_64(u64 val, unsigned int bits)
 	hash += n;
 #endif
 
+	if (__builtin_constant_p(bits > 32 || bits == 0)) {
+		BUILD_BUG_ON(bits > 32 || bits == 0);
+	} else {
+		WARN_ON(bits > 32 || bits == 0);
+	}
+
 	/* 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 +90,7 @@ 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