linux-kernel - [PATCH v2] bitops.h: correctly handle rol32 with 0 byte shift

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Message-Id: <1449198241-20654-1-git-send-email-sasha.levin@oracle.com>
Date:	Thu,  3 Dec 2015 22:04:01 -0500
From:	Sasha Levin <sasha.levin@...cle.com>
To:	torvalds@...ux-foundation.org
Cc:	mingo@...nel.org, linux-kernel@...r.kernel.org,
	Sasha Levin <sasha.levin@...cle.com>
Subject: [PATCH v2] bitops.h: correctly handle rol32 with 0 byte shift

ROL on a 32 bit integer with a shift of 32 or more is undefined and the
result is arch-dependent. Avoid this by handling the trivial case of
roling by 0 correctly.

The trivial solution of checking if shift is 0 breaks gcc's detection
of this code as a ROL instruction, which is unacceptable.

This bug was reported and fixed in GCC
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57157):

	The standard rotate idiom,

	  (x << n) | (x >> (32 - n))

	is recognized by gcc (for concreteness, I discuss only the case that x
	is an uint32_t here).

	However, this is portable C only for n in the range 0 < n < 32. For n
	== 0, we get x >> 32 which gives undefined behaviour according to the
	C standard (6.5.7, Bitwise shift operators). To portably support n ==
	0, one has to write the rotate as something like

	  (x << n) | (x >> ((-n) & 31))

	And this is apparently not recognized by gcc.

Note that this is broken on older GCCs and will result in slower ROL.

Acked-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Sasha Levin <sasha.levin@...cle.com>
---
 include/linux/bitops.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/bitops.h b/include/linux/bitops.h
index 2b8ed12..defeaac 100644
--- a/include/linux/bitops.h
+++ b/include/linux/bitops.h
@@ -107,7 +107,7 @@ static inline __u64 ror64(__u64 word, unsigned int shift)
  */
 static inline __u32 rol32(__u32 word, unsigned int shift)
 {
-	return (word << shift) | (word >> (32 - shift));
+	return (word << shift) | (word >> ((-shift) & 31));
 }
 
 /**
-- 
2.5.0

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