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authorSasha Levin <sasha.levin@oracle.com>2015-12-03 22:04:01 -0500
committerLinus Torvalds <torvalds@linux-foundation.org>2015-12-09 10:35:16 -0800
commitd7e35dfa2531b53618b9e6edcd8752ce988ac555 (patch)
treeb8cf70f1d314c519030b6ae387d76313a0698616 /include/linux/bitops.h
parent626d114f46fedb66f56bfbea4743ab68ab88e590 (diff)
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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@linux-foundation.org> Signed-off-by: Sasha Levin <sasha.levin@oracle.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'include/linux/bitops.h')
-rw-r--r--include/linux/bitops.h2
1 files changed, 1 insertions, 1 deletions
diff --git a/include/linux/bitops.h b/include/linux/bitops.h
index 2b8ed123ad36..defeaac0745f 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));
}
/**