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author | Sasha Levin <sasha.levin@oracle.com> | 2015-12-03 22:04:01 -0500 |
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committer | Linus Torvalds <torvalds@linux-foundation.org> | 2015-12-09 10:35:16 -0800 |
commit | d7e35dfa2531b53618b9e6edcd8752ce988ac555 (patch) | |
tree | b8cf70f1d314c519030b6ae387d76313a0698616 /include/linux/bitops.h | |
parent | 626d114f46fedb66f56bfbea4743ab68ab88e590 (diff) | |
download | linux-stable-d7e35dfa2531b53618b9e6edcd8752ce988ac555.tar.gz linux-stable-d7e35dfa2531b53618b9e6edcd8752ce988ac555.tar.bz2 linux-stable-d7e35dfa2531b53618b9e6edcd8752ce988ac555.zip |
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.h | 2 |
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)); } /** |