Messages in this thread Patch in this message | | | From | Florian La Roche <> | Subject | fix int_sqrt() for very large numbers | Date | Sat, 19 Jan 2019 16:14:50 +0100 |
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If an input number x for int_sqrt() has the highest bit set, then __ffs(x) is 64. (1UL << 64) is an overflow and breaks the algorithm.
Just subtracting 1 is an even better guess for the initial value of m and that's what also used to be done in earlier versions of this code.
best regards,
Florian La Roche
Signed-off-by: Florian La Roche <Florian.LaRoche@googlemail.com> --- lib/int_sqrt.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/lib/int_sqrt.c b/lib/int_sqrt.c index 14436f4ca6bd..ea00e84dc272 100644 --- a/lib/int_sqrt.c +++ b/lib/int_sqrt.c @@ -23,7 +23,7 @@ unsigned long int_sqrt(unsigned long x) if (x <= 1) return x; - m = 1UL << (__fls(x) & ~1UL); + m = 1UL << ((__fls(x) - 1) & ~1UL); while (m != 0) { b = y + m; y >>= 1; @@ -52,7 +52,7 @@ u32 int_sqrt64(u64 x) if (x <= ULONG_MAX) return int_sqrt((unsigned long) x); - m = 1ULL << (fls64(x) & ~1ULL); + m = 1ULL << ((fls64(x) - 1) & ~1ULL); while (m != 0) { b = y + m; y >>= 1; -- 2.17.1
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