Messages in this thread Patch in this message | | | Subject | Re: [PATCH] Reduce the number of expensive division instructions done by _parse_integer() | From | Eric Dumazet <> | Date | Thu, 09 Feb 2012 17:28:40 +0100 |
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Le jeudi 09 février 2012 à 15:48 +0000, David Howells a écrit : > _parse_integer() does one or two division instructions (which are slow) per > digit parsed to perform the overflow check. > > Furthermore, these are particularly expensive examples of division instruction > as the number of clock cycles required to complete them may go up with the > position of the most significant set bit in the dividend: > > if (*res > div_u64(ULLONG_MAX - val, base)) > > which is as maximal as possible. > > Worse, on 32-bit arches, more than one of these division instructions may be > required per digit. > > So, assuming we don't support a base of more than 16, skip the check if the > top nibble of the result is not set at this point. > > Signed-off-by: David Howells <dhowells@redhat.com> > --- > > lib/kstrtox.c | 2 +- > 1 files changed, 1 insertions(+), 1 deletions(-) > > diff --git a/lib/kstrtox.c b/lib/kstrtox.c > index 7a94c8f..f80c896 100644 > --- a/lib/kstrtox.c > +++ b/lib/kstrtox.c > @@ -64,7 +64,7 @@ unsigned int _parse_integer(const char *s, unsigned int base, unsigned long long > > if (val >= base) > break; > - if (*res > div_u64(ULLONG_MAX - val, base)) > + if (unlikely(*res >> 60) && *res > div_u64(ULLONG_MAX - val, base)) > overflow = 1; > *res = *res * base + val; > rv++; > > -
You could avoid the divide and have cleaner code I think.
unsigned long long next = *res * base + val;
if (next < *res) overflow = 1; *res = next;
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