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Date: Thu, 16 Jan 2014 00:17:50 +0000 From: Ben Hutchings <bhutchings@...arflare.com> To: Daniel Borkmann <dborkman@...hat.com> CC: <davem@...emloft.net>, <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>, Hannes Frederic Sowa <hannes@...essinduktion.org>, Eric Dumazet <eric.dumazet@...il.com>, "Austin S Hemmelgarn" <ahferroin7@...il.com>, Jesse Gross <jesse@...ira.com>, "Jamal Hadi Salim" <jhs@...atatu.com>, Stephen Hemminger <stephen@...workplumber.org>, Matt Mackall <mpm@...enic.com>, Pekka Enberg <penberg@...nel.org>, Christoph Lameter <cl@...ux-foundation.org>, "Andy Gospodarek" <andy@...yhouse.net>, Veaceslav Falico <vfalico@...hat.com>, "Jay Vosburgh" <fubar@...ibm.com>, Jakub Zawadzki <darkjames-ws@...kjames.pl> Subject: Re: [PATCH net-next 2/2] reciprocal_divide: correction/update of the algorithm On Thu, 2014-01-16 at 00:23 +0100, Daniel Borkmann wrote: [...] > --- a/include/linux/reciprocal_div.h > +++ b/include/linux/reciprocal_div.h [...] > + * RECIPROCAL_VALUE_TO_ZERO can be used to express an element, which > + * used as the argument to reciprocal_divide always yields zero. > */ [...] > +#define RECIPROCAL_VALUE_RESULT_TO_ZERO ((struct reciprocal_value){.sh1 = 32}) [...] > +static inline u32 reciprocal_divide(u32 a, struct reciprocal_value R) > { > - return (u32)(((u64)A * R) >> 32); > + u32 t = (u32)(((u64)a * R.m) >> 32); > + return (t + ((a - t) >> R.sh1)) >> R.sh2; [...] (a - t) has type u32. So (a - t) >> 32 has undefined behaviour. Ben. -- Ben Hutchings, Staff Engineer, Solarflare Not speaking for my employer; that's the marketing department's job. They asked us to note that Solarflare product names are trademarked. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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