From: Guenter Roeck <linux@roeck-us.net>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-kernel@vger.kernel.org, Pekka Enberg <penberg@kernel.org>,
Ingo Molnar <mingo@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
Jean Delvare <khali@linux-fr.org>,
lm-sensors@lm-sensors.org
Subject: Re: [lm-sensors] [PATCH v3] linux/kernel.h: Fix DIV_ROUND_CLOSEST to support negative operands
Date: Fri, 31 Aug 2012 01:12:16 +0000 [thread overview]
Message-ID: <20120831011216.GA22010@roeck-us.net> (raw)
In-Reply-To: <20120830173531.291e7b6d.akpm@linux-foundation.org>
On Thu, Aug 30, 2012 at 05:35:31PM -0700, Andrew Morton wrote:
> On Thu, 30 Aug 2012 17:10:47 -0700 Guenter Roeck <linux@roeck-us.net> wrote:
>
> > DIV_ROUND_CLOSEST returns a bad result for dividends with different sign:
> > DIV_ROUND_CLOSEST(-2, 2) = 0
> >
> > Most of the time this does not matter. However, in the hardware monitoring
> > subsystem, DIV_ROUND_CLOSEST is sometimes used on integers which can be
> > negative (such as temperatures).
> >
> > ...
> >
> > --- a/include/linux/kernel.h
> > +++ b/include/linux/kernel.h
> > @@ -84,8 +84,11 @@
> > )
> > #define DIV_ROUND_CLOSEST(x, divisor)( \
> > { \
> > - typeof(divisor) __divisor = divisor; \
> > - (((x) + ((__divisor) / 2)) / (__divisor)); \
> > + typeof(x) __x = x; \
> > + typeof(divisor) __d = divisor; \
> > + ((__x) < 0) = ((__d) < 0) ? \
> > + (((__x) + ((__d) / 2)) / (__d)) : \
> > + (((__x) - ((__d) / 2)) / (__d)); \
> > } \
> > )
>
> Your v2 had that sneaky little "(typeof(x))-1 >= 0" trick in it, so
> half the code gets elided at compile time if `x' (why isn't this called
> "dividend") has an unsigned type.
>
> Would retaining that be of any benefit? We do want to avoid doing the
> compare-and-branch in as many cases as possible.
>
DIV_ROUND_CLOSEST(0,-2)=1
This also happens if I keep the sneaky code. The v3 code does not have this
problem. I know it is a bit theoretic, but still there. Of course, I could
simply ignore the divisor's sign entirely, assuming (and documenting) that
negative divisors are just too odd to deal with. Commentss welcome ...
> Also, this would be a great opportunity to document the macro's beahviour
> (I do go on). That would be a useful thing to do, given that we're now
> handling the four +/+, +/-, -/+, -/- cases and the behaviour for each
> case isn't terribly obvious.
>
Ok.
Guenter
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WARNING: multiple messages have this Message-ID (diff)
From: Guenter Roeck <linux@roeck-us.net>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-kernel@vger.kernel.org, Pekka Enberg <penberg@kernel.org>,
Ingo Molnar <mingo@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
Jean Delvare <khali@linux-fr.org>,
lm-sensors@lm-sensors.org
Subject: Re: [PATCH v3] linux/kernel.h: Fix DIV_ROUND_CLOSEST to support negative operands
Date: Thu, 30 Aug 2012 18:12:16 -0700 [thread overview]
Message-ID: <20120831011216.GA22010@roeck-us.net> (raw)
In-Reply-To: <20120830173531.291e7b6d.akpm@linux-foundation.org>
On Thu, Aug 30, 2012 at 05:35:31PM -0700, Andrew Morton wrote:
> On Thu, 30 Aug 2012 17:10:47 -0700 Guenter Roeck <linux@roeck-us.net> wrote:
>
> > DIV_ROUND_CLOSEST returns a bad result for dividends with different sign:
> > DIV_ROUND_CLOSEST(-2, 2) = 0
> >
> > Most of the time this does not matter. However, in the hardware monitoring
> > subsystem, DIV_ROUND_CLOSEST is sometimes used on integers which can be
> > negative (such as temperatures).
> >
> > ...
> >
> > --- a/include/linux/kernel.h
> > +++ b/include/linux/kernel.h
> > @@ -84,8 +84,11 @@
> > )
> > #define DIV_ROUND_CLOSEST(x, divisor)( \
> > { \
> > - typeof(divisor) __divisor = divisor; \
> > - (((x) + ((__divisor) / 2)) / (__divisor)); \
> > + typeof(x) __x = x; \
> > + typeof(divisor) __d = divisor; \
> > + ((__x) < 0) == ((__d) < 0) ? \
> > + (((__x) + ((__d) / 2)) / (__d)) : \
> > + (((__x) - ((__d) / 2)) / (__d)); \
> > } \
> > )
>
> Your v2 had that sneaky little "(typeof(x))-1 >= 0" trick in it, so
> half the code gets elided at compile time if `x' (why isn't this called
> "dividend") has an unsigned type.
>
> Would retaining that be of any benefit? We do want to avoid doing the
> compare-and-branch in as many cases as possible.
>
DIV_ROUND_CLOSEST(0,-2)=1
This also happens if I keep the sneaky code. The v3 code does not have this
problem. I know it is a bit theoretic, but still there. Of course, I could
simply ignore the divisor's sign entirely, assuming (and documenting) that
negative divisors are just too odd to deal with. Commentss welcome ...
> Also, this would be a great opportunity to document the macro's beahviour
> (I do go on). That would be a useful thing to do, given that we're now
> handling the four +/+, +/-, -/+, -/- cases and the behaviour for each
> case isn't terribly obvious.
>
Ok.
Guenter
next prev parent reply other threads:[~2012-08-31 1:12 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-08-31 0:10 [lm-sensors] [PATCH v3] linux/kernel.h: Fix DIV_ROUND_CLOSEST to support negative operands Guenter Roeck
2012-08-31 0:10 ` Guenter Roeck
2012-08-31 0:35 ` [lm-sensors] " Andrew Morton
2012-08-31 0:35 ` Andrew Morton
2012-08-31 1:12 ` Guenter Roeck [this message]
2012-08-31 1:12 ` Guenter Roeck
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