* [PATCH v3] linux/kernel.h: Fix DIV_ROUND_CLOSEST to support negative operands
@ 2012-08-31 0:10 Guenter Roeck
2012-08-31 0:35 ` Andrew Morton
0 siblings, 1 reply; 3+ messages in thread
From: Guenter Roeck @ 2012-08-31 0:10 UTC (permalink / raw)
To: linux-kernel
Cc: Andrew Morton, Pekka Enberg, Ingo Molnar, H. Peter Anvin,
Jean Delvare, lm-sensors, Guenter Roeck
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).
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
---
v3: Instead of adding a new macro, fix DIV_ROUND_CLOSEST.
This version works for negative dividend and divisor.
v2: v1 did not work if typeof(divisor) was an unsigned variable type
(which can obviously not be negative).
Rework to revert to DIV_ROUND_CLOSEST if the dividend is unsigned,
or if it is signed but non-negative.
include/linux/kernel.h | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)
diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 6043821..4b180de 100644
--- 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)); \
} \
)
--
1.7.9.7
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH v3] linux/kernel.h: Fix DIV_ROUND_CLOSEST to support negative operands
2012-08-31 0:10 [PATCH v3] linux/kernel.h: Fix DIV_ROUND_CLOSEST to support negative operands Guenter Roeck
@ 2012-08-31 0:35 ` Andrew Morton
2012-08-31 1:12 ` Guenter Roeck
0 siblings, 1 reply; 3+ messages in thread
From: Andrew Morton @ 2012-08-31 0:35 UTC (permalink / raw)
To: Guenter Roeck
Cc: linux-kernel, Pekka Enberg, Ingo Molnar, H. Peter Anvin,
Jean Delvare, lm-sensors
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.
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.
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH v3] linux/kernel.h: Fix DIV_ROUND_CLOSEST to support negative operands
2012-08-31 0:35 ` Andrew Morton
@ 2012-08-31 1:12 ` Guenter Roeck
0 siblings, 0 replies; 3+ messages in thread
From: Guenter Roeck @ 2012-08-31 1:12 UTC (permalink / raw)
To: Andrew Morton
Cc: linux-kernel, Pekka Enberg, Ingo Molnar, H. Peter Anvin,
Jean Delvare, lm-sensors
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
^ permalink raw reply [flat|nested] 3+ messages in thread
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2012-08-31 0:10 [PATCH v3] linux/kernel.h: Fix DIV_ROUND_CLOSEST to support negative operands Guenter Roeck
2012-08-31 0:35 ` Andrew Morton
2012-08-31 1:12 ` Guenter Roeck
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