Re: MSEC_TO_JIFFIES is messed up...

[Paul Wagland]

Nit pick I'm sure... but....

On May 13, 2004, at 0:18, William Lee Irwin III wrote:

> Optimize the cases where HZ is a divisor of 1000 or vice-versa in
> JIFFIES_TO_MSECS() and MSECS_TO_JIFFIES() by allowing the 
> nonvanishing(!)
> integral ratios to appear as a parenthesized expressions eligible for
> constant folding optimizations.
>
>
> -- wli
>
>
> Index: linux-2.5/include/linux/time.h
> ===================================================================
> --- linux-2.5.orig/include/linux/time.h	2004-05-12 15:04:10.000000000 
> -0700
> +++ linux-2.5/include/linux/time.h	2004-05-12 15:12:49.000000000 -0700
> @@ -184,12 +184,12 @@
>   * Avoid unnecessary multiplications/divisions in the
>   * two most common HZ cases:
>   */
> -#if HZ == 1000
> -# define JIFFIES_TO_MSECS(x)	(x)
> -# define MSECS_TO_JIFFIES(x)	(x)
> -#elif HZ == 100
> -# define JIFFIES_TO_MSECS(x)	((x) * 10)
> -# define MSECS_TO_JIFFIES(x)	(((x) + 9) / 10)
> +#if HZ <= 1000 && !(1000 % HZ)
> +# define JIFFIES_TO_MSECS(j)	((1000/HZ)*(j))
> +# define MSECS_TO_JIFFIES(m)	(((m) + (1000/HZ) - 1)/(1000/HZ))
> +#elif HZ > 1000 && !(HZ % 1000)
> +# define JIFFIES_TO_MSECS(j)	(((j) + (HZ/1000) - 1)/(HZ/1000))
> +# define MSECS_TO_JIFFIES(m)	((m)*(HZ/1000))
>  #else
>  # define JIFFIES_TO_MSECS(x)	(((x) * 1000) / HZ)
>  # define MSECS_TO_JIFFIES(x)	(((x) * HZ + 999) / 1000)

Also, can we keep the same parameter name across all of the macros?

This changes behaviour when HZ==(z)000

JIFFIES_TO_MSECS  goes from
((x) * 1000) / (z)000  to (((x) + (z) - 1)/(z))

i.e. for x=1, z=2 this goes from ((1)*1000)/2000)=0 to (((1)+(2)-1)/2)=1

However, MSECS_TO_JIFFIES remains the same going from
(((x) * (z)000 + 999) / 1000) to ((x)*(z))

I.e. they basically reduce down to the same thing (modulo overflows)

All of the other permuations look correct to me...

Cheers,
Paul

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