[PATCH 2/4] dsmark: get rid of trivial function

[PATCH 2/4] dsmark: get rid of trivial function

[Stephen Hemminger]

Replace loop in dsmark_valid_indices with equivalent bit math.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>

--- a/net/sched/sch_dsmark.c	2008-01-20 13:07:58.000000000 -0800
+++ b/net/sched/sch_dsmark.c	2008-01-20 13:22:54.000000000 -0800
@@ -45,13 +45,8 @@ struct dsmark_qdisc_data {
 
 static inline int dsmark_valid_indices(u16 indices)
 {
-	while (indices != 1) {
-		if (indices & 1)
-			return 0;
-		indices >>= 1;
-	}
-
-	return 1;
+	/* Must have only one bit set */
+	return (indices & (indices - 1)) == 0;
 }
 
 static inline int dsmark_valid_index(struct dsmark_qdisc_data *p, u16 index)

Re: [PATCH 2/4] dsmark: get rid of trivial function

[Patrick McHardy]

Stephen Hemminger wrote:
> Replace loop in dsmark_valid_indices with equivalent bit math.
> 
> Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
> 
> --- a/net/sched/sch_dsmark.c	2008-01-20 13:07:58.000000000 -0800
> +++ b/net/sched/sch_dsmark.c	2008-01-20 13:22:54.000000000 -0800
> @@ -45,13 +45,8 @@ struct dsmark_qdisc_data {
>  
>  static inline int dsmark_valid_indices(u16 indices)
>  {
> -	while (indices != 1) {
> -		if (indices & 1)
> -			return 0;
> -		indices >>= 1;
> -	}
> -
> -	return 1;
> +	/* Must have only one bit set */
> +	return (indices & (indices - 1)) == 0;


hweight seems easier to understand, it took me a bit
to realize that the comment matches the code :)

Re: [PATCH 2/4] dsmark: get rid of trivial function

[Ilpo Järvinen]

On Mon, 21 Jan 2008, Patrick McHardy wrote:

> Stephen Hemminger wrote:
> > Replace loop in dsmark_valid_indices with equivalent bit math.
> > 
> > Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
> > 
> > --- a/net/sched/sch_dsmark.c	2008-01-20 13:07:58.000000000 -0800
> > +++ b/net/sched/sch_dsmark.c	2008-01-20 13:22:54.000000000 -0800
> > @@ -45,13 +45,8 @@ struct dsmark_qdisc_data {
> >  
> >  static inline int dsmark_valid_indices(u16 indices)
> >  {
> > -	while (indices != 1) {
> > -		if (indices & 1)
> > -			return 0;
> > -		indices >>= 1;
> > -	}
> > -
> > -	return 1;
> > +	/* Must have only one bit set */
> > +	return (indices & (indices - 1)) == 0;

Isn't there some magic under include/linux to do that btw, I suppose 
that if the caller side zero check is pushed down there too, the
is_power_of_2() is 100% match? :-) 

> hweight seems easier to understand, it took me a bit
> to realize that the comment matches the code :)

In addition, the original seems infinite loop with zero indices given
but luckily that was checked at the caller site already...


-- 
 i.

Re: [PATCH 2/4] dsmark: get rid of trivial function

[David Miller]

From: Patrick McHardy <kaber@trash.net>
Date: Mon, 21 Jan 2008 01:16:32 +0100

> Stephen Hemminger wrote:
> > Replace loop in dsmark_valid_indices with equivalent bit math.
> > 
> > Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
> > 
> > --- a/net/sched/sch_dsmark.c	2008-01-20 13:07:58.000000000 -0800
> > +++ b/net/sched/sch_dsmark.c	2008-01-20 13:22:54.000000000 -0800
> > @@ -45,13 +45,8 @@ struct dsmark_qdisc_data {
> >  
> >  static inline int dsmark_valid_indices(u16 indices)
> >  {
> > -	while (indices != 1) {
> > -		if (indices & 1)
> > -			return 0;
> > -		indices >>= 1;
> > -	}
> > -
> > -	return 1;
> > +	/* Must have only one bit set */
> > +	return (indices & (indices - 1)) == 0;
> 
> hweight seems easier to understand, it took me a bit
> to realize that the comment matches the code :)

Sounds good.  Here is what I ended up checking in.

[PKT_SCHED] dsmark: Use hweight32() instead of convoluted loop.

Based upon a patch by Stephen Hemminger and suggestions
from Patrick McHardy.

Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/sched/sch_dsmark.c b/net/sched/sch_dsmark.c
index a9732ae..d96eaf0 100644
--- a/net/sched/sch_dsmark.c
+++ b/net/sched/sch_dsmark.c
@@ -10,6 +10,7 @@
 #include <linux/errno.h>
 #include <linux/skbuff.h>
 #include <linux/rtnetlink.h>
+#include <linux/bitops.h>
 #include <net/pkt_sched.h>
 #include <net/dsfield.h>
 #include <net/inet_ecn.h>
@@ -43,17 +44,6 @@ struct dsmark_qdisc_data {
 	int			set_tc_index;
 };
 
-static inline int dsmark_valid_indices(u16 indices)
-{
-	while (indices != 1) {
-		if (indices & 1)
-			return 0;
-		indices >>= 1;
-	}
-
-	return 1;
-}
-
 static inline int dsmark_valid_index(struct dsmark_qdisc_data *p, u16 index)
 {
 	return (index <= p->indices && index > 0);
@@ -348,7 +338,8 @@ static int dsmark_init(struct Qdisc *sch, struct rtattr *opt)
 		goto errout;
 
 	indices = RTA_GET_U16(tb[TCA_DSMARK_INDICES-1]);
-	if (!indices || !dsmark_valid_indices(indices))
+
+	if (hweight32(indices) != 1)
 		goto errout;
 
 	if (tb[TCA_DSMARK_DEFAULT_INDEX-1])

Re: [PATCH 2/4] dsmark: get rid of trivial function

[Stephen Hemminger]

On Mon, 21 Jan 2008 02:22:23 -0800 (PST)
David Miller <davem@davemloft.net> wrote:

> From: Patrick McHardy <kaber@trash.net>
> Date: Mon, 21 Jan 2008 01:16:32 +0100
> 
> > Stephen Hemminger wrote:
> > > Replace loop in dsmark_valid_indices with equivalent bit math.
> > > 
> > > Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
> > > 
> > > --- a/net/sched/sch_dsmark.c	2008-01-20 13:07:58.000000000 -0800
> > > +++ b/net/sched/sch_dsmark.c	2008-01-20 13:22:54.000000000 -0800
> > > @@ -45,13 +45,8 @@ struct dsmark_qdisc_data {
> > >  
> > >  static inline int dsmark_valid_indices(u16 indices)
> > >  {
> > > -	while (indices != 1) {
> > > -		if (indices & 1)
> > > -			return 0;
> > > -		indices >>= 1;
> > > -	}
> > > -
> > > -	return 1;
> > > +	/* Must have only one bit set */
> > > +	return (indices & (indices - 1)) == 0;
> > 
> > hweight seems easier to understand, it took me a bit
> > to realize that the comment matches the code :)
> 
> Sounds good.  Here is what I ended up checking in.
> 
> [PKT_SCHED] dsmark: Use hweight32() instead of convoluted loop.
> 
> Based upon a patch by Stephen Hemminger and suggestions
> from Patrick McHardy.
> 
> Signed-off-by: David S. Miller <davem@davemloft.net>
> 
> diff --git a/net/sched/sch_dsmark.c b/net/sched/sch_dsmark.c
> index a9732ae..d96eaf0 100644
> --- a/net/sched/sch_dsmark.c
> +++ b/net/sched/sch_dsmark.c
> @@ -10,6 +10,7 @@
>  #include <linux/errno.h>
>  #include <linux/skbuff.h>
>  #include <linux/rtnetlink.h>
> +#include <linux/bitops.h>
>  #include <net/pkt_sched.h>
>  #include <net/dsfield.h>
>  #include <net/inet_ecn.h>
> @@ -43,17 +44,6 @@ struct dsmark_qdisc_data {
>  	int			set_tc_index;
>  };
>  
> -static inline int dsmark_valid_indices(u16 indices)
> -{
> -	while (indices != 1) {
> -		if (indices & 1)
> -			return 0;
> -		indices >>= 1;
> -	}
> -
> -	return 1;
> -}
> -
>  static inline int dsmark_valid_index(struct dsmark_qdisc_data *p, u16 index)
>  {
>  	return (index <= p->indices && index > 0);
> @@ -348,7 +338,8 @@ static int dsmark_init(struct Qdisc *sch, struct rtattr *opt)
>  		goto errout;
>  
>  	indices = RTA_GET_U16(tb[TCA_DSMARK_INDICES-1]);
> -	if (!indices || !dsmark_valid_indices(indices))
> +
> +	if (hweight32(indices) != 1)
>  		goto errout;

Come on Dave, that is a step backwards.
So you took a two instruction thing that any programmer who ever had one of those
technical trick interviews would surely understand, and made it call a function...
Seems like the thing you would consul others against.

Please use !is_power_of_2(indices) instead.

Re: [PATCH 2/4] dsmark: get rid of trivial function

[David Miller]

From: Stephen Hemminger <stephen.hemminger@vyatta.com>
Date: Mon, 21 Jan 2008 19:47:51 -0800

> >  
> >  	indices = RTA_GET_U16(tb[TCA_DSMARK_INDICES-1]);
> > -	if (!indices || !dsmark_valid_indices(indices))
> > +
> > +	if (hweight32(indices) != 1)
> >  		goto errout;
> 
> Come on Dave, that is a step backwards.

Absolutely not.

> So you took a two instruction thing that any programmer who ever had
> one of those technical trick interviews would surely understand, and
> made it call a function...  Seems like the thing you would consul
> others against.

It's counting bits, "hamming weight" is a count of bits.

That is more understandable to me than:

	Oh BTW, power of two values also just so happen to
	have only 1 bit set.

Testing for a power of two obfuscates the meaning of the
test.  It doesn't want a power-of-two, it wants a bitmask
with only one bit set.