[lttng-dev] [patch] Staging: lttng: dubious one-bit signed bitfields

[Mathieu Desnoyers]

* walter harms (wharms at bfs.de) wrote:
> hi,
> This patch looks ok to me but this design is ugly by itself.
> It should be replaced by an uchar uint whatever or use a
> real bool (obviously not preferred by this programmes).

bool :1, uchar :1 or uint :1 could make sense. uchar:1/bool:1 won't save
any space here, because the surrounding fields are either uint or
pointers, so alignment will just add padding.

I try to use int/uint whenever possible because x86 CPUs tend to get
less register false-dependencies when using instructions modifying the
whole register (generated by using int/uint types) rather than only part
of it (uchar/char/bool). I only use char/uchar/bool when there is a
clear wanted space gain.

The reason why I never use the bool type within a structure when I want
a compact representation is that bool takes a whole byte just to
represent one bit:

#include <stdio.h>
#include <stdbool.h>

struct usebitfield {
        int a;
        unsigned int f:1, g:1, h:1, i:1, j:1;
        int b;
};

struct usebool {
        int a;
        bool f, g, h, i, j;
        int b;
};

struct useboolbf {
        int a;
        bool f:1, g:1, h:1, i:1, j:1;
        int b;
};

int main()
{
        printf("bitfield %d bytes, bool %d bytes, boolbitfield %d bytes\n",
                sizeof(struct usebitfield), sizeof(struct usebool),
                sizeof(struct useboolbf));
}

result:

bitfield 12 bytes, bool 16 bytes, boolbitfield 12 bytes

This is because each bool takes one byte, while the bitfields are put in
units of "unsigned int" (or bool for the 3rd struct). So in this
example, we need 5 bytes + 3 bytes alignment for the bool, but only 4
bytes to hold the "unsigned int" unit for the bitfields.

The choice between bool and bitfields must also take into account the
frequency of access to the variable, because bitfields require mask
operations to access the selected bit(s). You will notice that none of
these bitfields are accessed on the tracing fast-path: only in
slow-paths. Therefore, space gain is more important than speed here.

One might argue that I have so few of these fields here that it does not
make an actual difference to go for bitfield or bool. I am just trying
to choose types best suited for their intended purpose, ensuring they
are future-proof and will allow simply adding more fields using the same
type, as needed.

So I guess I'll go for uint :1.

Thanks,

Mathieu

> 
> re,
>  wh
> 
> Am 01.12.2011 10:37, schrieb Dan Carpenter:
> > Sparse complains that these signed bitfields look "dubious".  The
> > problem is that instead of being either 0 or 1 like people would expect,
> > signed one bit variables like this are either 0 or -1.  It doesn't cause
> > a problem in this case but it's ugly so lets fix them.
> > 
> > Signed-off-by: Dan Carpenter <dan.carpenter at oracle.com>
> > ---
> > I just did this against linux next but it applies fine on top of
> > Mathieu's recent patches.
> > 
> > diff --git a/drivers/staging/lttng/lib/ringbuffer/backend_types.h b/drivers/staging/lttng/lib/ringbuffer/backend_types.h
> > index 1d301de..019929a 100644
> > --- a/drivers/staging/lttng/lib/ringbuffer/backend_types.h
> > +++ b/drivers/staging/lttng/lib/ringbuffer/backend_types.h
> > @@ -65,7 +65,7 @@ struct channel_backend {
> >  					 * for writer.
> >  					 */
> >  	unsigned int buf_size_order;	/* Order of buffer size */
> > -	int extra_reader_sb:1;		/* Bool: has extra reader subbuffer */
> > +	unsigned int extra_reader_sb:1;	/* Bool: has extra reader subbuffer */
> >  	struct lib_ring_buffer *buf;	/* Channel per-cpu buffers */
> >  
> >  	unsigned long num_subbuf;	/* Number of sub-buffers for writer */
> > diff --git a/drivers/staging/lttng/lib/ringbuffer/frontend_types.h b/drivers/staging/lttng/lib/ringbuffer/frontend_types.h
> > index 5c7437f..9086c58 100644
> > --- a/drivers/staging/lttng/lib/ringbuffer/frontend_types.h
> > +++ b/drivers/staging/lttng/lib/ringbuffer/frontend_types.h
> > @@ -60,8 +60,8 @@ struct channel {
> >  	struct notifier_block cpu_hp_notifier;	/* CPU hotplug notifier */
> >  	struct notifier_block tick_nohz_notifier; /* CPU nohz notifier */
> >  	struct notifier_block hp_iter_notifier;	/* hotplug iterator notifier */
> > -	int cpu_hp_enable:1;			/* Enable CPU hotplug notif. */
> > -	int hp_iter_enable:1;			/* Enable hp iter notif. */
> > +	unsigned int cpu_hp_enable:1;		/* Enable CPU hotplug notif. */
> > +	unsigned int hp_iter_enable:1;		/* Enable hp iter notif. */
> >  	wait_queue_head_t read_wait;		/* reader wait queue */
> >  	wait_queue_head_t hp_wait;		/* CPU hotplug wait queue */
> >  	int finalized;				/* Has channel been finalized */
> > @@ -94,8 +94,8 @@ struct lib_ring_buffer_iter {
> >  		ITER_NEXT_RECORD,
> >  		ITER_PUT_SUBBUF,
> >  	} state;
> > -	int allocated:1;
> > -	int read_open:1;		/* Opened for reading ? */
> > +	unsigned int allocated:1;
> > +	unsigned int read_open:1;		/* Opened for reading ? */
> >  };
> >  
> >  /* ring buffer state */
> > @@ -138,9 +138,9 @@ struct lib_ring_buffer {
> >  	unsigned long get_subbuf_consumed;	/* Read-side consumed */
> >  	unsigned long prod_snapshot;	/* Producer count snapshot */
> >  	unsigned long cons_snapshot;	/* Consumer count snapshot */
> > -	int get_subbuf:1;		/* Sub-buffer being held by reader */
> > -	int switch_timer_enabled:1;	/* Protected by ring_buffer_nohz_lock */
> > -	int read_timer_enabled:1;	/* Protected by ring_buffer_nohz_lock */
> > +	unsigned int get_subbuf:1;		/* Sub-buffer being held by reader */
> > +	unsigned int switch_timer_enabled:1;	/* Protected by ring_buffer_nohz_lock */
> > +	unsigned int read_timer_enabled:1;	/* Protected by ring_buffer_nohz_lock */
> >  };
> >  
> >  static inline
> > diff --git a/drivers/staging/lttng/ltt-events.h b/drivers/staging/lttng/ltt-events.h
> > index 36b281a..3fc355d 100644
> > --- a/drivers/staging/lttng/ltt-events.h
> > +++ b/drivers/staging/lttng/ltt-events.h
> > @@ -191,7 +191,7 @@ struct ltt_event {
> >  		} ftrace;
> >  	} u;
> >  	struct list_head list;		/* Event list */
> > -	int metadata_dumped:1;
> > +	unsigned int metadata_dumped:1;
> >  };
> >  
> >  struct ltt_channel_ops {
> > @@ -251,7 +251,7 @@ struct ltt_channel {
> >  	struct ltt_event *sc_compat_unknown;
> >  	struct ltt_event *sc_exit;	/* for syscall exit */
> >  	int header_type;		/* 0: unset, 1: compact, 2: large */
> > -	int metadata_dumped:1;
> > +	unsigned int metadata_dumped:1;
> >  };
> >  
> >  struct ltt_session {
> > @@ -264,7 +264,7 @@ struct ltt_session {
> >  	struct list_head list;		/* Session list */
> >  	unsigned int free_chan_id;	/* Next chan ID to allocate */
> >  	uuid_le uuid;			/* Trace session unique ID */
> > -	int metadata_dumped:1;
> > +	unsigned int metadata_dumped:1;
> >  };
> >  
> >  struct ltt_session *ltt_session_create(void);
> > --
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> > 
> > 

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com