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

[walter harms]

Hello Mathieu,
nice to hear someone is concerned about space.
since you plan to go for uint perhaps we can drop that bitfield stuff at all ?

re,
 wh


Am 01.12.2011 15:20, schrieb 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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>