As we add more than one of those "flags", not using bitfields will grow
the memory footprint of these structures, so I don't think this is
advised here.
Thanks,
Mathieu
* walter harms (wharms@xxxxxx) wrote:
> 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@xxxxxx) 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@xxxxxxxxxx>
> >>> ---
> >>> 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);
> >>> --
> >>> To unsubscribe from this list: send the line "unsubscribe kernel-janitors" in
> >>> the body of a message to majordomo@xxxxxxxxxxxxxxx
> >>> More majordomo info at http://vger.kernel.org/majordomo-info.html
> >>>
> >>>
> >
--
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
_______________________________________________
devel mailing list
devel@xxxxxxxxxxxxxxxxxxxxxx
http://driverdev.linuxdriverproject.org/mailman/listinfo/devel
