Re: [PATCH 06/14] bitsperlong.h: introduce SMALL_CONST() macro
On Fri, Feb 19, 2021 at 12:07:27AM +0100, Rasmus Villemoes wrote:
> On 18/02/2021 05.05, Yury Norov wrote:
> > Many algorithms become simpler if they are passed with relatively small
> > input values. One example is bitmap operations when the whole bitmap fits
> > into one word. To implement such simplifications, linux/bitmap.h declares
> > small_const_nbits() macro.
> >
> > Other subsystems may also benefit from optimizations of this sort, like
> > find_bit API in the following patches. So it looks helpful to generalize
> > the macro and extend it's visibility.
>
> Perhaps, but SMALL_CONST is too generic a name, it needs to keep "bits"
> somewhere in there. So why not just keep it at small_const_nbits?
>
> > Signed-off-by: Yury Norov
> > ---
> > include/asm-generic/bitsperlong.h | 2 ++
> > include/linux/bitmap.h| 33 ++-
> > 2 files changed, 17 insertions(+), 18 deletions(-)
> >
> > diff --git a/include/asm-generic/bitsperlong.h
> > b/include/asm-generic/bitsperlong.h
> > index 3905c1c93dc2..0eeb77544f1d 100644
> > --- a/include/asm-generic/bitsperlong.h
> > +++ b/include/asm-generic/bitsperlong.h
> > @@ -23,4 +23,6 @@
> > #define BITS_PER_LONG_LONG 64
> > #endif
> >
> > +#define SMALL_CONST(n) (__builtin_constant_p(n) && (unsigned long)(n) <
> > BITS_PER_LONG)
> > +
> > #endif /* __ASM_GENERIC_BITS_PER_LONG */
> > diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
> > index adf7bd9f0467..e89f1dace846 100644
> > --- a/include/linux/bitmap.h
> > +++ b/include/linux/bitmap.h
> > @@ -224,9 +224,6 @@ extern int bitmap_print_to_pagebuf(bool list, char *buf,
> > * so make such users (should any ever turn up) call the out-of-line
> > * versions.
> > */
> > -#define small_const_nbits(nbits) \
> > - (__builtin_constant_p(nbits) && (nbits) <= BITS_PER_LONG && (nbits) > 0)
> > -
> > static inline void bitmap_zero(unsigned long *dst, unsigned int nbits)
> > {
> > unsigned int len = BITS_TO_LONGS(nbits) * sizeof(unsigned long);
> > @@ -278,7 +275,7 @@ extern void bitmap_to_arr32(u32 *buf, const unsigned
> > long *bitmap,
> > static inline int bitmap_and(unsigned long *dst, const unsigned long *src1,
> > const unsigned long *src2, unsigned int nbits)
> > {
> > - if (small_const_nbits(nbits))
> > + if (SMALL_CONST(nbits - 1))
>
> Please don't force most users to be changed to something less readable.
> What's wrong with just keeping small_const_nbits() the way it is,
> avoiding all this churn and keeping the readability?
The wrong thing is that it's defined in include/linux/bitmap.h, and I
cannot use it in include/asm-generic/bitops/find.h, so I have to either
move it to a separate header, or generalize and share with find.h and
other users this way. I prefer the latter option, thougt it's more
verbose.
> At a quick reading, one of the very few places where you end up not
> passing nbits-1 but just nbits is this
>
> unsigned long find_next_zero_bit_le(const void *addr, unsigned
> long size, unsigned long offset)
> {
> + if (SMALL_CONST(size)) {
> + unsigned long val = *(const unsigned long *)addr;
> +
> + if (unlikely(offset >= size))
> + return size;
>
> which is a regression, for much the same reason the nbits==0 case was
> excluded from small_const_nbits in the first place. If size is 0, we
> used to just return 0 early in _find_next_bit. But you've introduced a
> dereference of addr before that check is now done, which is
> theoretically an oops.
>
> If find_next_zero_bit_le cannot handle nbits==BITS_PER_LONG efficiently
> but requires one off-limits bit position, fine, so be it, add an extra
> "small_const_nbits() && nbits < BITS_PER_LONG" (and a comment).
Sure, it's my bad. I need to fix it.
What would you prefer, moving the small_const_nbits() to a separate
header, or introduce something like
#define small_const_size(n) SMALL_CONST((n) - 1)
to avoid mistakes like this? I think small_const_size() is better
because it might be potentially useful for someone else, but I can
go either way.
Re: [PATCH 06/14] bitsperlong.h: introduce SMALL_CONST() macro
On 12/03/2021 06.28, Yury Norov wrote:
> On Fri, Feb 19, 2021 at 12:07:27AM +0100, Rasmus Villemoes wrote:
>> On 18/02/2021 05.05, Yury Norov wrote:
>>> Many algorithms become simpler if they are passed with relatively small
>>> input values. One example is bitmap operations when the whole bitmap fits
>>> into one word. To implement such simplifications, linux/bitmap.h declares
>>> small_const_nbits() macro.
>>>
>>> Other subsystems may also benefit from optimizations of this sort, like
>>> find_bit API in the following patches. So it looks helpful to generalize
>>> the macro and extend it's visibility.
>>
>> Perhaps, but SMALL_CONST is too generic a name, it needs to keep "bits"
>> somewhere in there. So why not just keep it at small_const_nbits?
>>
>>> Signed-off-by: Yury Norov
>>> ---
>>> include/asm-generic/bitsperlong.h | 2 ++
>>> include/linux/bitmap.h| 33 ++-
>>> 2 files changed, 17 insertions(+), 18 deletions(-)
>>>
>>> diff --git a/include/asm-generic/bitsperlong.h
>>> b/include/asm-generic/bitsperlong.h
>>> index 3905c1c93dc2..0eeb77544f1d 100644
>>> --- a/include/asm-generic/bitsperlong.h
>>> +++ b/include/asm-generic/bitsperlong.h
>>> @@ -23,4 +23,6 @@
>>> #define BITS_PER_LONG_LONG 64
>>> #endif
>>>
>>> +#define SMALL_CONST(n) (__builtin_constant_p(n) && (unsigned long)(n) <
>>> BITS_PER_LONG)
>>> +
>>> #endif /* __ASM_GENERIC_BITS_PER_LONG */
>>> diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
>>> index adf7bd9f0467..e89f1dace846 100644
>>> --- a/include/linux/bitmap.h
>>> +++ b/include/linux/bitmap.h
>>> @@ -224,9 +224,6 @@ extern int bitmap_print_to_pagebuf(bool list, char *buf,
>>> * so make such users (should any ever turn up) call the out-of-line
>>> * versions.
>>> */
>>> -#define small_const_nbits(nbits) \
>>> - (__builtin_constant_p(nbits) && (nbits) <= BITS_PER_LONG && (nbits) > 0)
>>> -
>>> static inline void bitmap_zero(unsigned long *dst, unsigned int nbits)
>>> {
>>> unsigned int len = BITS_TO_LONGS(nbits) * sizeof(unsigned long);
>>> @@ -278,7 +275,7 @@ extern void bitmap_to_arr32(u32 *buf, const unsigned
>>> long *bitmap,
>>> static inline int bitmap_and(unsigned long *dst, const unsigned long *src1,
>>> const unsigned long *src2, unsigned int nbits)
>>> {
>>> - if (small_const_nbits(nbits))
>>> + if (SMALL_CONST(nbits - 1))
>>
>> Please don't force most users to be changed to something less readable.
>> What's wrong with just keeping small_const_nbits() the way it is,
>> avoiding all this churn and keeping the readability?
>
> The wrong thing is that it's defined in include/linux/bitmap.h, and I
> cannot use it in include/asm-generic/bitops/find.h, so I have to either
> move it to a separate header, or generalize and share with find.h and
> other users this way. I prefer the latter option, thougt it's more
> verbose.
The logical place would be the same place the BITS_PER_LONG macro is
defined, no? No need to introduce a new header for that, and all current
users of small_const_nbits() must already (very possibly indirectly)
include asm-generic/bitsperlong.h.
I do prefer to keep both the name small_const_nbits() and its current
semantics, which, although not currently spelled out that way anywhere,
is "is BITMAP_SIZE(nbits) known at compile time and equal to 1", which
is precisely what allows the static inlines to unconditionally
dereference the pointer (that's the "exclude the 0 case") and just deal
with that one word.
I don't like either SMALL_CONST or small_const_size, because nothing in
there says it has anything to do with bit ops. As I said, if you have
some special place that for some reason cannot handle
nbits==BITS_PER_LONG, then just add that as an additional constraint
with a comment why.
Rasmus
Re: [PATCH 06/14] bitsperlong.h: introduce SMALL_CONST() macro
On Fri, Mar 12, 2021 at 10:12:22AM +0100, Rasmus Villemoes wrote:
> On 12/03/2021 06.28, Yury Norov wrote:
> > On Fri, Feb 19, 2021 at 12:07:27AM +0100, Rasmus Villemoes wrote:
> >> On 18/02/2021 05.05, Yury Norov wrote:
> >>> Many algorithms become simpler if they are passed with relatively small
> >>> input values. One example is bitmap operations when the whole bitmap fits
> >>> into one word. To implement such simplifications, linux/bitmap.h declares
> >>> small_const_nbits() macro.
> >>>
> >>> Other subsystems may also benefit from optimizations of this sort, like
> >>> find_bit API in the following patches. So it looks helpful to generalize
> >>> the macro and extend it's visibility.
> >>
> >> Perhaps, but SMALL_CONST is too generic a name, it needs to keep "bits"
> >> somewhere in there. So why not just keep it at small_const_nbits?
> >>
> >>> Signed-off-by: Yury Norov
> >>> ---
> >>> include/asm-generic/bitsperlong.h | 2 ++
> >>> include/linux/bitmap.h| 33 ++-
> >>> 2 files changed, 17 insertions(+), 18 deletions(-)
> >>>
> >>> diff --git a/include/asm-generic/bitsperlong.h
> >>> b/include/asm-generic/bitsperlong.h
> >>> index 3905c1c93dc2..0eeb77544f1d 100644
> >>> --- a/include/asm-generic/bitsperlong.h
> >>> +++ b/include/asm-generic/bitsperlong.h
> >>> @@ -23,4 +23,6 @@
> >>> #define BITS_PER_LONG_LONG 64
> >>> #endif
> >>>
> >>> +#define SMALL_CONST(n) (__builtin_constant_p(n) && (unsigned long)(n) <
> >>> BITS_PER_LONG)
> >>> +
> >>> #endif /* __ASM_GENERIC_BITS_PER_LONG */
> >>> diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
> >>> index adf7bd9f0467..e89f1dace846 100644
> >>> --- a/include/linux/bitmap.h
> >>> +++ b/include/linux/bitmap.h
> >>> @@ -224,9 +224,6 @@ extern int bitmap_print_to_pagebuf(bool list, char
> >>> *buf,
> >>> * so make such users (should any ever turn up) call the out-of-line
> >>> * versions.
> >>> */
> >>> -#define small_const_nbits(nbits) \
> >>> - (__builtin_constant_p(nbits) && (nbits) <= BITS_PER_LONG && (nbits) > 0)
> >>> -
> >>> static inline void bitmap_zero(unsigned long *dst, unsigned int nbits)
> >>> {
> >>> unsigned int len = BITS_TO_LONGS(nbits) * sizeof(unsigned long);
> >>> @@ -278,7 +275,7 @@ extern void bitmap_to_arr32(u32 *buf, const unsigned
> >>> long *bitmap,
> >>> static inline int bitmap_and(unsigned long *dst, const unsigned long
> >>> *src1,
> >>> const unsigned long *src2, unsigned int nbits)
> >>> {
> >>> - if (small_const_nbits(nbits))
> >>> + if (SMALL_CONST(nbits - 1))
> >>
> >> Please don't force most users to be changed to something less readable.
> >> What's wrong with just keeping small_const_nbits() the way it is,
> >> avoiding all this churn and keeping the readability?
> >
> > The wrong thing is that it's defined in include/linux/bitmap.h, and I
> > cannot use it in include/asm-generic/bitops/find.h, so I have to either
> > move it to a separate header, or generalize and share with find.h and
> > other users this way. I prefer the latter option, thougt it's more
> > verbose.
>
> The logical place would be the same place the BITS_PER_LONG macro is
> defined, no?
Yes. This where I placed SMALL_CONST() in current version.
> No need to introduce a new header for that, and all current
> users of small_const_nbits() must already (very possibly indirectly)
> include asm-generic/bitsperlong.h.
>
> I do prefer to keep both the name small_const_nbits() and its current
> semantics, which, although not currently spelled out that way anywhere,
> is "is BITMAP_SIZE(nbits) known at compile time and equal to 1", which
> is precisely what allows the static inlines to unconditionally
> dereference the pointer (that's the "exclude the 0 case") and just deal
> with that one word.
>
> I don't like either SMALL_CONST or small_const_size, because nothing in
> there says it has anything to do with bit ops. As I said, if you have
> some special place that for some reason cannot handle
> nbits==BITS_PER_LONG, then just add that as an additional constraint
> with a comment why.
OK, I'll move small_const_nbits() to the bitsperlong header and
resubmit shortly. My concern still is that nbits is too specific
for bitsperlong.h, but if you're good with it, I'm OK as well.
Re: [PATCH 06/14] bitsperlong.h: introduce SMALL_CONST() macro
On 18/02/2021 05.05, Yury Norov wrote:
> Many algorithms become simpler if they are passed with relatively small
> input values. One example is bitmap operations when the whole bitmap fits
> into one word. To implement such simplifications, linux/bitmap.h declares
> small_const_nbits() macro.
>
> Other subsystems may also benefit from optimizations of this sort, like
> find_bit API in the following patches. So it looks helpful to generalize
> the macro and extend it's visibility.
Perhaps, but SMALL_CONST is too generic a name, it needs to keep "bits"
somewhere in there. So why not just keep it at small_const_nbits?
> Signed-off-by: Yury Norov
> ---
> include/asm-generic/bitsperlong.h | 2 ++
> include/linux/bitmap.h| 33 ++-
> 2 files changed, 17 insertions(+), 18 deletions(-)
>
> diff --git a/include/asm-generic/bitsperlong.h
> b/include/asm-generic/bitsperlong.h
> index 3905c1c93dc2..0eeb77544f1d 100644
> --- a/include/asm-generic/bitsperlong.h
> +++ b/include/asm-generic/bitsperlong.h
> @@ -23,4 +23,6 @@
> #define BITS_PER_LONG_LONG 64
> #endif
>
> +#define SMALL_CONST(n) (__builtin_constant_p(n) && (unsigned long)(n) <
> BITS_PER_LONG)
> +
> #endif /* __ASM_GENERIC_BITS_PER_LONG */
> diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
> index adf7bd9f0467..e89f1dace846 100644
> --- a/include/linux/bitmap.h
> +++ b/include/linux/bitmap.h
> @@ -224,9 +224,6 @@ extern int bitmap_print_to_pagebuf(bool list, char *buf,
> * so make such users (should any ever turn up) call the out-of-line
> * versions.
> */
> -#define small_const_nbits(nbits) \
> - (__builtin_constant_p(nbits) && (nbits) <= BITS_PER_LONG && (nbits) > 0)
> -
> static inline void bitmap_zero(unsigned long *dst, unsigned int nbits)
> {
> unsigned int len = BITS_TO_LONGS(nbits) * sizeof(unsigned long);
> @@ -278,7 +275,7 @@ extern void bitmap_to_arr32(u32 *buf, const unsigned long
> *bitmap,
> static inline int bitmap_and(unsigned long *dst, const unsigned long *src1,
> const unsigned long *src2, unsigned int nbits)
> {
> - if (small_const_nbits(nbits))
> + if (SMALL_CONST(nbits - 1))
Please don't force most users to be changed to something less readable.
What's wrong with just keeping small_const_nbits() the way it is,
avoiding all this churn and keeping the readability?
At a quick reading, one of the very few places where you end up not
passing nbits-1 but just nbits is this
unsigned long find_next_zero_bit_le(const void *addr, unsigned
long size, unsigned long offset)
{
+ if (SMALL_CONST(size)) {
+ unsigned long val = *(const unsigned long *)addr;
+
+ if (unlikely(offset >= size))
+ return size;
which is a regression, for much the same reason the nbits==0 case was
excluded from small_const_nbits in the first place. If size is 0, we
used to just return 0 early in _find_next_bit. But you've introduced a
dereference of addr before that check is now done, which is
theoretically an oops.
If find_next_zero_bit_le cannot handle nbits==BITS_PER_LONG efficiently
but requires one off-limits bit position, fine, so be it, add an extra
"small_const_nbits() && nbits < BITS_PER_LONG" (and a comment).
Rasmus
