Re: [PATCH 06/14] bitsperlong.h: introduce SMALL_CONST() macro

[Rasmus Villemoes]

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 <yury.norov@xxxxxxxxx>
>>> ---
>>>  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