Re: [PATCH] xen/bitmap: Consistently use unsigned bits values

[Jan Beulich <jbeulich@xxxxxxxx>]

On 01.02.2024 11:33, Andrew Cooper wrote:
> Right now, most of the static inline helpers take an unsigned nbits quantity,
> and most of the library functions take a signed quanity.  Because
> BITMAP_LAST_WORD_MASK() is expressed as a divide, the compiler is forced to
> emit two different paths to get the correct semantics for signed division.
> 
> Swap all signed bit-counts to being unsigned bit-counts for the simple cases.
> This includes the return value of bitmap_weight().
> 
> Bloat-o-meter for a random x86 build reports:
>   add/remove: 0/0 grow/shrink: 8/19 up/down: 167/-413 (-246)
> 
> which all comes from compiler not emitting "dead" logic paths for negative bit
> counts.
> 
> No functional change.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
albeit with a question at the bottom.

> There is much more wanting cleaning up here, but we have to start somewhere.
> Some observations:
> 
>  * Various of the boolean-like return values have -1 for zero-length bitmaps.
>    I can't spot any callers which care, so this seems like a waste.
>  * bitmap_zero() and similar clear predate us switching to use
>    __builtin_memset(), because there's no need for bitmap_switch().
>  * Should we consolidate 'bits' vs 'nbits'?

This looks desirable to me.

>  * The internals of these helpers want converting too.  Other helpers need
>    more than just a parameter conversion.

This may or may not relate to my question; it's not exactly clear what you
mean here.

> --- a/xen/include/xen/bitmap.h
> +++ b/xen/include/xen/bitmap.h
> @@ -66,25 +66,25 @@
>   * lib/bitmap.c provides these functions:
>   */
>  
> -extern int __bitmap_empty(const unsigned long *bitmap, int bits);
> -extern int __bitmap_full(const unsigned long *bitmap, int bits);
> -extern int __bitmap_equal(const unsigned long *bitmap1,
> -                     const unsigned long *bitmap2, int bits);
> -extern void __bitmap_complement(unsigned long *dst, const unsigned long *src,
> -                     int bits);
> -extern void __bitmap_and(unsigned long *dst, const unsigned long *bitmap1,
> -                     const unsigned long *bitmap2, int bits);
> -extern void __bitmap_or(unsigned long *dst, const unsigned long *bitmap1,
> -                     const unsigned long *bitmap2, int bits);
> -extern void __bitmap_xor(unsigned long *dst, const unsigned long *bitmap1,
> -                     const unsigned long *bitmap2, int bits);
> -extern void __bitmap_andnot(unsigned long *dst, const unsigned long *bitmap1,
> -                     const unsigned long *bitmap2, int bits);
> -extern int __bitmap_intersects(const unsigned long *bitmap1,
> -                     const unsigned long *bitmap2, int bits);
> -extern int __bitmap_subset(const unsigned long *bitmap1,
> -                     const unsigned long *bitmap2, int bits);
> -extern int __bitmap_weight(const unsigned long *bitmap, int bits);
> +int __bitmap_empty(const unsigned long *bitmap, unsigned int bits);
> +int __bitmap_full(const unsigned long *bitmap, unsigned int bits);
> +int __bitmap_equal(const unsigned long *bitmap1,
> +                   const unsigned long *bitmap2, unsigned int bits);
> +void __bitmap_complement(unsigned long *dst, const unsigned long *src,
> +                         unsigned int bits);
> +void __bitmap_and(unsigned long *dst, const unsigned long *bitmap1,
> +                  const unsigned long *bitmap2, unsigned int bits);
> +void __bitmap_or(unsigned long *dst, const unsigned long *bitmap1,
> +                 const unsigned long *bitmap2, unsigned int bits);
> +void __bitmap_xor(unsigned long *dst, const unsigned long *bitmap1,
> +                  const unsigned long *bitmap2, unsigned int bits);
> +void __bitmap_andnot(unsigned long *dst, const unsigned long *bitmap1,
> +                     const unsigned long *bitmap2, unsigned int bits);
> +int __bitmap_intersects(const unsigned long *bitmap1,
> +                        const unsigned long *bitmap2, unsigned int bits);
> +int __bitmap_subset(const unsigned long *bitmap1,
> +                    const unsigned long *bitmap2, unsigned int bits);
> +unsigned int __bitmap_weight(const unsigned long *bitmap, unsigned int bits);
>  extern void __bitmap_set(unsigned long *map, unsigned int start, int len);
>  extern void __bitmap_clear(unsigned long *map, unsigned int start, int len);

What about these two, and the subsequent (in the .c file at least)
bitmap_*_region()? That last remark above may mean you deliberately
left them out for now (which is okay - as you say, this is merely a
1st step).

Jan