Re: [PATCH 11/13] lib: add fast path for find_first_*_bit() and find_last_bit()

[Guenter Roeck <linux@roeck-us.net>]

On Mon, Mar 15, 2021 at 06:54:22PM -0700, Yury Norov wrote:
> Similarly to bitmap functions, users would benefit if we'll handle
> a case of small-size bitmaps that fit into a single word.
> 
> While here, move the find_last_bit() declaration to bitops/find.h
> where other find_*_bit() functions sit.
> 
> Signed-off-by: Yury Norov <yury.norov@gmail.com>
> ---
>  include/asm-generic/bitops/find.h | 50 ++++++++++++++++++++++++++++---
>  include/linux/bitops.h            | 12 --------
>  lib/find_bit.c                    | 12 ++++----
>  3 files changed, 52 insertions(+), 22 deletions(-)
> 
> diff --git a/include/asm-generic/bitops/find.h b/include/asm-generic/bitops/find.h
> index 4148c74a1e4d..8d818b304869 100644
> --- a/include/asm-generic/bitops/find.h
> +++ b/include/asm-generic/bitops/find.h
> @@ -5,6 +5,9 @@
>  extern unsigned long _find_next_bit(const unsigned long *addr1,
>  		const unsigned long *addr2, unsigned long nbits,
>  		unsigned long start, unsigned long invert, unsigned long le);
> +extern unsigned long _find_first_bit(const unsigned long *addr, unsigned long size);
> +extern unsigned long _find_first_zero_bit(const unsigned long *addr, unsigned long size);
> +extern unsigned long _find_last_bit(const unsigned long *addr, unsigned long size);
>  
>  #ifndef find_next_bit
>  /**
> @@ -102,8 +105,17 @@ unsigned long find_next_zero_bit(const unsigned long *addr, unsigned long size,
>   * Returns the bit number of the first set bit.
>   * If no bits are set, returns @size.
>   */
> -extern unsigned long find_first_bit(const unsigned long *addr,
> -				    unsigned long size);
> +static inline
> +unsigned long find_first_bit(const unsigned long *addr, unsigned long size)
> +{
> +	if (small_const_nbits(size)) {
> +		unsigned long val = *addr & BITS_FIRST(size - 1);
> +
> +		return val ? __ffs(val) : size;

This patch results in:

include/asm-generic/bitops/find.h: In function 'find_last_bit':
include/asm-generic/bitops/find.h:164:16: error: implicit declaration of function '__fls'; did you mean '__ffs'?

and:

./include/asm-generic/bitops/__fls.h: At top level:
./include/asm-generic/bitops/__fls.h:13:38: error: conflicting types for '__fls'

when building scripts/mod/devicetable-offsets.o.

Seen with h8300 builds.

Guenter

> +	}
> +
> +	return _find_first_bit(addr, size);
> +}
>  
>  /**
>   * find_first_zero_bit - find the first cleared bit in a memory region
> @@ -113,8 +125,17 @@ extern unsigned long find_first_bit(const unsigned long *addr,
>   * Returns the bit number of the first cleared bit.
>   * If no bits are zero, returns @size.
>   */
> -extern unsigned long find_first_zero_bit(const unsigned long *addr,
> -					 unsigned long size);
> +static inline
> +unsigned long find_first_zero_bit(const unsigned long *addr, unsigned long size)
> +{
> +	if (small_const_nbits(size)) {
> +		unsigned long val = *addr | ~BITS_FIRST(size - 1);
> +
> +		return val == ~0UL ? size : ffz(val);
> +	}
> +
> +	return _find_first_zero_bit(addr, size);
> +}
>  #else /* CONFIG_GENERIC_FIND_FIRST_BIT */
>  
>  #ifndef find_first_bit
> @@ -126,6 +147,27 @@ extern unsigned long find_first_zero_bit(const unsigned long *addr,
>  
>  #endif /* CONFIG_GENERIC_FIND_FIRST_BIT */
>  
> +#ifndef find_last_bit
> +/**
> + * find_last_bit - find the last set bit in a memory region
> + * @addr: The address to start the search at
> + * @size: The number of bits to search
> + *
> + * Returns the bit number of the last set bit, or size.
> + */
> +static inline
> +unsigned long find_last_bit(const unsigned long *addr, unsigned long size)
> +{
> +	if (small_const_nbits(size)) {
> +		unsigned long val = *addr & BITS_FIRST(size - 1);
> +
> +		return val ? __fls(val) : size;
> +	}
> +
> +	return _find_last_bit(addr, size);
> +}
> +#endif
> +
>  /**
>   * find_next_clump8 - find next 8-bit clump with set bits in a memory region
>   * @clump: location to store copy of found clump
> diff --git a/include/linux/bitops.h b/include/linux/bitops.h
> index a5a48303b0f1..26bf15e6cd35 100644
> --- a/include/linux/bitops.h
> +++ b/include/linux/bitops.h
> @@ -286,17 +286,5 @@ static __always_inline void __assign_bit(long nr, volatile unsigned long *addr,
>  })
>  #endif
>  
> -#ifndef find_last_bit
> -/**
> - * find_last_bit - find the last set bit in a memory region
> - * @addr: The address to start the search at
> - * @size: The number of bits to search
> - *
> - * Returns the bit number of the last set bit, or size.
> - */
> -extern unsigned long find_last_bit(const unsigned long *addr,
> -				   unsigned long size);
> -#endif
> -
>  #endif /* __KERNEL__ */
>  #endif
> diff --git a/lib/find_bit.c b/lib/find_bit.c
> index 2470ae390f3c..e2c301d28568 100644
> --- a/lib/find_bit.c
> +++ b/lib/find_bit.c
> @@ -75,7 +75,7 @@ EXPORT_SYMBOL(_find_next_bit);
>  /*
>   * Find the first set bit in a memory region.
>   */
> -unsigned long find_first_bit(const unsigned long *addr, unsigned long size)
> +unsigned long _find_first_bit(const unsigned long *addr, unsigned long size)
>  {
>  	unsigned long idx;
>  
> @@ -86,14 +86,14 @@ unsigned long find_first_bit(const unsigned long *addr, unsigned long size)
>  
>  	return size;
>  }
> -EXPORT_SYMBOL(find_first_bit);
> +EXPORT_SYMBOL(_find_first_bit);
>  #endif
>  
>  #ifndef find_first_zero_bit
>  /*
>   * Find the first cleared bit in a memory region.
>   */
> -unsigned long find_first_zero_bit(const unsigned long *addr, unsigned long size)
> +unsigned long _find_first_zero_bit(const unsigned long *addr, unsigned long size)
>  {
>  	unsigned long idx;
>  
> @@ -104,11 +104,11 @@ unsigned long find_first_zero_bit(const unsigned long *addr, unsigned long size)
>  
>  	return size;
>  }
> -EXPORT_SYMBOL(find_first_zero_bit);
> +EXPORT_SYMBOL(_find_first_zero_bit);
>  #endif
>  
>  #ifndef find_last_bit
> -unsigned long find_last_bit(const unsigned long *addr, unsigned long size)
> +unsigned long _find_last_bit(const unsigned long *addr, unsigned long size)
>  {
>  	if (size) {
>  		unsigned long val = BITS_FIRST_MASK(size - 1);
> @@ -124,7 +124,7 @@ unsigned long find_last_bit(const unsigned long *addr, unsigned long size)
>  	}
>  	return size;
>  }
> -EXPORT_SYMBOL(find_last_bit);
> +EXPORT_SYMBOL(_find_last_bit);
>  #endif
>  
>  unsigned long find_next_clump8(unsigned long *clump, const unsigned long *addr,