Re: [PATCH v2 4/6] bitops: define const_*() versions of the non-atomics

[Yury Norov <yury.norov@gmail.com>]

On Fri, Jun 10, 2022 at 01:34:25PM +0200, Alexander Lobakin wrote:
> Define const_*() variants of the non-atomic bitops to be used when
> the input arguments are compile-time constants, so that the compiler
> will be always to resolve those to compile-time constants as well.

will be always able?

> Those are mostly direct aliases for generic_*() with one exception
> for const_test_bit(): the original one is declared atomic-safe and
> thus doesn't discard the `volatile` qualifier, so in order to let
> optimize the code, define it separately disregarding the qualifier.
> Add them to the compile-time type checks as well just in case.
> 
> Suggested-by: Marco Elver <elver@google.com>
> Signed-off-by: Alexander Lobakin <alexandr.lobakin@intel.com>
> ---
>  .../asm-generic/bitops/generic-non-atomic.h   | 31 +++++++++++++++++++
>  include/linux/bitops.h                        |  3 +-
>  2 files changed, 33 insertions(+), 1 deletion(-)
> 
> diff --git a/include/asm-generic/bitops/generic-non-atomic.h b/include/asm-generic/bitops/generic-non-atomic.h
> index 3ce0fa0ab35f..9a77babfff35 100644
> --- a/include/asm-generic/bitops/generic-non-atomic.h
> +++ b/include/asm-generic/bitops/generic-non-atomic.h
> @@ -121,4 +121,35 @@ generic_test_bit(unsigned long nr, const volatile unsigned long *addr)
>  	return 1UL & (addr[BIT_WORD(nr)] >> (nr & (BITS_PER_LONG-1)));
>  }
>  
> +/*
> + * const_*() definitions provide good compile-time optimizations when
> + * the passed arguments can be resolved at compile time.
> + */
> +#define const___set_bit			generic___set_bit
> +#define const___clear_bit		generic___clear_bit
> +#define const___change_bit		generic___change_bit
> +#define const___test_and_set_bit	generic___test_and_set_bit
> +#define const___test_and_clear_bit	generic___test_and_clear_bit
> +#define const___test_and_change_bit	generic___test_and_change_bit
> +
> +/**
> + * const_test_bit - Determine whether a bit is set
> + * @nr: bit number to test
> + * @addr: Address to start counting from
> + *
> + * A version of generic_test_bit() which discards the `volatile` qualifier to
> + * allow the compiler to optimize code harder. Non-atomic and to be used only
> + * for testing compile-time constants, e.g. from the corresponding macro, or
> + * when you really know what you are doing.

Not sure I understand the last sentence... Can you please rephrase?

> + */
> +static __always_inline bool
> +const_test_bit(unsigned long nr, const volatile unsigned long *addr)
> +{
> +	const unsigned long *p = (const unsigned long *)addr + BIT_WORD(nr);
> +	unsigned long mask = BIT_MASK(nr);
> +	unsigned long val = *p;
> +
> +	return !!(val & mask);
> +}
> +
>  #endif /* __ASM_GENERIC_BITOPS_GENERIC_NON_ATOMIC_H */
> diff --git a/include/linux/bitops.h b/include/linux/bitops.h
> index 87087454a288..51c22b8667b4 100644
> --- a/include/linux/bitops.h
> +++ b/include/linux/bitops.h
> @@ -36,7 +36,8 @@ extern unsigned long __sw_hweight64(__u64 w);
>  
>  /* Check that the bitops prototypes are sane */
>  #define __check_bitop_pr(name)						\
> -	static_assert(__same_type(arch_##name, generic_##name) &&	\
> +	static_assert(__same_type(const_##name, generic_##name) &&	\
> +		      __same_type(arch_##name, generic_##name) &&	\
>  		      __same_type(name, generic_##name))
>  
>  __check_bitop_pr(__set_bit);
> -- 
> 2.36.1