linux-kernel - Re: [PATCH v3 4/7] bitops: define const

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Date:   Mon, 20 Jun 2022 11:49:50 +0200
From:   Marco Elver <elver@...gle.com>
To:     Alexander Lobakin <alexandr.lobakin@...el.com>
Cc:     Arnd Bergmann <arnd@...db.de>, Yury Norov <yury.norov@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Mark Rutland <mark.rutland@....com>,
        Matt Turner <mattst88@...il.com>,
        Brian Cain <bcain@...cinc.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>,
        "David S. Miller" <davem@...emloft.net>,
        Kees Cook <keescook@...omium.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Borislav Petkov <bp@...e.de>, Tony Luck <tony.luck@...el.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-alpha@...r.kernel.org, linux-hexagon@...r.kernel.org,
        linux-ia64@...r.kernel.org, linux-m68k@...ts.linux-m68k.org,
        linux-sh@...r.kernel.org, sparclinux@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 4/7] bitops: define const_*() versions of the non-atomics

On Fri, 17 Jun 2022 at 19:21, Alexander Lobakin
<alexandr.lobakin@...el.com> 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 able to resolve those to compile-time constants as
> well. 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 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@...gle.com>
> Signed-off-by: Alexander Lobakin <alexandr.lobakin@...el.com>

Reviewed-by: Marco Elver <elver@...gle.com>

> ---
>  .../asm-generic/bitops/generic-non-atomic.h   | 31 +++++++++++++++++++
>  include/linux/bitops.h                        |  1 +
>  2 files changed, 32 insertions(+)
>
> diff --git a/include/asm-generic/bitops/generic-non-atomic.h b/include/asm-generic/bitops/generic-non-atomic.h
> index b85b8a2ac239..3d5ebd24652b 100644
> --- a/include/asm-generic/bitops/generic-non-atomic.h
> +++ b/include/asm-generic/bitops/generic-non-atomic.h
> @@ -127,4 +127,35 @@ 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 a compiler to optimize code harder. Non-atomic and to be called only
> + * for testing compile-time constants, e.g. by the corresponding macros, not
> + * directly from "regular" code.
> + */
> +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..d393297287d5 100644
> --- a/include/linux/bitops.h
> +++ b/include/linux/bitops.h
> @@ -37,6 +37,7 @@ 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) &&       \
> +                     __same_type(const_##name, generic_##name) &&      \
>                       __same_type(name, generic_##name))
>
>  __check_bitop_pr(__set_bit);
> --
> 2.36.1
>