Re: [PATCH v3 6/7] bitops: let optimize out non-atomic bitops on compile-time constants

[Marco Elver]

On Fri, 17 Jun 2022 at 19:00, Alexander Lobakin
<alexandr.lobakin@xxxxxxxxx> wrote:
>
> Currently, many architecture-specific non-atomic bitop
> implementations use inline asm or other hacks which are faster or
> more robust when working with "real" variables (i.e. fields from
> the structures etc.), but the compilers have no clue how to optimize
> them out when called on compile-time constants. That said, the
> following code:
>
>         DECLARE_BITMAP(foo, BITS_PER_LONG) = { }; // -> unsigned long foo[1];
>         unsigned long bar = BIT(BAR_BIT);
>         unsigned long baz = 0;
>
>         __set_bit(FOO_BIT, foo);
>         baz |= BIT(BAZ_BIT);
>
>         BUILD_BUG_ON(!__builtin_constant_p(test_bit(FOO_BIT, foo));
>         BUILD_BUG_ON(!__builtin_constant_p(bar & BAR_BIT));
>         BUILD_BUG_ON(!__builtin_constant_p(baz & BAZ_BIT));
>
> triggers the first assertion on x86_64, which means that the
> compiler is unable to evaluate it to a compile-time initializer
> when the architecture-specific bitop is used even if it's obvious.
> In order to let the compiler optimize out such cases, expand the
> bitop() macro to use the "constant" C non-atomic bitop
> implementations when all of the arguments passed are compile-time
> constants, which means that the result will be a compile-time
> constant as well, so that it produces more efficient and simple
> code in 100% cases, comparing to the architecture-specific
> counterparts.
>
> The savings are architecture, compiler and compiler flags dependent,
> for example, on x86_64 -O2:
>
> GCC 12: add/remove: 78/29 grow/shrink: 332/525 up/down: 31325/-61560 (-30235)
> LLVM 13: add/remove: 79/76 grow/shrink: 184/537 up/down: 55076/-141892 (-86816)
> LLVM 14: add/remove: 10/3 grow/shrink: 93/138 up/down: 3705/-6992 (-3287)
>
> and ARM64 (courtesy of Mark):
>
> GCC 11: add/remove: 92/29 grow/shrink: 933/2766 up/down: 39340/-82580 (-43240)
> LLVM 14: add/remove: 21/11 grow/shrink: 620/651 up/down: 12060/-15824 (-3764)
>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Signed-off-by: Alexander Lobakin <alexandr.lobakin@xxxxxxxxx>

Reviewed-by: Marco Elver <elver@xxxxxxxxxx>

> ---
>  include/linux/bitops.h | 18 +++++++++++++++++-
>  1 file changed, 17 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/bitops.h b/include/linux/bitops.h
> index 3c3afbae1533..26a43360c4ae 100644
> --- a/include/linux/bitops.h
> +++ b/include/linux/bitops.h
> @@ -33,8 +33,24 @@ extern unsigned long __sw_hweight64(__u64 w);
>
>  #include <asm-generic/bitops/generic-non-atomic.h>
>
> +/*
> + * Many architecture-specific non-atomic bitops contain inline asm code and due
> + * to that the compiler can't optimize them to compile-time expressions or
> + * constants. In contrary, gen_*() helpers are defined in pure C and compilers
> + * optimize them just well.
> + * Therefore, to make `unsigned long foo = 0; __set_bit(BAR, &foo)` effectively
> + * equal to `unsigned long foo = BIT(BAR)`, pick the generic C alternative when
> + * the arguments can be resolved at compile time. That expression itself is a
> + * constant and doesn't bring any functional changes to the rest of cases.
> + * The casts to `uintptr_t` are needed to mitigate `-Waddress` warnings when
> + * passing a bitmap from .bss or .data (-> `!!addr` is always true).
> + */
>  #define bitop(op, nr, addr)                                            \
> -       op(nr, addr)
> +       ((__builtin_constant_p(nr) &&                                   \
> +         __builtin_constant_p((uintptr_t)(addr) != (uintptr_t)NULL) && \
> +         (uintptr_t)(addr) != (uintptr_t)NULL &&                       \
> +         __builtin_constant_p(*(const unsigned long *)(addr))) ?       \
> +        const##op(nr, addr) : op(nr, addr))
>
>  #define __set_bit(nr, addr)            bitop(___set_bit, nr, addr)
>  #define __clear_bit(nr, addr)          bitop(___clear_bit, nr, addr)
> --
> 2.36.1
>