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

Andy Shevchenko
2022-06-20T10:05:51Z

On Fri, Jun 17, 2022 at 04:40:30PM +0200, Alexander Lobakin 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)

...

> +/*
> + * 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

generic_*() ?

> + * 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).
> + */

--
With Best Regards,
Andy Shevchenko