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

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Message-ID: <YrBGUqfS7r9m0eAf@smile.fi.intel.com>
Date:   Mon, 20 Jun 2022 13:05:06 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Alexander Lobakin <alexandr.lobakin@...el.com>
Cc:     Arnd Bergmann <arnd@...db.de>, Yury Norov <yury.norov@...il.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>,
        Marco Elver <elver@...gle.com>, 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 6/7] bitops: let optimize out non-atomic bitops on
 compile-time constants

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