From mboxrd@z Thu Jan 1 00:00:00 1970 From: Alexander Lobakin Subject: [PATCH v3 6/7] bitops: let optimize out non-atomic bitops on compile-time constants Date: Fri, 17 Jun 2022 16:40:30 +0200 Message-ID: <20220617144031.2549432-7-alexandr.lobakin@intel.com> References: <20220617144031.2549432-1-alexandr.lobakin@intel.com> Mime-Version: 1.0 Content-Transfer-Encoding: 8bit Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1655476854; x=1687012854; h=from:to:cc:subject:date:message-id:in-reply-to: references:mime-version:content-transfer-encoding; bh=cKDaIk+tyuEQhyGKnxAqlVtiqCqt8rLzZ/7eP9mJcAA=; b=LBxnQ+AASnMGic5zS3LVDe5hl7z66d31vJTr6PQqVGN54XgfKGg68x9L jJoBdLNIaTOHTtTEdQ7iLG5bH2GOXLA07CM8apQGw8yBvcvy7tonP10kB 66/FB/o5ducfK9zGQoR3nxczbkOBLDlRkOoHt46IFwjUNO2IDy8xFU6Hm XwrUDTLB5kD4MxFjD00+qklK+l6FWPtrpqLCfDzopPIH6Ru3TD/AjrbUT ii56pX3+0i8LmIBeD7NA9+Sy3Gq1Qun2EERwlYt6K5Zj0tZV5/MS6er/D eMC0H45EVSQWVd9e7rdUGJEfLWweHZwpfxnRHVBqX4a7CCszJu2RBCE6H Q==; In-Reply-To: <20220617144031.2549432-1-alexandr.lobakin@intel.com> List-ID: Content-Type: text/plain; charset="us-ascii" To: Arnd Bergmann , Yury Norov Cc: Alexander Lobakin , Andy Shevchenko , Mark Rutland , Matt Turner , Brian Cain , Geert Uytterhoeven , Yoshinori Sato , Rich Felker , "David S. Miller" , Kees Cook , "Peter Zijlstra (Intel)" , Marco Elver , Borislav Petkov , Tony Luck , Maciej Fijalkowski , Jesse Brandeburg , Greg Kroah-Hartman , linux-alpha@vger.kernel.org, linux-hexagon@vger.kernel.org, linux-ia64 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 Signed-off-by: Alexander Lobakin --- 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 +/* + * 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