From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id B74B2CCA47B for ; Fri, 10 Jun 2022 11:36:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1348768AbiFJLgB (ORCPT ); Fri, 10 Jun 2022 07:36:01 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:47040 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1346749AbiFJLfz (ORCPT ); Fri, 10 Jun 2022 07:35:55 -0400 Received: from mga09.intel.com (mga09.intel.com [134.134.136.24]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 65B6370921 for ; Fri, 10 Jun 2022 04:35:38 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1654860938; x=1686396938; h=from:to:cc:subject:date:message-id:in-reply-to: references:mime-version:content-transfer-encoding; bh=7c74IH5zXtMHvz1DnqxXwajlv0u5rM/yQqyI4XwsmRY=; b=ilu6GbUZdx5qVj4QUW0SLHZzUjKOLFjAgbDl1809DlW11YOO2hnx8r+n iIPtG5DvW6JxVeqQd+x3jAaginoizKv1M9+gUwWQpYczS5NgEIsgQH8oS FEzdHsmut5t2+ewtJ62473/djSeENd9L4duZphQupML25w2dU/o1JhSzs EKTzWRhO4jh6Zyh/cioa00WesdGdiwjRgCSYHv0hQHrZUpnt3IxExw7hk Kzy9Rv/PfUnFxrGWQ4hl9erlZvEAT1bDg7Tw+v7x1kBbGFRQLclEoM9Tu yQU9m6Du9cLymRLfiOGuBAJ+EVI1a32Qz+Bfx1GHBgrhIFTW/8GC8fVbf Q==; X-IronPort-AV: E=McAfee;i="6400,9594,10373"; a="278402975" X-IronPort-AV: E=Sophos;i="5.91,290,1647327600"; d="scan'208";a="278402975" Received: from orsmga006.jf.intel.com ([10.7.209.51]) by orsmga102.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 10 Jun 2022 04:35:37 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.91,290,1647327600"; d="scan'208";a="556339717" Received: from irvmail001.ir.intel.com ([10.43.11.63]) by orsmga006.jf.intel.com with ESMTP; 10 Jun 2022 04:35:31 -0700 Received: from newjersey.igk.intel.com (newjersey.igk.intel.com [10.102.20.203]) by irvmail001.ir.intel.com (8.14.3/8.13.6/MailSET/Hub) with ESMTP id 25ABZKim030333; Fri, 10 Jun 2022 12:35:29 +0100 From: Alexander Lobakin 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 , Greg Kroah-Hartman , linux-alpha@vger.kernel.org, linux-hexagon@vger.kernel.org, linux-ia64@vger.kernel.org, linux-m68k@lists.linux-m68k.org, linux-sh@vger.kernel.org, sparclinux@vger.kernel.org, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH v2 6/6] bitops: let optimize out non-atomic bitops on compile-time constants Date: Fri, 10 Jun 2022 13:34:27 +0200 Message-Id: <20220610113427.908751-7-alexandr.lobakin@intel.com> X-Mailer: git-send-email 2.36.1 In-Reply-To: <20220610113427.908751-1-alexandr.lobakin@intel.com> References: <20220610113427.908751-1-alexandr.lobakin@intel.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-m68k@vger.kernel.org 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 753f98e0dcf5..364bdc3606b4 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