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 6CB2DC6FA82 for ; Tue, 20 Sep 2022 20:00:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230337AbiITUAd (ORCPT ); Tue, 20 Sep 2022 16:00:33 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:55262 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229870AbiITUAZ (ORCPT ); Tue, 20 Sep 2022 16:00:25 -0400 Received: from galois.linutronix.de (Galois.linutronix.de [193.142.43.55]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 25B8240BF2; Tue, 20 Sep 2022 13:00:22 -0700 (PDT) Date: Tue, 20 Sep 2022 20:00:19 -0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1663704021; h=from:from:sender:sender:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=td/0Lmoinj5QP/RSW+m2L52+twM1EDigUVdjwGaT5EQ=; b=NxkDySEsXZryEsfWdfAl+vc/sosE0IWQwkHHxySO5r3Xy0tFoAIXFhN0UktjUcK4H/CIum 22XxdPb9zhvAAtutMmbE83Q75mRD8C2flwqLR8fXQz7u9+jiocj3L4pG72Oz/ZaYttGqnO Wn8FHwe3U5wFuatzd1+usLfHewUzKYCnwJn77VH5qu/f2gCcmAl6QIbDqmrmc3lDAH+QXH K08i6rKuFqJiLWZuRnY1bjvjhgATR9Quz3PX9RWvV5sSHGVPA0hKQxoof8tqlVhFgAhpWU BeYq6kwWnbul4NoIx2zTkTVR0b7Websldrjnc5SbbtD2TWFj5ymN+MESiUGBpg== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1663704021; h=from:from:sender:sender:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=td/0Lmoinj5QP/RSW+m2L52+twM1EDigUVdjwGaT5EQ=; b=ABA6KXS2pHFFYv2/aCc/wGXC1WCdKD1YlLRLDafGrbT1BZFDxMWIas65sz5atL60gomCVY iX3LDCTzGQUl7VCw== From: "tip-bot2 for Vincent Mailhol" Sender: tip-bot2@linutronix.de Reply-to: linux-kernel@vger.kernel.org To: linux-tip-commits@vger.kernel.org Subject: [tip: x86/asm] x86/asm/bitops: Use __builtin_ffs() to evaluate constant expressions Cc: Vincent Mailhol , Borislav Petkov , Nick Desaulniers , Yury Norov , x86@kernel.org, linux-kernel@vger.kernel.org In-Reply-To: <20220511160319.1045812-1-mailhol.vincent@wanadoo.fr> References: <20220511160319.1045812-1-mailhol.vincent@wanadoo.fr> MIME-Version: 1.0 Message-ID: <166370401997.401.11967078990556002751.tip-bot2@tip-bot2> Robot-ID: Robot-Unsubscribe: Contact to get blacklisted from these emails Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org The following commit has been merged into the x86/asm branch of tip: Commit-ID: 146034fed6ee75ec09cf8f996165e2296ceae0bb Gitweb: https://git.kernel.org/tip/146034fed6ee75ec09cf8f996165e2296ceae0bb Author: Vincent Mailhol AuthorDate: Wed, 07 Sep 2022 18:09:34 +09:00 Committer: Borislav Petkov CommitterDate: Tue, 20 Sep 2022 15:31:17 +02:00 x86/asm/bitops: Use __builtin_ffs() to evaluate constant expressions For x86_64, the current ffs() implementation does not produce optimized code when called with a constant expression. On the contrary, the __builtin_ffs() functions of both GCC and clang are able to fold the expression into a single instruction. ** Example ** Consider two dummy functions foo() and bar() as below: #include #define CONST 0x01000000 unsigned int foo(void) { return ffs(CONST); } unsigned int bar(void) { return __builtin_ffs(CONST); } GCC would produce below assembly code: 0000000000000000 : 0: ba 00 00 00 01 mov $0x1000000,%edx 5: b8 ff ff ff ff mov $0xffffffff,%eax a: 0f bc c2 bsf %edx,%eax d: 83 c0 01 add $0x1,%eax 10: c3 ret 0000000000000020 : 20: b8 19 00 00 00 mov $0x19,%eax 25: c3 ret And clang would produce: 0000000000000000 : 0: b8 ff ff ff ff mov $0xffffffff,%eax 5: 0f bc 05 00 00 00 00 bsf 0x0(%rip),%eax # c c: 83 c0 01 add $0x1,%eax f: c3 ret 0000000000000010 : 10: b8 19 00 00 00 mov $0x19,%eax 15: c3 ret Both examples clearly demonstrate the benefit of using __builtin_ffs() instead of the kernel's asm implementation for constant expressions. However, for non constant expressions, the kernel's ffs() asm version remains better for x86_64 because, contrary to GCC, it doesn't emit the CMOV assembly instruction, c.f. [1] (noticeably, clang is able optimize out the CMOV call). Use __builtin_constant_p() to select between the kernel's ffs() and the __builtin_ffs() depending on whether the argument is constant or not. As a side benefit, replacing the ffs() function declaration by a macro also removes below -Wshadow warning: ./arch/x86/include/asm/bitops.h:283:28: warning: declaration of 'ffs' shadows a built-in function [-Wshadow] 283 | static __always_inline int ffs(int x) ** Statistics ** On a allyesconfig, before...: $ objdump -d vmlinux.o | grep bsf | wc -l 1081 ...and after: $ objdump -d vmlinux.o | grep bsf | wc -l 792 So, roughly 26.7% of the calls to ffs() were using constant expressions and could be optimized out. (tests done on linux v5.18-rc5 x86_64 using GCC 11.2.1) [1] commit ca3d30cc02f7 ("x86_64, asm: Optimise fls(), ffs() and fls64()") [ bp: Massage commit message. ] Signed-off-by: Vincent Mailhol Signed-off-by: Borislav Petkov Reviewed-by: Nick Desaulniers Reviewed-by: Yury Norov Link: https://lore.kernel.org/r/20220511160319.1045812-1-mailhol.vincent@wanadoo.fr --- arch/x86/include/asm/bitops.h | 26 ++++++++++++++------------ 1 file changed, 14 insertions(+), 12 deletions(-) diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h index 0fe9de5..879238e 100644 --- a/arch/x86/include/asm/bitops.h +++ b/arch/x86/include/asm/bitops.h @@ -292,18 +292,7 @@ static __always_inline unsigned long __fls(unsigned long word) #undef ADDR #ifdef __KERNEL__ -/** - * ffs - find first set bit in word - * @x: the word to search - * - * This is defined the same way as the libc and compiler builtin ffs - * routines, therefore differs in spirit from the other bitops. - * - * ffs(value) returns 0 if value is 0 or the position of the first - * set bit if value is nonzero. The first (least significant) bit - * is at position 1. - */ -static __always_inline int ffs(int x) +static __always_inline int variable_ffs(int x) { int r; @@ -334,6 +323,19 @@ static __always_inline int ffs(int x) } /** + * ffs - find first set bit in word + * @x: the word to search + * + * This is defined the same way as the libc and compiler builtin ffs + * routines, therefore differs in spirit from the other bitops. + * + * ffs(value) returns 0 if value is 0 or the position of the first + * set bit if value is nonzero. The first (least significant) bit + * is at position 1. + */ +#define ffs(x) (__builtin_constant_p(x) ? __builtin_ffs(x) : variable_ffs(x)) + +/** * fls - find last set bit in word * @x: the word to search *