From nobody Sun May 10 09:55:06 2026 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 4E4A1C433EF for ; Thu, 12 May 2022 00:04:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1349536AbiELAEY (ORCPT ); Wed, 11 May 2022 20:04:24 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41220 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1349526AbiELAEQ (ORCPT ); Wed, 11 May 2022 20:04:16 -0400 Received: from mail-pl1-x62e.google.com (mail-pl1-x62e.google.com [IPv6:2607:f8b0:4864:20::62e]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 644E579806 for ; Wed, 11 May 2022 17:04:14 -0700 (PDT) Received: by mail-pl1-x62e.google.com with SMTP id m12so3364798plb.4 for ; Wed, 11 May 2022 17:04:14 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=sender:from:to:cc:subject:date:message-id:in-reply-to:references :mime-version:content-transfer-encoding; bh=ZJV9qCkaOxpO84xo6ebk43WZP210trdb6TDi7Y9RbbU=; b=cYDsj7gf4wAZNvatYUNPaypENVCUXAVHkGpYkETxpTFfEoUlTCFjPivD76VDqlg7hi eJasiUp+bH4A3bDHDpkLJBURCA5EdgZQunSULBqYNhBaZJFFsA+7AEO8wCZt2IQTW86T ub3XmPJAe1HX54hf3rcI7T/AoDIq+S1nc+aJr5OSQx+/1hv2C8VXuS4/kFtLIyBb/Ns5 LzO3i9GshcitFY2NK/BEcdRRP9nJjd3dEpQqeuF8s94ZFiOSzmxiQcHCufi+RTW6P5a0 nSC4LPyBLOBnqQ0Mgn3B0K9CyTrtu7ECT49xQvDKSwZEp3L+Su5iHyiCv45oZ7c1YNZm +c1A== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:sender:from:to:cc:subject:date:message-id :in-reply-to:references:mime-version:content-transfer-encoding; bh=ZJV9qCkaOxpO84xo6ebk43WZP210trdb6TDi7Y9RbbU=; b=k5gZAkEhJFpiouRrAHHIJJUZhdYNZu5qwhbSIbnVEp4I4GkgRJG+Fwfd8xZP/zVF+i N8FqYx/uj9jGT04tpXINGmnsKjYydwvGc2PiAWmyxT9rr/0JAjFqveZ6GlOEpj6keuoS kZAdAz2aSS6iE5EKLiHdAOO/ai/4TNtSKwggZhtL69zLhid2Wi1LbDwF5bF/UZDvczN+ J5EEVi2QgOk4R8czVIqGBhdkBaTjRcSfu0KN1rPEGca6EPUMredfmepx0Um5DaB58s0R driMEKnc6BUM+KKq9VIiooxaqksXBAtimmnqW3aTHlvs35rEqsupuoIWbk2nJPfMbCJu TMPA== X-Gm-Message-State: AOAM530D8R9QfPdSWs3R5NY5CS/keyEx341gohMSNcGp+hMW7o8KM7WZ nLQJUNAxr5Xwk6QOTIRbNxs= X-Google-Smtp-Source: ABdhPJz/sIUB1FWNgKw07AVyLZvVee4bfNwaHe8dvDQdVVeqiYVBi7q0hxsTBcqhHy/H/3h3g++leg== X-Received: by 2002:a17:902:f54e:b0:15e:9158:1237 with SMTP id h14-20020a170902f54e00b0015e91581237mr27271569plf.37.1652313853862; Wed, 11 May 2022 17:04:13 -0700 (PDT) Received: from localhost.localdomain (124x33x176x97.ap124.ftth.ucom.ne.jp. [124.33.176.97]) by smtp.gmail.com with ESMTPSA id e2-20020a170902d38200b0015f300ed0b7sm2491462pld.76.2022.05.11.17.04.11 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 11 May 2022 17:04:13 -0700 (PDT) Sender: Vincent Mailhol From: Vincent Mailhol To: Nick Desaulniers , Thomas Gleixner , Ingo Molnar , Borislav Petkov Cc: Dave Hansen , x86@kernel.org, "H . Peter Anvin" , Nathan Chancellor , Tom Rix , linux-kernel@vger.kernel.org, llvm@lists.linux.dev, David Howells , Jan Beulich , Vincent Mailhol Subject: [PATCH v3 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate constant expressions Date: Thu, 12 May 2022 09:03:39 +0900 Message-Id: <20220512000340.1171988-2-mailhol.vincent@wanadoo.fr> X-Mailer: git-send-email 2.35.1 In-Reply-To: <20220512000340.1171988-1-mailhol.vincent@wanadoo.fr> References: <20220511160319.1045812-1-mailhol.vincent@wanadoo.fr> <20220512000340.1171988-1-mailhol.vincent@wanadoo.fr> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" For x86_64, the current ffs() implementation does not produce optimized code when called with a constant expression. On the contrary, the __builtin_ffs() function of both GCC and clang is able to simplify the expression into a single instruction. * Example * Let's 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 For both example, we clearly see the benefit of using __builtin_ffs() instead of the kernel's asm implementation for constant expressions. However, for non constant expressions, the ffs() asm version of the kernel 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). This patch uses the __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, this patch also removes below -Wshadow warning: | ./arch/x86/include/asm/bitops.h:283:28: warning: declaration of 'ffs' sha= dows a built-in function [-Wshadow] | 283 | static __always_inline int ffs(int x) And finally, Nick Desaulniers pointed out in [2] that this also fixes a constant propagation missed-optimization in clang. ** Statistics ** On a allyesconfig, before applying this patch...: | $ 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()") http://lkml.kernel.org/r/20111213145654.14362.39868.stgit@warthog.procyon.o= rg.uk [2] https://lore.kernel.org/all/CAKwvOdnH_gYv4qRN9pKY7jNTQK95xNeH1w1KZJJmvC= kh8xJLBg@mail.gmail.com/ Reviewed-by: Nick Desaulniers Signed-off-by: Vincent Mailhol --- 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 a288ecd230ab..6ed979547086 100644 --- a/arch/x86/include/asm/bitops.h +++ b/arch/x86/include/asm/bitops.h @@ -269,18 +269,7 @@ static __always_inline unsigned long __fls(unsigned lo= ng word) #undef ADDR =20 #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; =20 @@ -310,6 +299,19 @@ static __always_inline int ffs(int x) return r + 1; } =20 +/** + * 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 --=20 2.35.1 From nobody Sun May 10 09:55:06 2026 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 AAB44C433F5 for ; Thu, 12 May 2022 00:04:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1349545AbiELAEd (ORCPT ); Wed, 11 May 2022 20:04:33 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41302 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1349533AbiELAEV (ORCPT ); Wed, 11 May 2022 20:04:21 -0400 Received: from mail-pl1-x62b.google.com (mail-pl1-x62b.google.com [IPv6:2607:f8b0:4864:20::62b]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 490B479820 for ; Wed, 11 May 2022 17:04:19 -0700 (PDT) Received: by mail-pl1-x62b.google.com with SMTP id i1so3349642plg.7 for ; Wed, 11 May 2022 17:04:19 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=sender:from:to:cc:subject:date:message-id:in-reply-to:references :mime-version:content-transfer-encoding; bh=/PaByGRhEGBhwCNS2nARgA0uB7mRgubG621cnM8Ytdw=; b=Ncprh/EGurdgo/9UVMCgqqAl2hyO8i1oIstb1T6EL9tz1wTbCLp3ZWWeZwJZQmyeZQ Q8Cf8TZau1SQOsvwH9sg6O3K8op7IDLl6jgMMB1tCYyeAuIon//1AmmN/ztdFaSg0LLK 8euqSn5C+XdXw5x7jHmLM4RvC13zDu9vbw5UF8Ynn0l0B/GuYuppmA3cVRO/mB69s+ZI QEMS+8Yfn3EJUge2uaCJNDYqRk1kywMuOd9XulJt0zvNyi0YupwAJtLRM0gvxRev2qSw GPtrIrWkHxqrnjcanC8XDyrJ7Qm2KTfR32HR89F4TfZSzaTcinXwiLNAHqWlUmtYKYcW +ufg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:sender:from:to:cc:subject:date:message-id :in-reply-to:references:mime-version:content-transfer-encoding; bh=/PaByGRhEGBhwCNS2nARgA0uB7mRgubG621cnM8Ytdw=; b=n6h5cyRUc9xswTqoZs2GdoMC+eXPE4T+mhZX+orYXLsj4hFfS13Vop/EfOaignbzzA r9zXQtDnhALlFu/PCjAUUlmkyfOprt4czoG4gLBsaKLr4n9Z5hmmk7xDt8sZLIWO0gtO 10ynMco0orqxZDMDrGfKkI5H3VdX2YKZNGwlmcblUAkIU7joKeK5Za3erP/EU5Qcwkpj ZZrhnRZW0q+676x/7xGk1hBcqsQJLFAhbWGtsqu7x8zs+lOgOGv/AQytt4E0jZqHt5eg NJuBtLh7Fp+a9e/BIL4oxzu76B0LV2HmV5H9de4k6pYBfgHXebnSmItYhfzp7Y2sMLR6 jf+w== X-Gm-Message-State: AOAM530Gfop4Ji8wRZs56+W94+CKt7cevIVrSQVOqMBZBoFRSSx9Jy0F zyM5UJf5FpqaKz0P/Lj1z94= X-Google-Smtp-Source: ABdhPJzT9/j/Ej+xBb7CJ8SwsJc0KuFRpL2Sy6A7Y+0ZtUfeeqrA08eYDXjkdzYM/Hk/MxK6CHTOMw== X-Received: by 2002:a17:903:2289:b0:15e:8d2c:5b44 with SMTP id b9-20020a170903228900b0015e8d2c5b44mr27290441plh.113.1652313858617; Wed, 11 May 2022 17:04:18 -0700 (PDT) Received: from localhost.localdomain (124x33x176x97.ap124.ftth.ucom.ne.jp. [124.33.176.97]) by smtp.gmail.com with ESMTPSA id e2-20020a170902d38200b0015f300ed0b7sm2491462pld.76.2022.05.11.17.04.15 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 11 May 2022 17:04:18 -0700 (PDT) Sender: Vincent Mailhol From: Vincent Mailhol To: Nick Desaulniers , Thomas Gleixner , Ingo Molnar , Borislav Petkov Cc: Dave Hansen , x86@kernel.org, "H . Peter Anvin" , Nathan Chancellor , Tom Rix , linux-kernel@vger.kernel.org, llvm@lists.linux.dev, David Howells , Jan Beulich , Vincent Mailhol Subject: [PATCH v3 2/2] x86/asm/bitops: __ffs,ffz: use __builtin_ctzl to evaluate constant expressions Date: Thu, 12 May 2022 09:03:40 +0900 Message-Id: <20220512000340.1171988-3-mailhol.vincent@wanadoo.fr> X-Mailer: git-send-email 2.35.1 In-Reply-To: <20220512000340.1171988-1-mailhol.vincent@wanadoo.fr> References: <20220511160319.1045812-1-mailhol.vincent@wanadoo.fr> <20220512000340.1171988-1-mailhol.vincent@wanadoo.fr> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" __ffs(x) is equivalent to (unsigned long)__builtin_ctzl(x) and ffz(x) is equivalent to (unsigned long)__builtin_ctzl(~x). Because __builting_ctzl() returns an int, a cast to (unsigned long) is necessary to avoid potential warnings on implicit casts. For x86_64, the current __ffs() and ffz() implementations do not produce optimized code when called with a constant expression. On the contrary, the __builtin_ctzl() gets simplified into a single instruction. However, for non constant expressions, the __ffs() and ffz() asm versions of the kernel remains slightly better than the code produced by GCC (it produces a useless instruction to clear eax). This patch uses the __builtin_constant_p() to select between the kernel's __ffs()/ffz() and the __builtin_ctzl() depending on whether the argument is constant or not. ** Statistics ** On a allyesconfig, before applying this patch...: | $ objdump -d vmlinux.o | grep tzcnt | wc -l | 3607 ...and after: | $ objdump -d vmlinux.o | grep tzcnt | wc -l | 2600 So, roughly 27.9% of the calls to either __ffs() or ffz() were using constant expressions and could be optimized out. (tests done on linux v5.18-rc5 x86_64 using GCC 11.2.1) Note: on x86_64, the asm bsf instruction produces tzcnt when used with the ret prefix (which is why we grep tzcnt instead of bsf in above benchmark). c.f. [1] [1] commit e26a44a2d618 ("x86: Use REP BSF unconditionally") http://lkml.kernel.org/r/5058741E020000780009C014@nat28.tlf.novell.com CC: Nick Desaulniers Signed-off-by: Vincent Mailhol Reviewed-by: Nick Desaulniers --- arch/x86/include/asm/bitops.h | 38 ++++++++++++++++++++++------------- 1 file changed, 24 insertions(+), 14 deletions(-) diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h index 6ed979547086..fb0d7cd9f957 100644 --- a/arch/x86/include/asm/bitops.h +++ b/arch/x86/include/asm/bitops.h @@ -224,13 +224,7 @@ static __always_inline bool variable_test_bit(long nr,= volatile const unsigned l ? constant_test_bit((nr), (addr)) \ : variable_test_bit((nr), (addr))) =20 -/** - * __ffs - find first set bit in word - * @word: The word to search - * - * Undefined if no bit exists, so code should check against 0 first. - */ -static __always_inline unsigned long __ffs(unsigned long word) +static __always_inline unsigned long variable___ffs(unsigned long word) { asm("rep; bsf %1,%0" : "=3Dr" (word) @@ -238,13 +232,18 @@ static __always_inline unsigned long __ffs(unsigned l= ong word) return word; } =20 -/** - * ffz - find first zero bit in word - * @word: The word to search - * - * Undefined if no zero exists, so code should check against ~0UL first. - */ -static __always_inline unsigned long ffz(unsigned long word) +/** + * __ffs - find first set bit in word + * @word: The word to search + * + * Undefined if no bit exists, so code should check against 0 first. + */ +#define __ffs(word) \ + (__builtin_constant_p(word) ? \ + (unsigned long)__builtin_ctzl(word) : \ + variable___ffs(word)) + +static __always_inline unsigned long variable_ffz(unsigned long word) { asm("rep; bsf %1,%0" : "=3Dr" (word) @@ -252,6 +251,17 @@ static __always_inline unsigned long ffz(unsigned long= word) return word; } =20 +/** + * ffz - find first zero bit in word + * @word: The word to search + * + * Undefined if no zero exists, so code should check against ~0UL first. + */ +#define ffz(word) \ + (__builtin_constant_p(word) ? \ + (unsigned long)__builtin_ctzl(~word) : \ + variable_ffz(word)) + /* * __fls: find last set bit in word * @word: The word to search --=20 2.35.1