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 bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 0CE39D778B1 for ; Mon, 26 Jan 2026 00:33:24 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:Content-Type:Cc:To:From: Subject:Message-ID:References:Mime-Version:In-Reply-To:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=26BDCJEdJLp7RpCdQh1+Gie1UpBf3nEsfBaL4sC4xLw=; b=Mof5sy2lu5lVQ0bt5aziMQgPAA T43FVEK2HHX3r7oZIc9s7Ekmtp58Xja5lhXKands4nGJcNrDA0Y0hY2ANDcI2fXwlgHFwMD3Y4EtA oyp04eNBOoTNxqyQ9UpHERq6R1d0Vn3MCoLD9DFus+8oM83amwr9eZzM3KkYVCG1NC16hEqOlTi2/ KoJiBQn/Q8vADPCVSKFfK6KKwM9BTw/AI5RsHRuvl/W3j6sES1M2qVxaI+G35C8CryMcH5P4L8k+L 8trwbbmKOQcjnE/4JX4998WSgRMEt64j4qrWLQrGhMB6jbA9sif5cSZOUv9PMEN82xswxTUyuVM+T Sb+xNY7Q==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1vkAXd-0000000BiP0-3UCY; Mon, 26 Jan 2026 00:33:17 +0000 Received: from mail-wm1-x349.google.com ([2a00:1450:4864:20::349]) by bombadil.infradead.org with esmtps (Exim 4.98.2 #2 (Red Hat Linux)) id 1vkAXY-0000000BiNb-2LNa for linux-arm-kernel@lists.infradead.org; Mon, 26 Jan 2026 00:33:13 +0000 Received: by mail-wm1-x349.google.com with SMTP id 5b1f17b1804b1-47ee8808ffbso29378285e9.2 for ; Sun, 25 Jan 2026 16:33:11 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1769387590; x=1769992390; darn=lists.infradead.org; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:from:to:cc:subject:date:message-id:reply-to; bh=26BDCJEdJLp7RpCdQh1+Gie1UpBf3nEsfBaL4sC4xLw=; b=ULZY/WNog2/9xK4r3JYjjIUrYCC564/p4V+gRtC5oEJpqZkwqN1Bg5kOm+eYBBrTOW iti44NRvQ2oUWI0aHprOJK96PlG9LIIwMNUpke26vJJZjwSpqaR4u0p+3T9fPLZK5o5J GiEvHKN5AkQ/LKMh532WGG2gvVD0Ons6YJg6mMqDRHqxGTgquFhscohMylkGg76OwmrE ye5reKF9mukI845i5gUWQLv9OqIw9MA22jydnG+E+iR4xD6efHj+LOwphw6Lc2qPcccx oLig6ClUyFlxB6EPIQrziBA1Y+SQz14l8PK9wxAzTOEBtjtZwqRDlIdKnOBZzGnVeihR ZCzQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1769387590; x=1769992390; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=26BDCJEdJLp7RpCdQh1+Gie1UpBf3nEsfBaL4sC4xLw=; b=HA61TFYrfptR6jPo8XvsWF9+Oot7rAd0VkIykx5+C7z9kNMff+Aj67YGCAPzoUJuYw MPrSJc4Z1/jmvo+GPtuJ/Edck9EnS6JgHwvW8qEjPbGFf9kT9ZiYWGVMvYS5BKoZy20k /GkR7RRFdt3Ql9vQ7fa5t3GEWcFSIGSK7keqdPRLv2u6hhkvBNa5PT4wUFx4yUPjJ+6X YoYYCVT5G/ZBp8uW5K+8ZkOu5Q098NlFrEemWJGDHlBE7/7JNVoJQTIPmJSAuQBPam7I oRP9cE9wRx4ieChVd7Dxd2DeSAh+AaGmClW0Y0vVLHJb3i/Xm1K7EZxvMuhaZVYd7VJ4 DE9g== X-Forwarded-Encrypted: i=1; AJvYcCXMDyHen363on9ucTnqMEAnEN8MwtjeFPVODFyTvUbvc4XcxE0SSUk+v4/F/pvqojgEjHWKZ05ZCyQ9e7vKSSst@lists.infradead.org X-Gm-Message-State: AOJu0YwQki3K6pqlIze3cG8/r+SxUsT/JOUnNBp7D5/qhJa8ISqsB/uU gyg/vAWAaQlkfcoEyLShR1aNBw2w6KJ7TWxdxsET23BpdL6HiUJlT2RTehLAM7gszkPxqSM+3wo 2cQ== X-Received: from wmbjw5.prod.google.com ([2002:a05:600c:5745:b0:47e:e4f5:36ca]) (user=elver job=prod-delivery.src-stubby-dispatcher) by 2002:a05:600c:190c:b0:477:9dc1:b706 with SMTP id 5b1f17b1804b1-4805ce67eb4mr48399755e9.19.1769387590378; Sun, 25 Jan 2026 16:33:10 -0800 (PST) Date: Mon, 26 Jan 2026 01:25:11 +0100 In-Reply-To: <20260126002936.2676435-1-elver@google.com> Mime-Version: 1.0 References: <20260126002936.2676435-1-elver@google.com> X-Mailer: git-send-email 2.52.0.457.g6b5491de43-goog Message-ID: <20260126002936.2676435-3-elver@google.com> Subject: [PATCH 2/3] arm64: Optimize __READ_ONCE() with CONFIG_LTO=y From: Marco Elver To: elver@google.com, Peter Zijlstra , Will Deacon Cc: Ingo Molnar , Thomas Gleixner , Boqun Feng , Waiman Long , Bart Van Assche , llvm@lists.linux.dev, Catalin Marinas , Arnd Bergmann , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org Content-Type: text/plain; charset="UTF-8" X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260125_163312_622498_5A20BA65 X-CRM114-Status: GOOD ( 18.30 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org Rework arm64 LTO __READ_ONCE() to improve code generation as follows: 1. Replace the _Generic-based __unqual_scalar_typeof() with the builtin typeof_unqual(). This strips qualifiers from all types, not just integer types, which is required to be able to assign (must be non-const) to __u.__val in the non-atomic case (required for #2). One subtle point here is that non-integer types of __val could be const or volatile within the union with the old __unqual_scalar_typeof(), if the passed variable is const or volatile. This would then result in a forced load from the stack if __u.__val is volatile; in the case of const, it does look odd if the underlying storage changes, but the compiler is told said member is "const" -- it smells like UB. 2. Eliminate the atomic flag and ternary conditional expression. Move the fallback volatile load into the default case of the switch, ensuring __u is unconditionally initialized across all paths. The statement expression now unconditionally returns __u.__val. This refactoring appears to help the compiler improve (or fix) code generation. With a defconfig + LTO + debug options builds, we observe different codegen for the following functions: btrfs_reclaim_sweep (708 -> 1032 bytes) btrfs_sinfo_bg_reclaim_threshold_store (200 -> 204 bytes) check_mem_access (3652 -> 3692 bytes) [inlined bpf_map_is_rdonly] console_flush_all (1268 -> 1264 bytes) console_lock_spinning_disable_and_check (180 -> 176 bytes) igb_add_filter (640 -> 636 bytes) igb_config_tx_modes (2404 -> 2400 bytes) kvm_vcpu_on_spin (480 -> 476 bytes) map_freeze (376 -> 380 bytes) netlink_bind (1664 -> 1656 bytes) nmi_cpu_backtrace (404 -> 400 bytes) set_rps_cpu (516 -> 520 bytes) swap_cluster_readahead (944 -> 932 bytes) tcp_accecn_third_ack (328 -> 336 bytes) tcp_create_openreq_child (1764 -> 1772 bytes) tcp_data_queue (5784 -> 5892 bytes) tcp_ecn_rcv_synack (620 -> 628 bytes) xen_manage_runstate_time (944 -> 896 bytes) xen_steal_clock (340 -> 296 bytes) Increase of some functions are due to more aggressive inlining due to better codegen (in this build, e.g. bpf_map_is_rdonly is no longer present due to being inlined completely). Signed-off-by: Marco Elver --- arch/arm64/include/asm/rwonce.h | 7 +++---- 1 file changed, 3 insertions(+), 4 deletions(-) diff --git a/arch/arm64/include/asm/rwonce.h b/arch/arm64/include/asm/rwonce.h index fc0fb42b0b64..9963948f4b44 100644 --- a/arch/arm64/include/asm/rwonce.h +++ b/arch/arm64/include/asm/rwonce.h @@ -32,8 +32,7 @@ #define __READ_ONCE(x) \ ({ \ typeof(&(x)) __x = &(x); \ - int atomic = 1; \ - union { __unqual_scalar_typeof(*__x) __val; char __c[1]; } __u; \ + union { TYPEOF_UNQUAL(*__x) __val; char __c[1]; } __u; \ switch (sizeof(x)) { \ case 1: \ asm volatile(__LOAD_RCPC(b, %w0, %1) \ @@ -56,9 +55,9 @@ : "Q" (*__x) : "memory"); \ break; \ default: \ - atomic = 0; \ + __u.__val = *(volatile typeof(*__x) *)__x; \ } \ - atomic ? (typeof(*__x))__u.__val : (*(volatile typeof(*__x) *)__x);\ + __u.__val; \ }) #endif /* !BUILD_VDSO */ -- 2.52.0.457.g6b5491de43-goog