From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 73E0323FC54; Mon, 2 Feb 2026 15:36:46 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770046606; cv=none; b=G9PmwwC9J4Ra3QWabb++YEKxYbybK3nCDsWm72Y41i3sTxdYuDpn1fSAl8hXrTQ26U7k+43AkCHEr0lv4d86s4vscgD6cfMPGb9jOBuDOTONOCvuA0dPK5bjhVu2SQeN0V2HKLX33uev6ErZDvKn9kPgZ2i4cXDaRU/lq6bJU4I= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770046606; c=relaxed/simple; bh=WI6zutmVBSxRxobYaH3/CHHeRqahghvOWUS1hAR6f9s=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=tu+hwh9/ImjvvUeliB49xYObZtZd6z5YMMRKBy0qxABIMGwvEN+qKP1Bivt6crlIKfSxMRg687+C0/xmGb0UmKcsC8ucCskTTBCOY9N1uz9cAsBpGT6eaIlfoyk9peUXWtquXxHuFC1uftSL+lYaYJqkJNr/yrqs+2u8VUrIYh8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=tZeEBiyj; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="tZeEBiyj" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 6AACDC2BC86; Mon, 2 Feb 2026 15:36:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1770046606; bh=WI6zutmVBSxRxobYaH3/CHHeRqahghvOWUS1hAR6f9s=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=tZeEBiyj67lav4qkYYDtamQukkvU4UhwkXxxUd7IGB3J8tok8/UGdeU/bYWf8vcy1 32lYKKSfiDF5MyL3k8+rkj4WtW+JC0MMGaAzyQkV4jZ1cZ1/6ikLAKeiIMZQjWx658 ZLmkrIIW2u9BX9XSVZNdcZlJEtcuD/HChCtuu9h30eP/MD6+npqcEFjzQ6QF1ObyTn XMNHY6odUN/gBuMisa1sTHMMYv77QjN3Udd2qRMED7S9Gt1zNEmzTInajbXFFeD97Y tpVYTswxHkfSUb+5QmeBy3Y1PxgsIkEKV9cGSmcfC7w8JNGAk9x3cUZVe9dglCI/qR MO8x6HRGKnbfQ== Date: Mon, 2 Feb 2026 15:36:40 +0000 From: Will Deacon To: Marco Elver Cc: Peter Zijlstra , Ingo Molnar , Thomas Gleixner , Boqun Feng , Waiman Long , Bart Van Assche , llvm@lists.linux.dev, David Laight , Catalin Marinas , Arnd Bergmann , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v3 2/3] arm64: Optimize __READ_ONCE() with CONFIG_LTO=y Message-ID: References: <20260130132951.2714396-1-elver@google.com> <20260130132951.2714396-3-elver@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260130132951.2714396-3-elver@google.com> On Fri, Jan 30, 2026 at 02:28:25PM +0100, Marco Elver wrote: > Rework arm64 LTO __READ_ONCE() to improve code generation as follows: > > 1. Replace _Generic-based __unqual_scalar_typeof() with more complete > __rwonce_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). > > Once our minimum compiler versions are bumped, this just becomes > TYPEOF_UNQUAL() (or typeof_unqual() should we decide to adopt C23 > naming). Sadly the fallback version of __rwonce_typeof_unqual() cannot > be used as a general TYPEOF_UNQUAL() fallback (see code comments). > > 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 > --- > v3: > * Comment. > > v2: > * Add __rwonce_typeof_unqual() as fallback for old compilers. > --- > arch/arm64/include/asm/rwonce.h | 21 +++++++++++++++++---- > 1 file changed, 17 insertions(+), 4 deletions(-) > > diff --git a/arch/arm64/include/asm/rwonce.h b/arch/arm64/include/asm/rwonce.h > index fc0fb42b0b64..42c9e8429274 100644 > --- a/arch/arm64/include/asm/rwonce.h > +++ b/arch/arm64/include/asm/rwonce.h > @@ -19,6 +19,20 @@ > "ldapr" #sfx "\t" #regs, \ > ARM64_HAS_LDAPR) > > +#ifdef USE_TYPEOF_UNQUAL > +#define __rwonce_typeof_unqual(x) TYPEOF_UNQUAL(x) > +#else > +/* > + * Fallback for older compilers (Clang < 19). > + * > + * Uses the fact that, for all supported Clang versions, 'auto' correctly drops > + * qualifiers. Unlike typeof_unqual(), the type must be completely defined, i.e. > + * no forward-declared struct pointer dereferences. The array-to-pointer decay > + * case does not matter for usage in READ_ONCE() either. > + */ > +#define __rwonce_typeof_unqual(x) typeof(({ auto ____t = (x); ____t; })) > +#endif I know that CONFIG_LTO practically depends on Clang, but it's a bit grotty relying on that assumption here. Ideally, it would be straightforward to enable the strong READ_ONCE() semantics on arm64 regardless of the compiler. > /* > * When building with LTO, there is an increased risk of the compiler > * converting an address dependency headed by a READ_ONCE() invocation > @@ -32,8 +46,7 @@ > #define __READ_ONCE(x) \ > ({ \ > typeof(&(x)) __x = &(x); \ > - int atomic = 1; \ > - union { __unqual_scalar_typeof(*__x) __val; char __c[1]; } __u; \ > + union { __rwonce_typeof_unqual(*__x) __val; char __c[1]; } __u; \ > switch (sizeof(x)) { \ > case 1: \ > asm volatile(__LOAD_RCPC(b, %w0, %1) \ > @@ -56,9 +69,9 @@ > : "Q" (*__x) : "memory"); \ > break; \ > default: \ > - atomic = 0; \ > + __u.__val = *(volatile typeof(*__x) *)__x; \ Since we're not providing acquire semantics for the non-atomic case, what we really want is the generic definition of __READ_ONCE() from include/asm-generic/rwonce.h here. The header inclusion mess prevents that, but why can't we just inline that definition here for the 'default' case? If TYPEOF_UNQUAL() leads to better codegen, shouldn't we use that to implement __unqual_scalar_typeof() when it is available? I fear I'm missing something here, but it just feels like we're optimising a pretty niche case (arm64 + LTO + non-atomic __READ_ONCE()) in a way that looks more generally applicable. Will