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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-435e10edfccsm21939238f8f.17.2026.01.30.07.11.31 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 30 Jan 2026 07:11:31 -0800 (PST) Date: Fri, 30 Jan 2026 15:11:30 +0000 From: David Laight To: Marco Elver Cc: Peter Zijlstra , Will Deacon , 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 Subject: Re: [PATCH v3 2/3] arm64: Optimize __READ_ONCE() with CONFIG_LTO=y Message-ID: <20260130151130.6026999b@pumpkin> In-Reply-To: <20260130132951.2714396-3-elver@google.com> References: <20260130132951.2714396-1-elver@google.com> <20260130132951.2714396-3-elver@google.com> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: llvm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Fri, 30 Jan 2026 14:28:25 +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 Having most of the comment in the commit message and a short one in the code look good. I think it will also fix a 'bleat' from min() about a signed v unsigned compare. The ?: causes 'u8' to be promoted to 'int' with the expected outcome. Reviewed-by: David Laight @gmail.com > --- > 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 > + > /* > * 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; \ > } \ > - atomic ? (typeof(*__x))__u.__val : (*(volatile typeof(*__x) *)__x);\ > + __u.__val; \ > }) > > #endif /* !BUILD_VDSO */