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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-48de0aaaad1sm11373355f8f.17.2026.10.14.05.27.37 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 14 Oct 2026 05:27:37 -0700 (PDT) Date: Wed, 14 Oct 2026 13:27:36 +0100 From: David Laight To: Alexander Gordeev Cc: Heiko Carstens , Ilya Leoshkevich , Sven Schnelle , Vasily Gorbik , Christian Borntraeger , linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org Subject: Re: [PATCH 2/4] s390/percpu: Drop unused output register from arch_this_cpu_add() Message-ID: <20261014132736.10a32cfe@pumpkin> In-Reply-To: <4907dbfd-3c5c-483b-be25-b0df83af2d20-agordeev@linux.ibm.com> References: <20261012114249.2898572-1-hca@linux.ibm.com> <20261012114249.2898572-3-hca@linux.ibm.com> <4907dbfd-3c5c-483b-be25-b0df83af2d20-agordeev@linux.ibm.com> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: linux-s390@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Mon, 12 Oct 2026 15:24:20 +0200 Alexander Gordeev wrote: > On Mon, Oct 12, 2026 at 01:42:47PM +0200, Heiko Carstens wrote: > > The laa/laag instructions return the old memory content, but > > arch_this_cpu_add() does not need it. Instead of allocating a dedicated > > output register old__ just to discard the returned value, change the > > val__ constraint from "d" to "+&d" so it is used as input/output > > register. > > > > This reduces register pressure. > > Do you know whether the compiler is may be smart enough to select the > same register for old and val himself? Given gcc tends to allocate a new register for x++ I doubt it (and then has to do a reg-reg move to put it back). David