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Thu, 08 Jul 2021 08:48:13 -0700 (PDT) Received: from [192.168.1.11] ([71.212.149.176]) by smtp.gmail.com with ESMTPSA id k10sm3425013pfp.63.2021.07.08.08.48.12 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 08 Jul 2021 08:48:12 -0700 (PDT) Subject: Re: [PATCH v2 8/8] linux-user: Simplify host <-> target errno conversion using macros To: Laurent Vivier , =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= , qemu-devel@nongnu.org References: <20210708141121.1731691-1-f4bug@amsat.org> <20210708141121.1731691-9-f4bug@amsat.org> From: Richard Henderson Message-ID: <8567ef72-e1d5-6789-ad3f-fdefccd2a7d2@linaro.org> Date: Thu, 8 Jul 2021 08:48:11 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=2607:f8b0:4864:20::1034; envelope-from=richard.henderson@linaro.org; helo=mail-pj1-x1034.google.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Taylor Simpson Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 7/8/21 8:30 AM, Laurent Vivier wrote: > Le 08/07/2021 à 16:11, Philippe Mathieu-Daudé a écrit : >> Convert the host_to_target_errno_table[] array to a switch case >> to allow compiler optimizations. Extract the errnos list as to >> a new includible unit, using a generic macro. Remove the code >> related to target_to_host_errno_table[] initialization. >> > > Is there some performance penalties by using a switch() rather than an array[] ? In many cases, definitely not. The compiler does notice the identity function when host and guest errnos match. In the other case, I would doubt (without evidence) there's much penalty. The switch becomes a table lookup + indirect branch + immediate load + return. Or a really good compiler transforms to a different array lookup. r~