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Mon, 21 Jun 2021 07:02:12 -0700 (PDT) Received: from [192.168.1.11] ([71.212.149.176]) by smtp.gmail.com with ESMTPSA id y66sm5812437pgb.4.2021.06.21.07.02.12 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 21 Jun 2021 07:02:12 -0700 (PDT) Subject: Re: [PATCH 01/28] tcg: Add flags argument to bswap opcodes To: Peter Maydell References: <20210614083800.1166166-1-richard.henderson@linaro.org> <20210614083800.1166166-2-richard.henderson@linaro.org> From: Richard Henderson Message-ID: Date: Mon, 21 Jun 2021 07:02:11 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=2607:f8b0:4864:20::102b; envelope-from=richard.henderson@linaro.org; helo=mail-pj1-x102b.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: QEMU Developers Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 6/21/21 6:51 AM, Peter Maydell wrote: > On Mon, 21 Jun 2021 at 14:41, Peter Maydell wrote: >> >> On Mon, 14 Jun 2021 at 09:43, Richard Henderson >> wrote: >>> >>> This will eventually simplify front-end usage, and will allow >>> backends to unset TCG_TARGET_HAS_MEMORY_BSWAP without loss of >>> optimization. >>> >>> The argument is added during expansion, not currently exposed >>> to the front end translators. Non-zero values are not yet >>> supported by any backends. >> >> Here we say non-zero values are not yet supported by the backend... > > Looking at the tcg/README docs, I think what you mean is that > at the moment all the backends assume/require that the caller passes > one of TCG_BSWAP_IZ or (TCG_BSWAP_IZ | TCG_BSWAP_OZ), since the > pre-flags implementation requires the top bytes to be zero and leaves > them that way. Correct. > But then the parts of your patch that pass in a zero > flags word would be wrong... The parts that pass in a zero flags word are covered by, from the README: > The flags are ignored -- the argument is present > for consistency with the smaller bswaps. r~