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[187.189.51.144]) by smtp.gmail.com with ESMTPSA id z14sm1423772oot.5.2020.12.10.17.36.46 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 10 Dec 2020 17:36:46 -0800 (PST) Subject: Re: [PATCH 1/4] clock: Introduce clock_ticks_to_ns() To: Peter Maydell References: <20201208181554.435-1-peter.maydell@linaro.org> <20201208181554.435-2-peter.maydell@linaro.org> From: Richard Henderson Message-ID: <4fe28a52-40a2-47d5-9c3d-44e5ad9f95c3@linaro.org> Date: Thu, 10 Dec 2020 19:36:44 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=2607:f8b0:4864:20::344; envelope-from=richard.henderson@linaro.org; helo=mail-ot1-x344.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: Damien Hedde , Aleksandar Rikalo , QEMU Developers , Luc Michel , =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 12/10/20 2:47 PM, Peter Maydell wrote: >> With the shift, you're discarding the high 32 bits of the result. You'll lose >> those same bits if you shift one of the inputs left by 32, and use only the >> high part of the result, e.g. >> >> mulu(&discard, &ret, clk->period, ticks << 32); >> return ret; >> >> Which on some hosts, e.g. aarch64, only requires umulh and not two multiply >> instructions. > > We can't do this if we want to allow a full 64-bit 'ticks' input, right? Correct. > So I think my plan for v2 of this series is just to add in the > saturation-to-INT64_MAX logic. Sounds good. r~