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Failed to connect to MySQL: (1203) User howardkn already has more than 'max_user_connections' active connectionsPath: news.eternal-september.org!eternal-september.org!.POSTED!not-for-mail From: David Brown Newsgroups: comp.lang.c Subject: Re: Fast division (was Re: Suggested method for returning a string from a C program?) Date: Wed, 26 Mar 2025 17:55:13 +0100 Organization: A noiseless patient Spider Lines: 25 Message-ID: References: <868qp1ra5f.fsf@linuxsc.com> <20250319115550.0000676f@yahoo.com> <20250319201903.00005452@yahoo.com> <86r02roqdq.fsf@linuxsc.com> <20250321113316.506@kylheku.com> <20250321210228.508@kylheku.com> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Injection-Date: Wed, 26 Mar 2025 17:55:13 +0100 (CET) Injection-Info: dont-email.me; posting-host="9e00a15ae379d2cae7edd65ac25a9709"; logging-data="2252747"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX1+9mYe6eUYoHLgfe1w041xhosnTM5QnPLM=" User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.11.0 Cancel-Lock: sha1:KtXLpyc7EauP4LWKzn2vyU2MQ+I= Content-Language: en-GB In-Reply-To: On 26/03/2025 14:47, Janis Papanagnou wrote: > On 26.03.2025 12:40, David Brown wrote: >>>> >>>> [ substituting a division by multiplication (and some primitives) ] >> >> Roughly speaking, when you want division of "y" by a fixed - i.e., >> constant value known at compile time - number "x", you can do it by >> pre-calculating z = 2^n / x and then you implement "y / x" by "y * z / >> 2^n". (There's also some stuff to handle correct rounding, especially >> with signed types.) > > Thanks for the terse explanation; the formulas helped me to detect > that the constants in Waldek's assembler code are the pre-computed > reciprocals. > > First I thought that there's a trick to avoid division of non-const > expressions. > For non-constant divisors, it's a bit more difficult! If you are going to use the same divisor several times, it can be worth computing the scaled reciprocal, so that you only have one hard operation. I believe that hardware floating point units do that anyway for division.