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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: RFC:  Generic Fixed-Point IIR Filter
Date: Thu, 26 Mar 2015 16:12:28 -0500
Organization: Jacob Sparre Andersen Research & Innovation
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Date: 2015-03-26T16:12:28-05:00
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"Jeffrey Carter" <spam.jrcarter.not@spam.not.acm.org> wrote in message 
news:mf1qo9$dqb$3@dont-email.me...
> On 03/26/2015 01:34 PM, Randy Brukardt wrote:
>>
>> A result of universal-fixed always has to be converted immediately to
>> another type. It's not legal to use it directly. Specifically, 
>> 4.5.5(19.1/2)
>> says: "The above two fixed-fixed multiplying operators shall not be used 
>> in
>> a context where the expected type for the result is itself 
>> universal_fixed
>> ".
>
> So there's always an expected subtype? I thought in a string of operations 
> in an
> expression, there was no expected type until the conversion of the result 
> to the
> expected subtype of the entire expression.

Right. The rule specifically does not allow something like:
    A * B * C
because that could take an infinite number of bits to evaluate (just keeping 
adding the * operators) - the number of bits needed is the sum of the 
maximum bits for each value. Universal-fixed is not supposed to overflow, 
some something has to make it implementable. In addition, most machines have 
a double precision multiply operation (like the 32*32 bits gives 64 bits 
operation the OP mentioned and a matching divide). So there's an obvious 
implementation in many cases (not on the margins, as I noted), so long as we 
are talking about a single multiply. Ada runtime math isn't supposed to 
require some complex infinite precision operation (which admittedly is 
interesting, since the compiler is required to do exactly that).

                                     Randy.