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From: mitchalsup@aol.com (MitchAlsup1)
Newsgroups: comp.arch
Subject: Re: Continuations
Date: Wed, 17 Jul 2024 16:17:22 +0000
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On Wed, 17 Jul 2024 1:41:12 +0000, John Savard wrote:

> On Tue, 16 Jul 2024 20:51:17 +0000, mitchalsup@aol.com (MitchAlsup1)
> wrote:
>
>>I think that history shows that those string facilities were, at best,
>>overkill. None of the RISC machines had any of that and were faster
>>and at least as general purpose as 360.
>>
>>Especially the COBOL stuff like EDIT and EDIT-and-MARK.
>
> I will agree with that; the 68000 and the x86 don't have those things
> either.
>
> Even if my personal inclination has been to include them too.
>
>>On the other hand, I am advocating that some Elementary Functions be
>>raised to first class citizens of ISA so that SIM() is 20 cycles instead
>>of 150 cycles. Why, because today, we understand enough about the cal-
>>culations, polynomials, error sources, to build in HW things that are
>>just as accurate as SW can build and nearly 7×-10× faster.
>
> I thought this was already the case; the 8087 had transcendental
> functions, and so did the 486 DX and from that onwards, and this was
> also the case with the 68881 and 68882.
>
> Of course, even if they're citizens of the ISA, they could be
> second-class citizens, whatever the definition of that might be.

While they were "in" the ISAs, they would take thousands of cycles
while FP would only take hundreds of cycles.

What I am talking about is to improve their performance until a
sin() takes about the same number of cycles of FDIV, not 10× more.
>
> Here, my personal inclination is to simply include the raw CORDIC
> calculation as an instruction, with bounds checking, special cases,
> and any fancy stuff in software. On the grounds that some things are
> too complicated for a classic old-style ISA.

This is a tradeoff in size versus cycles. I choose to stop when
sin() was as fast as FDIV.
>
> That, too, is of course a bad decision from a practical perspective.

Imagine a situation where::

     s = sin(x);
     c = cos(x);

is faster and more accurate than:

     s = sin(x);
     c = sqrt(1-s^2);

!!!
{{ignoring the fact that cos() can be negative where sqrt() cannot.}}
>
> John Savard