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From: cdl@chiton.ucsd.edu (Carl Lowenstein)
Newsgroups: comp.sys.next
Subject: Re: 68040 Math / Why so much system time?
Message-ID: <712@chiton.ucsd.edu>
Date: 17 Feb 91 20:00:19 GMT
References: <1991Feb14.160749.19048@batcomputer.tn.cornell.edu> <290@rosie.NeXT.COM> <1991Feb15.135340.2808@news.sara.nl> <1991Feb17.113656.5876@kithrup.COM>
Reply-To: cdl@chiton (Carl Lowenstein)
Organization: MPL of SIO at UCSD
Lines: 44

In article <1991Feb17.113656.5876@kithrup.COM> sef@kithrup.COM (Sean Eric Fagan) writes:
>In article <1991Feb15.135340.2808@news.sara.nl> toon@news.sara.nl writes:
>>Sorts of. Is trapping these instructions really faster than implementing
>>sin(), cos(), tan() and friends as run time library subroutines to the
>>(Objective-) C(++) compiler ? I would doubt it.
>
>Very good!  Perhaps you noticed the comment in this thread, a while ago,
>about 68040 fp-intensive programs being faster *when recompiled*?
>
>Or aren't you aware that there is no easy way to replace a single
>instruction, e.g., fsin?

I don't understand what point you are making here.  If there does exist
a math library that uses only fp instructions that are native to the
68040, then recompiling (or at least relinking) should make fp-intensive
programs run faster.

But the word from Morris Meyer at NeXT is that they didn't do it this
way, chosing to trap the un-implemented instructions and interpret them
in the kernel.  This does make it possible to run 68030/68882 programs
without relinking.  Conversely, it makes it possible to develop fp
programs on a 68040 system and have them work on the 68030/68882 with
no changes.

My proposal for a compromise solution is a math library that contains
two versions of the affected transcendental routines, one which is a
stub to call the 68882 hardware, and one using 68040 instructions.  The
68882 version can be the default.  If this traps as an un-implemented
instruction, the kernel trap handler can change a software switch so
that all subsequent calls to the routine get the 68040 version.

At the moment, I am not sufficiently fluent in 68040 to do this, but I
have seen very similar things done for PDP-11 floating point.







-- 
        carl lowenstein         marine physical lab     u.c. san diego
        {decvax|ucbvax} !ucsd!mpl!cdl                 cdl@mpl.ucsd.edu
                                                  clowenstein@ucsd.edu