From: jerry@connection.prospect.com (Jerry Shekhel)
Subject: Re: To Nish...answer, please.
Organization: Molecular Simulations, Inc.
Date: Fri, 17 Apr 1992 16:21:00 GMT

In article <1992Apr17.150319.1@cc.helsinki.fi> lindblad@cc.helsinki.fi writes:
>
>Nish, why have you not yet answered my question I've asked you about 5 times:
>what, in your opinion, makes 80486 superior in speed when compared to 68040? Is
>it the instruction set? I doubt that. Is it the amount of register shuffle? I
>don't think so. Is it because it has unified cache with no copyback? No, it
>can't be. Is it because it's instructions are 30-50% slower than equalent 68040
>instructions? What makes the 80486 a faster CPU in your mind?
>

Jarkko, I can't answer for Nish, especially since I don't have the opinion
that the 486 is faster than the 040; it's obviously significantly slower in
floating point, and they are about equal in integer.  However, I just wanted
to make a point here.  Although it's true that the 040 looks better on paper,
existing benchmarks and everyday applications seem to indicate that what
looks good on paper doesn't always translate into higher performance in the
real world.

Don't you agree that actual perceived performance is what's really important?
Whatever you think of compiled benchmarks, you must admit that they come
closer to estimating a system's real world performance than would some hand-
optimized assembly language loop.  Since the two CPU's in question appear to
perform identically on integer benchmarks, it's not up to Nish to explain
why the 486 is faster; in fact, the burden of explanation goes to you, as
you must try to explain what's causing the 040 to perform no better than
the 486 on the benchmarks.

One possible explanation has shown up on this newsgroup -- the claim that
compilers, including even multi-platform compilers like GNU C, tend to
produce code of higher quality on the 486 than on the 040; today's 040 code
generators have much room for improvement in terms of exploiting the
features of the chip.  Well, as much as I'd like to believe it, I can't
reconcile this claim with another one that people frequently make on this
newsgroup -- that the 680x0 line was designed with high-level languages in
mind, in order to ease the job of compiler writers, while the 80x86 line
makes it extremely difficult.  If the 680x0 makes it so easy, how come
current compilers are so bad?  Together, those two statements form a
contradiction, and I would honestly like to know what's really causing
the 040 to perform no better than the 486.

>
>Jarkko Lindblad
> 
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