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From: thomson@uthub.UUCP (Brian Thomson)
Newsgroups: net.micro.68k,net.arch
Subject: Re: FLAME!!! Re: EA orthogonality
Message-ID: <210@uthub.UUCP>
Date: Wed, 29-May-85 16:30:02 EDT
Article-I.D.: uthub.210
Posted: Wed May 29 16:30:02 1985
Date-Received: Mon, 3-Jun-85 13:05:28 EDT
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Organization: CSRG, University of Toronto
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Chris Shaw writes about orthogonality:
>When designing
>the 32032 then, the kind of programs the designers of the chip had in mind 
>were those that would be created by high-level languages. Thus, they made the
>instruction set as easy as possible to write compilers for. 
>On a general-purpose CPU, ... most programs will
>be created by programs (compilers), so it makes sense to tailor the instruction
>set to its intended programmers.

In my experience, the difficulty of (decent) compiler construction is affected
less by orthogonality than by the number of code sequences that must be
considered when implementing a given source language construct.
The C statement
		a = b * c + d + e;
might, in different contexts be implemented on your 32032 as:

		movd	_b,r0
		muld	_c,r0
		addd	_d,r0
		addd	_e,r0
		movd	r0,_a

or, if c is the constant 2, d a stack local, and e the constant 4,

		movd	_b,r0
		addr	4(-4(fp))[r0:w],_a

or even, if b, c, and d are all unsigned shorts, and e == b,

		movzwd	_b,r0
		indexw	r0,_c,_d	; b * (c+1) + d
		movd	r0,_a

Does that last one look ridiculous?  That's exactly my point: it's the
best code sequence under the given set of assumptions, and no compiler
would ever find it.  If these fancy addressing modes and high-level
language oriented instructions could be added without penalizing the
performance of bread-and-butter instructions, I'd be all for it, but
such is never the case.

If a machine forces me to put something in a data register before I
can add to it, and has no exceptions to this rule, it will be easy to
generate code.  It only gets tough when there are options.
-- 
		    Brian Thomson,	    CSRI Univ. of Toronto
		    {linus,ihnp4,uw-beaver,floyd,utzoo}!utcsrgv!uthub!thomson