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From: jans@mako.UUCP (Jan Steinman)
Newsgroups: net.arch
Subject: Re: better late... (really, fast NS32000 multiplies)
Message-ID: <836@mako.UUCP>
Date: Thu, 20-Jun-85 19:35:37 EDT
Article-I.D.: mako.836
Posted: Thu Jun 20 19:35:37 1985
Date-Received: Sat, 22-Jun-85 23:56:18 EDT
References: <254@greipa.UUCP>
Reply-To: jans@mako.UUCP (Jan Steinman)
Distribution: net
Organization: Tektronix, Wilsonville OR
Lines: 37
Summary: 

In article <254@greipa.UUCP> paul@greipa.UUCP (Paul A. Vixie) writes:
>Not being a compiler writer (yet :-), anyway) I don't see many other things
>a compiler could optimize for (except the "muld 2, tos" which could have been
>"ashd 1, tos" but only vax-11 C from DEC does this).

Note that if one operand is a constant small integer and the other is in a
register, the following multiplies are shorter and quicker:

	Instruction		Function		Clocks	Size
							(w/MMU)	(Bytes)
	addd	rx,rx		; 2*rx (destructive)	4	2
	addr	rx[rx:b],ry	; 2*rx			13	2
	addr	rx[rx:w],ry	; 3*rx			15	2
	addr	@0[rx:d],ry	; 4*rx			15	3
	addr	rx[rx:d],ry	; 5*rx			16	2
	addr	@0[rx:q],ry	; 8*rx			17	3
	addr	rx[rx:q],ry	; 9*rx			18	2

compared with:

	ashd	1,rx		; 2*rx (destructive)	23	7
	muld	2,rx		; 2*rx (destructive)	85	7

It's so much quicker, that the 9 clocks required to move the operand from the
stack into a register still keep it faster and smaller than the shifted vers.

I've noticed that the C compiler on our 6000 series boxes generate at least a
few of these.  (This will all change when the 32?32 with the barrell shifter
arrives!)  

I disagree with Henry Spencer in saying that multiplies are uncommon.  In
particular, the small integer multiply operation is used in index scaling,
particularly whenever index math is performed, and it is important that a
compiler do its best on these, as they are often inside loops.
-- 
:::::: Jan Steinman		Box 1000, MS 61-161	(w)503/685-2843 ::::::
:::::: tektronix!tekecs!jans	Wilsonville, OR 97070	(h)503/657-7703 ::::::