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From: jsdy@hadron.UUCP (Joseph S. D. Yao)
Newsgroups: net.lang.c
Subject: Re: C Floating point arithmetic
Message-ID: <128@hadron.UUCP>
Date: Wed, 18-Dec-85 03:06:14 EST
Article-I.D.: hadron.128
Posted: Wed Dec 18 03:06:14 1985
Date-Received: Fri, 20-Dec-85 02:15:08 EST
References: <706@lasspvax.UUCP> <7849@ucla-cs.ARPA> <984@turtlevax.UUCP>
Reply-To: jsdy@hadron.UUCP (Joseph S. D. Yao)
Distribution: net
Organization: Hadron, Inc., Fairfax, VA
Lines: 34
Keywords: VAX11 Architecture Handbook
Summary: One-instruction move & sxt/zero

In article <984@turtlevax.UUCP> ken@turtlevax.UUCP (Ken Turkowski) writes:
>However, if an int is a long, and your C compiler does all operations
>on longs, then it takes at least one extra instruction to either sign-
>or zero-extend a short or char to a long. So to add two shorts, you have:
>	move16	_a,r0
>	ext32	r0
>	move16	_b,r1
>	ext32	r1
>	add32	r1,r0
>	move16	r0,_a
>Rather than
>	move16	_a,r0
>	add16	_b,r0
>	move16	r0,_a
>Or
>	add16	_b,_a
>Can you say "order of magnitude in speed difference"?

How about:
	cvtwl	_a,r0
	cvtwl	_b,r1
	addl2	r1,r0
	cvtlw	r0,_a
Or, better,
	addw2	_b,_a
This is legitimate VAX code.  Can you say "efficient orthogonal
architecture"?  Five times fast?	;-)

[No, Cottrell hasn't converted me.  It just happens to be true.
Programs should still be written readably and, consequently,
portably.]
-- 

	Joe Yao		hadron!jsdy@seismo.{CSS.GOV,ARPA,UUCP}