Path: utzoo!attcan!uunet!igor!dsb@Rational.COM
From: dsb@Rational.COM (David S. Bakin)
Newsgroups: comp.lang.c
Subject: Re: "for" loops in C ...
Message-ID: <339@igor.Rational.COM>
Date: 8 Nov 88 19:19:04 GMT
References: <867@cernvax.UUCP> <645@quintus.UUCP>
Sender: news@igor.Rational.COM
Reply-To: dsb@Rational.COM (David S. Bakin)
Organization: Rational
Lines: 33
In-reply-to: ok@quintus.uucp (Richard A. O'Keefe)

[What's going on?  The article I'm replying to was signed by Chris Torek of
 uunet!mimsy!chris but the headers say it is from ok@quintus.uucp???]

In article <645@quintus.UUCP>, ok@quintus (Richard A. O'Keefe) writes:
>In article <192@libove.UUCP> root@libove.UUCP (Jay M. Libove) writes:
>>Looking at the above, I read it to be "shift an n-bit integer n bits left"
>>... Now, why is there any question as to the result?
>
>Because different machines implement shift-left differently.
>
> ... [omitted]
>
>On the VAX, the result of shifting by the (not known to be constant)
>value 32 is the same as the result of shifting by zero, because the VAX
>looks only at the 5 lowest order bits of the shift count.  It does this
>so that right shifts can be done with negative left shifts.
>-- 
>In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
>Domain:	chris@mimsy.umd.edu	Path:	uunet!mimsy!chris

I don't get it.  The operator << is defined in C as a left shift, what
does that have to do with a particular VAX instruction, ASHL, that happens
to mask its operand to the low-order 5 bits.  Doesn't this merely mean that
on the VAX when the argument of the left shift is not a known constant that
the single instruction ASH sequence must be replaced by a multiple instruction
sequence that does the right thing?

-- Dave

----------------------------------------------------------
Dave Bakin				    (408) 496-3600
c/o Rational; 3320 Scott Blvd.; Santa Clara, CA 95054-3197
Internet:  dsb@rational.com	 Uucp:  ...!uunet!igor!dsb