Path: utzoo!attcan!uunet!auspex!guy
From: guy@auspex.UUCP (Guy Harris)
Newsgroups: comp.lang.c
Subject: Re: "for" loops in C ...
Message-ID: <419@auspex.UUCP>
Date: 9 Nov 88 18:23:49 GMT
References: <867@cernvax.UUCP> <645@quintus.UUCP> <339@igor.Rational.COM>
Reply-To: guy@auspex.UUCP (Guy Harris)
Organization: Auspex Systems, Santa Clara
Lines: 13


 >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?

More precisely, "<<" is defined as a left shift that does unspecified
things if the shift count is larger than the size of the (appropriately
promoted) left-hand operand.  Thus, the single VAX ASHL instruction
implements what various language specs say "<<" means, and a left-shift
instruction that *didn't* mask the shift count would also do so.