Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!decwrl!sun!pitstop!sundc!seismo!uunet!microsoft!w-colinp
From: w-colinp@microsoft.UUCP (Colin Plumb)
Newsgroups: comp.arch
Subject: Re: quest for breakthroughs (long)
Message-ID: <730@microsoft.UUCP>
Date: 24 Feb 89 04:26:23 GMT
References: <740@tetons.UUCP> <76700068@p.cs.uiuc.edu> <671@oracle.oracle.com>
Reply-To: w-colinp@microsoft.uucp (Colin Plumb)
Organization: very little
Lines: 27

csimmons@oracle.UUCP (Charles Simmons) wrote:
> [The desired code was]:
> 
> 	nor $temp, $a, $b
> 	and $c, $c, $temp
> 
> The actual code generated [by the MIPS compiler] was:
> 
> 	or $temp, $a, $b
> 	nor $temp, $temp, $0
> 	and $c, $c, $temp
> 
> Since the 'nor' instruction doesn't directly map into the C language,
> I didn't really expect the compiler to handle this minor special case.)

Really?  I do.  Since the sequence "or $c, $a, $b; not $c, $c" is
exactly equivalent to "nor $c, $a, $b" and easily recognised by a
peephole optimiser.  If the code generator frequently uses extra
temporary registers (say the first sequence ends in "not $d, $c"),
you have to complicate this by checking to see if $c is dead after
this point, but I expect MIPS have already fixed this.

Well, guys?  Have you? :-)
-- 
	-Colin (uunet!microsoft!w-colinp)

"Don't listen to me.  I never do."