Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!uunet!husc6!rutgers!rochester!cornell!uw-beaver!tektronix!tekcrl!tekfdi!videovax!stever
From: stever@videovax.Tek.COM (Steven E. Rice, P.E.)
Newsgroups: comp.sys.amiga
Subject: Re: Manx C
Message-ID: <4529@videovax.Tek.COM>
Date: Mon, 10-Aug-87 15:30:05 EDT
Article-I.D.: videovax.4529
Posted: Mon Aug 10 15:30:05 1987
Date-Received: Wed, 12-Aug-87 01:43:39 EDT
References: <4540@jade.BERKELEY.EDU> <1836@vax135.UUCP>
Reply-To: stever@videovax.Tek.COM (Steven E. Rice, P.E.)
Organization: Tektronix Television Systems, Beaverton, Oregon
Lines: 53
Keywords: read the manual carefully
Summary: Oooops!

In article <4524@videovax.Tek.COM>, I wrote:

[ a bunch deleted ]

> There is no reason (other than incomplete optimization) that (ptr == 0)
> should be any larger than (!ptr) -- at least on the 68000 family!  If we
> ignore the limited arithmetic allowable on pointers, the general strategy
> for an equality test would be (using long words, because of the pointer
> arithmetic):
> 
>                  movea.l   <source>,Ax
>                  cmpa.l    #<constant>,Ax
>                  bne       not_equal
> 
>                  [ code to be executed if the two are equal ]
> 
>                  bra       around
> 
>     not_equal    [ code to be executed if the two are not equal ]
> 
>     around       [ next statement ]

So far, so good.  However, I then stick my foot into my mouth:
 
> But, when the optimizer (or for that matter, the compiler) sees it is
> generating a "movea.l  <source>,Ax" / "cmp.l  #0,Ax" sequence, it can
> immediately delete the "cmp" instruction, because the 68000 sets the
> condition codes when the register is loaded.

Thomas M. Breuel (<tmb@prep.ai.mit.edu>) wrote to point out that it is
true the condition codes are set -- but only when loading the data
registers!  Thus, while the full sequence would work, the shortened one
would not.

A reasonable alternative is to load the pointer value into a data register
(where the condition codes *are* set), and then branch:

                 move.l    <source>,Dx
                 bne       not_equal

                 etc.

Then, code in the leg executed when the pointer is non-zero can transfer
the pointer value to an address register, or use it directly as an index
value.

Thanks to Tom for pointing out my error.

					Steve Rice

-----------------------------------------------------------------------------
new: stever@videovax.tv.Tek.com
old: {decvax | hplabs | ihnp4 | uw-beaver | cae780}!tektronix!videovax!stever