[pups] 2.11BSD "make unix" aborts Error 141

[cdl@mpl.ucsd.edu (Carl Lowenstein)]

> From: "Steven M. Schultz" <sms at 2BSD.COM>
> To: pups at minnie.tuhs.org
> Subject: Re: [pups] 2.11BSD "make unix" aborts Error 141
> List-Archive: <http://minnie.tuhs.org/pipermail/pups/>
> Date: Wed, 19 Mar 2003 09:46:51 -0800 (PST)
> 
> Hi -
> 
> > From: David Evans <dfevans at bbcr.uwaterloo.ca>
> >   Ugh.
> > 
> >   Am I naive to suggest a rewrite of as?  :)
> 
> 	Just a little bit <g>
> 
> 	It's a thought that used to occur to me fairly often a long time 
> 	ago.  I cured myself of it though after I did the rewrite into
> 	its current form (if you think it's bad now you should have seen it
> 	before ;)).
> 
> 	The biggest problem are the SDI (Span Dependent Instructions).  The
> 	PDP-11 has 'jmp' (non conditional but no range limit) and 'br' (many
> 	flavors of conditional branches but with a +/- 128 byte range).  
> 	The compiler generates pseudo instructions like 'jeq' and leaves it
> 	up to the assembler to figure out if a branch will reach or if a
> 	branch around a jump is needed.   That's a PITA to handle and is
> 	I believe the cause of much ugliness in the assembler.

That, of course, is a _neat feature_ of AS.  On the other hand, how
much code bloat would result if AS always emitted (bne;jmp) pairs for
'jeq', etc.  Maybe only generate (bne;jmp) for forward 'jeq' where the
span is not yet known.  I am sure there are pathological instabilities
in this kind of code generation, where expanding one branch/jump
instruction makes another one go out of span range.

Answer:  3:1 for each unnecessary pair.  But I have no idea of the
statistics for real code.

    carl
-- 
    carl lowenstein         marine physical lab     u.c. san diego
                                                 clowenst at ucsd.edu