Path: utzoo!utgpu!cs.utexas.edu!wuarchive!uunet!stanford.edu!agate!agate!hughes
From: hughes@bigbang.Berkeley.EDU (Eric Hughes)
Newsgroups: alt.hackers
Subject: Re: credit 'fixing'
Message-ID: <HUGHES.91Mar9005027@bigbang.Berkeley.EDU>
Date: 9 Mar 91 05:50:27 GMT
References: <1991Feb21.154633.20781@bronze.ucs.indiana.edu>
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In-Reply-To: jkonrath@silver.ucs.indiana.edu's message of 21 Feb 91 15:46:33 GMT

In article <1991Feb21.154633.20781@bronze.ucs.indiana.edu> jkonrath@silver.ucs.indiana.edu (jon) said something like:

The "lack of knowledge around me" wrote:
   IF top = 8 THEN
     top = 0
   ELSE
     INC(top)
   END;
where i just did 
   top := (top+1)MOD8

==endquote==

Now wait!  The first code will (on most compilers) execute much faster, 
since there's no division instruction generated.  Put this fragment
inside a loop, and you've got a significant performance difference.


Hack:  a favorite C statement (conditional assignment)
           *( bool ? &x : &y ) = t ;
   I have broken two C compilers with this expression.
   

Hack:  For my work, I am currently converting an interpreter, which
takes a custom semi-text input format, and which is written in 8088
assembly, to a compiler/inner-interpreter pair.  Fine.

But we have a design constraint that all the existing high-level code
has to continue to run while the engines are being worked on.  Ick.

Solution: A custom language, with a compiler written in AWK, which
specifies the exchange language between the compiler and the
i-interpreter.  It allows the generation of text statements for those
verbs which are unconverted, and for postfix expressions for those
which are.  Now I can convert a keyword at a time, switch its output
syntax, reimplement the keyword in C, and the user is none the wiser.
Eventually all this scaffolding for conversion will be thrown out
when a full postfix conversion is completed.  Neat.

Particular gratitude to Jon Bentley, who inspired this approach.

Eric Hughes
P.O. 4789
Berkeley, CA 94704