From: anton@mips.complang.tuwien.ac.at   
      
   Programmers use value-flavoured words for various reasons.  In some   
   cases they want to have the address of the data otherwise accessed by   
   calling the value-flavoured word V or by using TO V.  Popular systems   
   have ADDR V or &OF V for taking the address.   
      
   However, taking the address of the data means that (in the absence of   
   complicated, expensive, and unreliable alias analysis) any memory   
   access can access the data, so we are very limited in   
   register-allocating the data and/or in reordering the accesses to the   
   data.   
      
   We want to avoid imposing such optimization obstacles when not   
   necessary.  Taking the address of data accessed through   
   value-flavoured words is rare: E.g., in the source of Gforth, there   
   are 130 uses of TO, but the only use of ADDR is for defining a synonym   
   (the SwiftForth name &OF), i.e., not to take the address of data.   
   Therefore we do not want to impose the optimization obstacles on all   
   value-flavoured words by supporting ADDR on all of them.   
      
   In the past, we introduced alternative defining words (VARUE etc.) for   
   defining value-flavoured words that support ADDR.  But we now have   
   around 20 words for defining VALUE-flavoured words that do not support   
   ADDR, and duplicating them for supporting ADDR seems excessive.  So   
   during the last days we implemented an alternative: If you want to   
   define a value-flavoured word which you can use with ADDR, you precede   
   the defining word with ADDRESSABLE:.  E.g.,   
      
   5 addressable: value foo   
   7 addr foo +!   
   foo . \ prints 12   
      
   or   
      
   : bar {: addressable: x -- y :}   
     5 addr x +! x ;   
      
   As you can see, it's hard to give simple examples for the use of ADDR   
   that are not contrived.   
      
   In the case of the locals definition, the defining word W: for a   
   cell-sized value-flavoured local is usually implied, and can still be   
   preceded by ADDRESSABLE:.   
      
   Currently Gforth does not perform any optimizations for which ADDR   
   would be an obstacle, so using ADDR on a non-ADDRESSABLE: word only   
   produces a warning, but once we implement such optimizations, these   
   warnings will turn into errors.   
      
   - anton   
   --   
   M. Anton Ertl  http://www.complang.tuwien.ac.at/anton/home.html   
   comp.lang.forth FAQs: http://www.complang.tuwien.ac.at/forth/faq/toc.html   
        New standard: https://forth-standard.org/   
   EuroForth 2023 proceedings: http://www.euroforth.org/ef23/papers/   
   EuroForth 2024 proceedings: http://www.euroforth.org/ef24/papers/   
      
   --- SoupGate-DOS v1.05   
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