From: anton@mips.complang.tuwien.ac.at   
      
   Stephen Fuld  writes:   
   >On 11/7/2025 2:09 AM, Anton Ertl wrote:   
   >> Stephen Fuld  writes:   
   >>> On 11/6/2025 12:46 AM, Anton Ertl wrote:   
   >>>> On such architectures switch would also be implemented by modifying   
   >>>> the code,   
   >>>   
   >>> I don't think so.  Switch can, and I understand usually is,implemented   
   >>> via an index into a jump table.  No self modifying code required.   
   >>   
   >> What does "index into a jump table" mean in one of those architectures   
   >> that did not have indirect jumps and used self-modifying code instead?   
   >   
   >For example, the following Fortran code   
   >   
   >	goto (10,20,30,40) I  @ will jump to label 10 if I =1, 20 if I = 2, etc   
   >   
   >would be compiled to something like (add any required "bounds checking"   
   >for I)   
   >   
   >	load 	R1,I   
   >	Jump 	$,R1	   
   >	Jump	10   
   >	Jump 	20   
   >	Jump 	30   
   >	Jump 	40   
      
   Which architecture ist that?   
      
   >No code modification nor indirection required .   
      
   The "Jump $,R1" is an indirect jump.  With that the assigned goto can   
   be implemented as (for "GOTO X")   
      
   load R1,X   
   Jump 0,R1   
      
   >>>> and indirect calls and method dispatch would also be   
   >>>> implemented by modifying the code.  If self-modifying code is "just   
   >>>> bad", and any language features that are implemented on some long-gone   
   >>>> architectures using self-modifying code are bad by association, then   
   >>>> we have to get rid of all of these language features ASAP.   
   >>>   
   >>> And, by an large they have.   
   >>   
   >> We have gotten rid of indirect calls, e.g., in higher-order functions   
   >> in functional programming languages?  We have gotten rid of dynamic   
   >> method dispatch in object-oriented programs.   
   >   
   >No, and I defer to you, or others here, on how these features are   
   >implemented, specifically whether code modification is required.  I was   
   >referring to features such as assigned goto in Fortran, and Alter goto   
   >in Cobol.   
      
   On modern architectures higher-order functions are implemented with   
   indirect branches or indirect calls (depending on whether it's a   
   tail-call or not); likewise for method dispatch.   
      
   I do not know how Lisp, FORTRAN, Algol 60 and other early languages   
   with higher-order functions were implemented on architectures that do   
   not have indirect branches; but if the assigned goto was implemented   
   with self-modifying code, the call to a function in a variable was   
   probably implemented like that, too.   
      
   >> Thinking about the things that self-modifying code has been used for   
   >> on some architecture, IIRC that also includes array indexing.  So have   
   >> we gotten rid of array indexing in programming languages?   
   >   
   >Of course not.  But I suspect that we have "gotten rid of" any   
   >architecture that *requires* code modification for array indexing.   
      
   We have also gotten rid of any architecture that requires   
   self-modifying code for implementing the assigned goto.   
      
   - anton   
   --   
   'Anyone trying for "industrial quality" ISA should avoid undefined behavior.'   
     Mitch Alsup,    
      
   --- SoupGate-Win32 v1.05   
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