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From: "Rod Pemberton" <dont_use_email@xnothavet.cqm>
Newsgroups: comp.lang.forth
Subject: Re: Epic Fail
Date: Wed, 30 Apr 2014 22:02:11 -0400
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On Wed, 30 Apr 2014 07:45:47 -0400, Andrew Haley  
<andrew29@littlepinkcloud.invalid> wrote:

> We need to talk about real examples.
>
> The problem is that if you have a Forth word like, say:
>
> : foo   9 * ;
>
> you could implement this in the JVM as a method:
>
>     public static long bar(long n) {
>         return n * 9;
>     }
>
>   [snip IR and assembly]
>
> but in Forth we have a stack that is accessed by every word,
> and can be left unbalanced, so we have to expose it as a
> real array:
>
>     public static int foo(long[] stack, int sp) {
>         stack[sp] *= 9;
>         return sp;
>     }
>
>   [snip IR and assembly]
>

Is there any reason to return 'sp'?  You didn't modify it...

Why didn't you do something like this?:

   public static int foo(long[] stack, int sp)
   {
     long n;
     n=stack[sp];
     n*=9;
     stack[sp]=n;
     return n;
   }

I.e., wouldn't the local variable 'n' _allow_ the compiler
to use a register thereby possibly reducing the code somewhat?
Try it...

I.e., in C, you might've chosen to pass a pointer to 'sp'
in the parameter list instead.  That would eliminate the
following line in the main code, replacing it with the next:

   sp=foo(stack,sp); /* eliminated */
   foo(stack,&sp); /* replacement */

Also, can you make 'stack' and 'sp' have file scope, be global,
so you don't need to pass them?  That's probably adding a small
amount of parameter passing code.  I.e., 'void' parameters,
at least in C, eliminates some passing overhead:

   public static void foo(void)
   {
     long n;
     n=stack[sp];
     n*=9;
     stack[sp]=n;
   }

Boundary checks on the stack pointer for underflow and overflow
probably adds some extra code too.  I haven't figured out a
clean or safe way to eliminate that from my interpreter, yet.
That would be nice if I could.

I don't see stack boundary checks in the code, but it looked
like some was in the assembly.  I'm assuming this is for C++
and some object-oriented feature was in play which inserted
the checks.


Rod Pemberton