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digitalmars.D.bugs - [Issue 1076] New: by using scope(exit) tail recursion ain't working

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=1076

           Summary: by using scope(exit) tail recursion ain't working
           Product: D
           Version: 1.009
          Platform: PC
        OS/Version: Windows
            Status: NEW
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: bugzilla digitalmars.com
        ReportedBy: davidl 126.com


int i;

    int scp( int n ){ 
      if( n==0 ) return 0; 
   //   scope(exit) printf("%d",n); 
      scope(exit) i++;
      return scp(n-1); 
    } 
    void main() { 
      scp(40000); 
      printf(`%d`,i);
    }
//stack overflow
change scope(exit) i++; by i++; then tail recursion works fine


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Mar 21 2007
next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=1076





------- Comment #1 from davidl 126.com  2007-03-21 13:03 -------
actually i have an idea of how to implement tail recursion with scope .

err the idea here is simple, when tail recursion , just loop to the point
after SEH structure has been pushed on the stack. coz we are tail recursion
the same func, the SEH structure except the next SEH structure part are the
same as the SEH structure we have now pushed on the stack.

1. if an exception the current func doesn't handle, then throwing it directly
to the caller wouldn't be a problem, coz it would be thrown by the os
recursively

2. if an exception in the current func is actually handled, then in the
handler,
which get called from our OS, would simply do its job, after execution of its
handler func , then back to continue tail recursion not return here.

it's quite a rough thought.
but i think it's possible to have scope() stuff support tail recursion


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Mar 21 2007
prev sibling next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=1076


bugzilla digitalmars.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |INVALID




------- Comment #2 from bugzilla digitalmars.com  2007-03-21 13:18 -------
Tail recursion is when the *last* thing a function does is call itself. With
scope (exit), this isn't the case, and so the function recurses normally.


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Mar 21 2007
prev sibling next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=1076


smjg iname.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |smjg iname.com




------- Comment #3 from smjg iname.com  2007-03-21 13:37 -------
(In reply to comment #1)

I don't see how that would help.  The scope(exit) code would have to be called
as many times as there are levels of recursion.  In this case, where it doesn't
touch any local variables, it could probably be rewritten as a for loop.  But
in the general case, you'd still need all the stack frames for it to work.


--
Mar 21 2007
prev sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=1076





------- Comment #4 from davidl 126.com  2007-03-22 00:39 -------
err, yep , i haven't considered the following:
void hello( int n)
{
    if (n==0) return;
    if (n%2==1)
    {
        scope(exit) printf(`%d odd`\n,n);
        hello(n-1);
    }
    if (n%2==0)
    {
        scope(exit) printf(`%d even`\n,n);
        hello(n-1);
    }
}


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Mar 21 2007