[Tcl-bugs] [ tcl-Patches-1998273 ] Making a proc arg's unshared

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Patches item #1998273, was opened at 2008-06-19 23:55
Message generated for change (Comment added) made by ferrieux
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Category: 10. Objects
Group: None
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: Alexandre Ferrieux (ferrieux)
Assigned to: miguel sofer (msofer)
Summary: Making a proc arg's unshared

Initial Comment:
Inside a proc, a value passed as argument typically has a refcount of three. Of the two unwanted ones, one is easily removed: the local var

         foo $x[unset x]

but the other is harder: it is in the CallFrame's objv, basically [info level 0].

The attached patch provides [info zaplevel] which prematurely neutralizes the [info level 0] data for that purpose. Thus, in

     proc pop x {
        info zaplevel
        lrange $x[unset x] 0 end-1
     }

The $x value is effectively passed unshared, provided it is passed unshared by the caller.

The patch works by downing the refcounts of all members of [info level 0] except the head (to keep the proc name visible, should someone call [info level 0] after [info zaplevel]), and replacing them with references to a shared empty Tcl_Obj. Hence an [info level 0] of "foo a b c" becomes "foo {} {} {}".
This gives us safety in the event of callers cacheing CallFrame->objc,objv in locals/registers.


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>Comment By: Alexandre Ferrieux (ferrieux)
Date: 2008-06-20 00:32

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No. [take x] is exactly the same as $x[unset x], but in less efficient
;-)

To witness that you still have an extra ref:
% proc K {x y} { set x }
% proc take {v} { upvar 1 $v x ; K $x [unset x] }
% proc pop x {lrange [::tcl::unsupported::debugobj [take x]] 0 end-1}
% pop [list d f g h]
Obj:0x9F2340(2) (list):0x9F1000-0x0 <<<?>>>
d f g

Now with [info zaplevel]:
% proc pop x {info zaplevel;lrange [::tcl::unsupported::debugobj [take x]]
0 end-1}
% pop [list d f g h]
Obj:0x9DB658(1) (list):0x9EF4C8-0x0 <<<?>>>
d f g

PS: A side note: the $x[unset x] idiom is the K-free K given to me by
Kevin IIRC. It is very efficient because the concatenation of the empty
string has a shortcut and causes no duplication nor shimmering.

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Comment By: Andreas Kupries (andreas_kupries)
Date: 2008-06-20 00:05

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Does the following do the same?

proc K {x y} { set x }
proc take {v} { upvar 1 $v x ; K $x [unset x] }
proc pop x { lrange [take x] 0 end-1 }


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