Unassign a variable
lazarus / 9-Editor

----------------------------------------------------------------------
LazaruX · 2007-11-30 16:46

Hi everybody,
with this code:

Code: [Select]
var
 number:integer


we assign a variable, this means that a little part of the memory is
reserved to be able to fill the maximal allowed integer size.

My question is, how do I unassign this variable?
How do I free this memory?

----------------------------------------------------------------------
antonio · 2007-11-30 19:55

Why do you need to do so?

----------------------------------------------------------------------
gerardus · 2007-12-02 10:37

Like this:

Code: [Select]
 { var
 number:integer }


:wink:

Seriously, there is no way, unless you assign memory dynamically for
all your variables.
But I don't understand why you would have all that trouble just to
save a few bytes.

Regards,

Gerard

----------------------------------------------------------------------
Zaher · 2007-12-03 05:44

Inside the function your variable will deallocated when when finish
the function, but if you talk about a huge global variable you can use
a memory functions.
But inside the function when you finish using a variable, with
optimize options for FPC the compiler automatic remove this variable
from memory.
Code: [Select]
function foo();
var
  x:Integer;
begin
...
...x :=10;
..writeln(x);//after this line x will deallocated, if you compile with
optimize mode.
...
..
..
..
end;

I hope i am aright.

----------------------------------------------------------------------
gerardus · 2007-12-03 09:05

Quote from: "zaher"
But inside the function when you finish using a variable, with
optimize options for FPC the compiler automatic remove this variable
from memory.


Interesting.
I thought that space was allocated on the stack for local variables
like integers. Does the compiler generate code to manipulate the stack
after every variable isn't used anymore?

Regards,

Gerard.

----------------------------------------------------------------------
Marc · 2007-12-03 10:43

The compiler reserves stackspace on entrance of the procedure.
Assigning a value doesn't allocate, the space is already there.
On exit the stackpointer is restored

----------------------------------------------------------------------
gerardus · 2007-12-03 11:31

Quote from: "Marc"The compiler reserves stackspace on entrance of the
procedure. Assigning a value doesn't allocate, the space is already
there.
On exit the stackpointer is restored

But the stack pointer isn't moved during procedure execution by some
magic optimizing trick, or is it?

----------------------------------------------------------------------
Marc · 2007-12-03 17:24

what would it optimize ?

----------------------------------------------------------------------
LazaruX · 2007-12-03 18:33

I already new about the variables in the procedure and functions, but
when they are assigned globally?

Last time I was talking to C programmers and they told me that in C
its possible to unassign variables and in Pascal not. I also need this
for some little applications.

----------------------------------------------------------------------
RudieD · 2007-12-03 18:59

Don't make it global if you don't want to use it global.

----------------------------------------------------------------------
Zaher · 2007-12-03 19:10

In functions when optimization enabled the compiler push in stack that
variable when first time used, then pop it when last time used (not
need it any mode). i test it in Delphi, in FPC not sure, but i think
it is more powerful than Delphi.

----------------------------------------------------------------------
gerardus · 2007-12-03 20:40

Quote from: "Marc"what would it optimize ?

Nothing. Un-optimize, maybe.

----------------------------------------------------------------------
Bart · 2007-12-03 22:51

Quote from: "BPsoftware"Last time I was talking to C programmers and
they told me that in C its possible to unassign variables and in
Pascal not. I also need this for some little applications.
Use pointers for everything then and allocate from the heap.
Real C style coders (also called NARG's) do this al the time  :lol:

Beware though, this makes it real easy to shoot yourself in the foot.
(No flame intended!!!)

Bart

----------------------------------------------------------------------
Marc · 2007-12-04 09:45

Quote from: "zaher"In functions when optimization enabled the compiler
push in stack that variable when first time used, then pop it when
last time used (not need it any mode). i test it in Delphi, in FPC not
sure, but i think it is more powerful than Delphi.

Modifying the stackpointer for each variable (un)usage is way more
inefficient than do it once on procedure entry/exit
So I won't call this an optimalization.

However, the compiler knows what the lifetime of a variable is, so it
can use the same stack location for different variables. You don't
have to unset anything, simply not using it is enough.

IIRC florian is/was working on this to add this to the compiler.