Re: Tail Recursion (Why it appears that Gnat 3.15p does support it)

[Jacob Sparre Andersen <sparre@nbi.dk>]

Chad  R. Meiners wrote:

> I have noticed that Gnat 3.15p does not optimize the tail recursion
> in the following program.
> 
> with Ada.Text_IO;
> use  Ada.Text_IO;
> 
> procedure Tail is
> 
> type Int is mod 2**64;
> 
> procedure Fib(First, Second : Int) is
> Next : constant Int := First + Second;
> begin
> Put(Int'Image(Next) & ", ");
> Fib(Second,Next);
> end Fib;
> 
> begin
> Put("1, 1, ");
> Fib(1,1);
> end Tail;
> 
> Does anyone know why GNAT (or any other compiler) does not always
> reuse stack frames for all subroutines that appear right before a
> return?

Could it be related to the parameter passing rules in Ada?  And would
it really be faster?

If the arguments for Fib are not copied in the order you have written
them in the procedure specification and a single stack frame is
reused, the program will not behave correctly.  But alternating
between two stack frames should be safe enough.

Jacob
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