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Date: Sat, 04 Dec 2010 09:17:08 -0500
From: "Peter C. Chapin" <PChapin@vtc.vsc.edu>
Organization: Vermont Technical College
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Subject: Re: Large arrays passed to arithmetic operators overflows GNAT stack
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Date: 2010-12-04T09:17:08-05:00
List-Id: <comp.lang.ada>

On 2010-12-04 01:32, Jerry wrote:

> with
>     Ada.Numerics.Long_Real_Arrays;
> use
>     Ada.Numerics.Long_Real_Arrays;
> procedure array_test is
>     type Real_Vector_Access    is access Real_Vector;
>     N : Integer := 1_048_130;
>     t_Ptr : Real_Vector_Access := new Real_Vector(0 .. N);
>       t : Real_Vector renames t_Ptr.all;

I don't think you need this renaming. If you do t_Ptr'Range or t_Ptr(i)
the compiler will forward those operations directly to the object.

>     t_Diff_Ptr : Real_Vector_Access := new Real_Vector(0 .. N - 1);
>       t_Diff : Real_Vector renames t_Diff_Ptr.all;
> begin
>     for i in t'range loop
>         t(i) := 1.0;
>     end loop;
>     t_Diff := t(1 .. N) - t(0 .. N - 1);

I believe a temporary object (on the stack) needs to be created here to
hold the result of the difference before it gets assigned to t_Diff. So
despite the fact that the result is ultimately stored on the heap it
ends up passing through the stack on its way. If I'm correct it's not
the subtraction operator itself that is causing the problem but rather
what is happening with the result.

In short "-" is returning the array by value and not an access to the
array. The compiler has to figure out what to do with that value before
assigning it to t_Diff and it is using a stack temporary.

Peter