Re: Large arrays (again), problem case for GNAT

["Randy Brukardt" <randy@rrsoftware.com>]

"Dr. Adrian Wrigley" <amtw@linuxchip.demon.co.uk.uk.uk> wrote in message
news:pan.2005.04.13.12.46.52.518681@linuxchip.demon.co.uk.uk.uk...
> Hi guys!
>
> A month or two back I hinted that I had been having problems
> the GNAT code accessing large data structures.  It seemed that
> incorrect code was silently being generated.  Since I didn't
> have a test case available, no progress could be made.

I tried to make an all-Ada version to test on Janus/Ada. But there is not
enough memory on my machine to run it (at least, I think that's why I'm
getting Storage_Error; the Janus/Ada runtime bypasses the system heap and
goes directly to the Windows virtual memory manager for large blocks of
memory, so the failure should indicate that Windows couldn't allocate it.)

I made three changes to the program:
    Added a type Big_Int to replace Integer (better portability, especially
for a compiler
       with Integer'Size = 16);
    Replaced the GNAT-specific "Address_Image" with
      System.Storage_Elements.Integer_Address (clunky, but pure Ada);
    Replaced the call to Malloc with New.

The latter was done simply because end-running the compiler's memory
management is likely to cause bugs. And ours, at least, is designed to work
with any reasonable-sized chunk of memory. Finally, if I hadn't changed
that, it would not have worked, as Janus/Ada does not allow random memory to
be used with a pool-specific type. (Really, the access type should be a
general access type if you are going to end-run Ada's heap management.)

The test works with small enough numbers to run on my machine with 384Meg of
RAM (the top value being about 130_000_000). If you were our customer, I'd
try to figure out why the Storage_Error is being raised. Probably the answer
really is to use a supported compiler; you do really get what you pay for.

                       Randy.


 with Text_IO;
with System.Storage_Elements; -- Added, RLB.

procedure TestBig2 is

-- Test devised by Adrian Wrigley, amtw@linuxchip.demon.co.uk.uk.uk (one uk
is e
-- I disclaim all rights to this code.  Do what you wish with it!

-- This test case illustrates a problem with code
-- compiled on GNAT 3.15p for x86 under Linux

-- It shows that the 'Address of elements of large
-- arrays can be silently incorrect

-- Redid by Randy Brukardt to work with Janus/Ada; added an appropriate
Integer
-- type, use New to allocate, and removed GNAT specific stuff.

-- Notes:
--
-- This test works if the word "constant" is removed
--
-- The test also works if the value is less than 200_000_000
-- Indexing element also works with computed index
   Size : constant := 210_000_000;
   type Big_Int is range 0 .. Size;

   type Big_T is array (Big_Int range 1 .. Size) of aliased Float;

   type Big_A is access Big_T;

   Big : Big_A;

--   function GetSomeMemory (X : Integer) return Big_A;
--   pragma Import (C, GetSomeMemory, "malloc");

begin

--   Big := GetSomeMemory (Size*4 + 16); -- This seems to work OK for now
   Big := new Big_T;

   Text_IO.Put_Line ("First element attribute " & Big_Int'Image
(Big_T'First));
   Text_IO.Put_Line ("Last element attribute  " & Big_Int'Image
(Big_T'Last));

   Text_IO.Put_Line ("Address of first element is " &
        System.Storage_Elements.Integer_Address'Image(
           System.Storage_Elements.To_Integer(Big (Big_T'First)'Address)));
   Text_IO.Put_Line ("Address of last element is  " &
        System.Storage_Elements.Integer_Address'Image(
           System.Storage_Elements.To_Integer(Big (Big_T'Last)'Address)));
   Text_IO.Put_Line ("(Last element should be at higher address than
first)");


   declare
      J : Big_Int := Size;
   begin
      Text_IO.Put_Line ("Address of last element is " &
        System.Storage_Elements.Integer_Address'Image(
           System.Storage_Elements.To_Integer(Big (J)'Address)));
   end;

   Text_IO.Put_Line ("Writing to all elements in loop");
   for I in Big_T'Range loop
      Big (I) := 0.0;
   end loop;

   Text_IO.Put_Line ("Writing to 'Last element");
--raised STORAGE_ERROR : stack overflow (or erroneous memory access):
   Big (Big_T'Last) := 0.0; -- Fails!!!!!

end TestBig2;