[Ada] Fix problem in assigning global variables with address clauses

[Arnaud Charlet <charlet at adacore dot com>]

The code for array assignment assumed that two library level objects
cannot overlap, but that's wrong if either of them has an address
clause.

Consider the following example:

with System.Storage_Elements;
procedure ACopy (Source, Target : System.Address;
                  Byte_Count : Natural) is

     package Sse renames System.Storage_Elements;

     Count : constant Sse.Storage_Count
               := Sse.Storage_Count (Byte_Count);

     subtype Chunk is Sse.Storage_Array (1 .. Count);

     Source_Chunk : Chunk;
     pragma Import (Ada, Source_Chunk);
     for Source_Chunk'Address use Source;

     Target_Chunk : Chunk;
     pragma Import (Ada, Target_Chunk);
     for Target_Chunk'Address use Target;

     use type System.Address;
     use type System.Storage_Elements.Storage_Offset;

     -- It is assumed that address wrapping is not an issue.
     -- We are not copying bytes either from or to
     -- a region that includes (or even abuts) address 0.

begin
        Target_Chunk := Source_Chunk;
end ACopy;

If this is compiled with -S -gnatdt -O2, the tree output should not
have Forwards_OK or Backwards_OK referenced anywhere and the asm file
should show that a memmove is being generated rather than a memcpy.

Tested on x86_64-pc-linux-gnu, committed on trunk

2009-04-17  Robert Dewar  <dewar@adacore.com>

	* exp_ch5.adb (Expand_Assign_Array): Do not set Forwards_OK and
	Backwards_OK if either operand has an address clause.

Attachment: difs
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