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From: Georg Bauhaus <sb463ba@l1-hrz.uni-duisburg.de>
Newsgroups: comp.lang.ada
Subject: Re: Improving Ada's image - Was: 7E7 Flight Controls Electronics
Date: Fri, 18 Jun 2004 14:44:13 +0000 (UTC)
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Brian May <bam@snoopy.apana.org.au> wrote:
:>>>>> "Georg" == Georg Bauhaus <sb463ba@l1-hrz.uni-duisburg.de> writes:
: 
:    Georg> (And add a note that new String'("bla") need not mean dynamic heap
:    Georg> allocation, for example at library level.)
: 
: Any chance you could please expand on this statement? How else could
: it be implemented? What does GNAT do?

An assembly listing for the following program shows no occurence
of an allocator for Lib.fixed, and one occurence for dyn in main.
Here is the library part.

package Lib is

   type String_Access is access constant String;

   fixed: constant String_Access := new String'("AZAZAZ");

end Lib;

; ************* Initialized part of the global segment :

    000000 lib.fixed        DD  __lcl.00000015
    000004 __lcl.00000015   DD  00000001
                            DD  00000006
                            DB  41
                            DB  5A
                            DB  41
                            DB  5A
                            DB  41
                            DB  5A
; ********* End of segment, Total size = 18

To me, this looks like the String "AZAZAZ", with bounds 1 and 6.
The listing has been produced by ObjectAda 722, GNAT appears to behave
similarly.

with Lib;

procedure main is
   dyn: Lib.String_Access;
   n: Positive := 4;
   --  to prevent the compiler from "knowing" that lib.fixed(2..4) can be
   --  determined in advance, see the following statement.
begin
   dyn := new String'(Lib.fixed(2..n));
end main;

For this, I get MOVements for dyn, and n, then a small calculation,
then a CALL rts_allocate.
Then some instructions referring to __lcl.00000015 from above the
lib.fixed character bytes, then a CALL _rts_block_move.

So my guess is that the allocation happens for dyn, but not for fixed.
(I don't read ASM fluently though, and neither Power PC assembler, so
take this with a grain of salt.)

IIRC, this has been discussed before.


-- Georg