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From: Vinzent 'Gadget' Hoefler <nntp-2004-04@t-domaingrabbing.de>
Newsgroups: comp.lang.ada
Subject: Re: Pass by reference
Date: Fri, 09 Apr 2004 21:33:20 +0200
Organization: JeLlyFish software
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Date: 2004-04-09T21:33:20+02:00
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Pat Rogers wrote:

>If you made the type volatile that would also force by-reference.

Are you sure? I'm just looking at the assembly ouput of GNAT for a
procedure that takes a volatile type (memory mapped register) and it
does't seem so. Even worse, it optimizes the sequence of assignment
which I need for this register. If you have any idea how can I avoid
using access types to make it right, I'd be glad to hear it.

Here's the procedure:

|   -- Write ------------------------------------------------------------
|   -- writes  a timer value (low byte/high byte in succession) to the --
|   -- memory mapped timer port given by its access                    --
|   ---------------------------------------------------------------------
|   procedure Write (Timer_Reg : in out Registers.Reg_8;
|                    Value     : in     i8254_Timer) is
|      Internal_Value : Interfaces.Unsigned_16 :=3D To_U16 (Value);
|   begin
|      Timer_Reg :=3D Lo_Byte (Internal_Value);
|      Timer_Reg :=3D Hi_Byte (Internal_Value);
|   end Write;

the assembly output is:

|_timer__write:
|        pushl %ebp
|        movl %esp,%ebp
|        movl 12(%ebp),%eax
|        movl %eax,%edx
|        cmpl $65536,%eax
|        jne L21
|        xorl %edx,%edx
|L21:
|        movl %edx,%eax
|        movl %ebp,%esp
|        shrw $8,%ax
|        popl %ebp
|        ret

which is not what it should be.

If I change it back to use access types:

|   procedure Write (Timer_Reg : in Registers.Reg_8_Access;
|                    Value     : in i8254_Timer) is
|      Internal_Value : Interfaces.Unsigned_16 :=3D To_U16 (Value);
|   begin
|      Timer_Reg.all :=3D Lo_Byte (Internal_Value);
|      Timer_Reg.all :=3D Hi_Byte (Internal_Value);
|   end Write;

everything is fine:

|_timer__write:
|        pushl %ebp
|        movl %esp,%ebp
|        movl 12(%ebp),%eax
|        movl 8(%ebp),%ecx
|        movl %eax,%edx
|        cmpl $65536,%eax
|        jne L21
|        xorl %edx,%edx
|L21:
|        movl %edx,%eax
|        movb %dl,(%ecx)
|        shrw $8,%ax
|        movl %ebp,%esp
|        movb %al,(%ecx)
|        popl %ebp
|        ret


Vinzent.