Re: Any way of persuading GNAT/GCC to implement a true overlay and not a pointer?

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Sat, 1 Apr 2006 19:19:48 +0100, Doobs wrote:

> "Doobs" <doobs@doobs.com> wrote in message 
> news:SdmdndA2Z9jbI7PZnZ2dnUVZ8qWdnZ2d@pipex.net...
>>
>> "Florian Weimer" <fw@deneb.enyo.de> wrote in message 
>> news:87odzl5ilt.fsf@mid.deneb.enyo.de...
>>>> I was under the impression that code of the following form :
>>>>
>>>> X : <Some Type>;
>>>> Y : <Some Type>;
>>>> for Y'Address use X'Address;
>>>>
>>>> would result in an overlay in the resulting code.
>>>
>>> Could you show some more code?  Probably initialization is causing
>>> your problems, which can be fixed with a pragma Import.
>>
>> The following is an example of the problem:
>>
>> package TestPackage
>> is
>>
>>   type MyRecordType is
>>   record
>>      element1 : Positive;
>>      element2 : Positive;
>>   end record;
>>
>>   type MyArrayType is array(1..2) of Positive;
>>
>>   myX : MyRecordType;
>>   pragma volatile(myX);
>>   myY : MyArrayType;
>>   pragma volatile(myY);
>>   for myY'Address use myX'Address;
>>
>>
>> end TestPackage;
>>
>> The following fragments of the resulting MAP file (GCC 3.4.2 mingw32) show 
>> the problem...
>>
>> .data          0x00472b00       0x10 ./testpackage.o
>>                0x00472b00                testpackage__myy
>> .data          0x00472b10       0x10 ./nextpackage.o
>>
>> <..... later in the file...>
>>
>> COMMON         0x004a2770       0x20 ./testpackage.o
>>                0x004a2770                testpackage__myx
>>                0x004a2780                testpackage_E
>>
>>
>> Clearly myX and myY do NOT share the same address.  I am also puzzled as 
>> to why the variables have been put in differen memory sections...
>>
> Note that including a pragma import did NOT solve the problem - the code 
> looked like this
> 
>    myX : MyRecordType;
>    pragma volatile(myX);
>    myY : MyArrayType;
>    pragma volatile(myY);
>    for myY'Address use myX'Address;
>    pragma Import (Ada, myY);
> 
> But the map file exhibited the same problem as before.

Hmm, with gnat 3.15p for Windows (no any pragmas) it gives:

    0x00414848                testpackage__myy
    0x0041484c                testpackage__myx

which seems to be fine, assuming MyY's dope. Both

   MyX.element1'Address = MyY (1)'Address
   MyX.element2'Address = MyY (2)'Address

evaluate true.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de