Re: Addressing in Object Ada v/s GNAT (2013) showing Vast Differences

[Lucas Redding <lucas.redding@gmail.com>]

On Monday, September 21, 2015 at 12:57:09 PM UTC+1, Jacob Sparre Andersen wrote:
> Lucas Redding wrote:
> 
> Your examples don't compile.  Please show us the actual code.
> 
> Greetings,
> 
> Jacob
> -- 
> "Very small. Go to sleep" - monster (not drooling)

Hi Jacob

thanks for that. I am avoiding having to flood this with tons of code. So here is a compiled example.

package interface_p
is

   type sixteen_bits is range 0..(2**16)-1 ; 
   type fourteen_bits is range 0..(2**14)-1; 
   type ten_bits is range 0..(2**10)-1; 
   type padding_bits is range 0..(2**8)-1 ; 

   type rec is record 
      a: fourteen_bits ; 
      b: ten_bits ; 
      padding : padding_bits ;
   end record ; 

   for rec use 
   record
      a at 0 range 0..13 ; 
      b at 0 range 14..23 ; 
      padding at 0 range 24..31;
   end record ; 
   for rec'size use 32; 
   for rec'alignment use 1; 

   subtype local_fourteen_bits is sixteen_bits range 0 .. (2 ** 14)-1;
   subtype local_ten_bits is sixteen_bits range 0 .. (2 ** 10)-1; 
   type local_rec is record
      a : local_fourteen_bits ; 
      b : local_ten_bits ; 
   end record; 

   for local_rec use 
   record
      a at 0 range 0 .. 13 ; 
      b at 2 range 0 .. 9 ; 
   end record; 

   for local_rec'size use 32 ; 
   for local_rec'Alignment use 1 ; 
 
   VAR_REC : rec := (a => 1, b => 2, padding => 0 ) ; 
   
   -- I explicitly assign here but in actual application it maps onto a 32 bit 
   -- record in memory. I have deliberately changed in order to avoid adding 
   -- tons of irrelevant code.
   
   
   function READ_RECORD
   return local_rec ;

end interface_p ;

package body INTERFACE_P
is
   function READ_RECORD
   return local_rec 
   is
      VAR_LOCAL_REC : local_rec ; 
   begin 
   
      -- We would like to copy the information once to a byte aligned 
      -- record to hopefully speed up 
      -- access to this item because it is accessed internally a large 
      -- number of times. 

      VAR_LOCAL_REC.a := local_fourteen_bits(VAR_REC.a) ;
      VAR_LOCAL_REC.b := local_ten_bits(VAR_REC.b) ;
      
      -- The problem is when I copy each record component to the local copies 
      -- (including the typecast) 
      -- my var_local_rec.a takes on a 16 bit value and thus includes the bit values 
      -- from var_rec.b ;       
      
      -- So if var_rec.a is 1 and var_rec.b is 2 I end up with the following in 
      -- var_local_rec in GNAT Ada : 

      -- var_local_rec.a = 32769 ; 
      -- var_local_rec.b = 2 ; 

      -- Clearly this is wrong. And although I have set all checks in the compiler I 
      -- don't get an overflow error which is what I would expect. 

      --      In Object Ada I get : 

      --      var_local_rec.a = 1 ; 
      --      var_local_rec.b = 2; 
      return VAR_LOCAl_REC ;
      
   end READ_RECORD ;

end INTERFACE_P ;