What code is generated for Ada Array of Records loop?

Question

For example for:

type PERSONCV is
   record
      name: String ( 1..4 );
      age: Integer;
      cvtext: String ( 1..2000 );
   end record;

N: constant := 40000;
persons : array ( 1..N ) of PERSONCV;

function jobcandidate return Boolean is
   iscandidate: Boolean := False;
begin
   for p of persons loop   -- what code is generated for this?
      if p.age >= 18 then
         iscandidate := true;
         exit;
      end if;
   end loop;
   return iscandidate;
end;

In C/C++ the loop part would typically be:

PERSONCV * p; // address pointer
int k = 0;
while ( k < N )
   {
   p = &persons [ k ]; // pointer to k'th record
   if ( p->age >= 18 )...  
   ...
   k++ ;
   }

I have read that Ada uses Value semantics for records. Does the Ada loop above copy the k'th record to loop variable p? e.g. like this is in C/C++ :

PERSONCV p; // object/variable
int k = 0;
while ( k < N )
   {
   memcpy ( &p, &persons [ k ], sizeof ( PERSONCV ) ); // copies k'th elem 
   if ( p.age >= 18 )...  
   ...
   k++ ;
   }

Simon Wright Simon Wright · Accepted Answer · 2017-01-19T18:09:27

Assuming you are using GNAT, there are two avenues of investigation.

The switch -gnatG will regenerate an Ada-like representation of what the front end of the compiler is going to pass to the back end (before any optimisations). In this case, I see

   function resander__jobcandidate return boolean is
      iscandidate : boolean := false;
      L_1 : label
   begin
      L_1 : for C8b in 1 .. 40000 loop
         p : resander__personcv renames resander__persons (C8b);
         if p.age >= 18 then
            iscandidate := true;
            exit;
         end if;
      end loop L_1;
      return iscandidate;
   end resander__jobcandidate;

so the question is, how does renames get translated? Given that the record size is 2008 bytes, the chances of the compiler generating a copy is pretty much zero.

The second investigatory approach is to keep the assembly code that the compiler normally emits to the assembler and then deletes, using the switch -S. This confirms that the code generated is like your first C++ version (for macOS).

As an interesting sidelight, Ada 2012 allows an alternate implementation of jobcandidate:

   function jobcandidate2 return Boolean is
     (for some p of persons => p.age >= 18);

which generates identical code.

What code is generated for Ada Array of Records loop?

3 Answers