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Date: Sun, 15 May 2016 12:42:48 -0700 (PDT)
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Subject: Re: Proof of array initialization in SPARK 2014
From: rieachus@comcast.net
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Xref: news.eternal-september.org comp.lang.ada:30416
Date: 2016-05-15T12:42:48-07:00
List-Id: <comp.lang.ada>

On Thursday, May 12, 2016 at 1:01:19 PM UTC+1, Dmitrij Novikov wrote:

> I don't know how to cope with complex array initializations in SPARK.

What is wrong with?

   function Initialize return My_Array
   is
     A : My_Array(1..8) :=3D (others =3D> True);=20
   begin=20
     A(A'Last) :=3D False;=20
     return A;
   end Initialize;

This code should be faster (if that matters), but has the advantage that A =
is initialized before beginning the loop (that went away).

You could also assign a constant array to A.  You don't want A to be a cons=
tant, so you do need an assignment.  Or you could initialize to True, True,=
 True, True, True, True, True, False.  But I assume that this example is a =
simplified version of a more complex problem.

Will the compiler generate a loop to initialize A, or build a constant into=
 the code?  Depends on a lot of things, so what you get is what you get. ;-=
)  In particular, if A fits in a register and is returned on the stack you =
will probably get fast initialization.  It is often worth defining a bit ar=
ray type:

         subtype Index is Integer range 0 .. 31;
         type Bit_Array is array (Index range <>) of Boolean;

Especially when working with hardware registers.  (Change 31 to 15 for some=
 older hardware, and maybe 63 for some newest hardware.)

It may seem that I worry more about speed than correctness, but in hard rea=
l time (like radar) when you are bit twiddling registers inside interrupts,=
 you want code that eliminates non-stack memory references.