Suppress warnings for unchecked conversion

Question

I have this piece of code :

with Ada.Unchecked
private package MyPackage is
    function UC_Bool_To_U8 is new Ada.Unchecked_Conversion (Source => Boolean, Target => T_U8);
end MyPackage;

Where T_U8 is :

type T_U8 is range 0 .. 2**7;

Function UC_Bool_To_U8 is working but I have warnings on compilation :

warning: types for unchecked conversion have different sizes

warning: size of "Boolean" is 1, size of "T_U8" is 8

warning: source will be extended with 7 high order zero bits

How can I suppress theses warnings ?

Answer 1

Warnings can be suppressed by using a pragma as shown in this Ada Gem blog post and the following example:

main.adb

with Ada.Unchecked_Conversion;

procedure Main is

   --  Using a modular type instead of an integer type. Result is the same.
   type T_U8 is mod 2**7;    
   
   pragma Warnings (Off, "types for unchecked conversion have different sizes");   
   
   function UC_Bool_To_U8 is new Ada.Unchecked_Conversion
     (Source => Boolean, Target => T_U8);
   
   pragma Warnings (On, "types for unchecked conversion have different sizes");
   
begin
   null;
end Main;

However, please also consider to not use Unchecked_Conversion when converting a boolean type to some integer or modular type. The compiler will completely optimize away a simple if-statement as can be seen in the Compiler Explorer:

input to Compiler Explorer

pragma Source_File_Name (To_U8, Body_File_Name => "example.adb");

with Interfaces.C.Extensions;

function To_U8 (B : Boolean) return Interfaces.C.Extensions.Unsigned_8 with Inline is
begin
    return (if B then 1 else 0);
end To_U8;

ouput of Compiler Explorer (using compiler switch -O1 or -O2)

_ada_to_u8:
        mov     eax, edi  # b, tmp86
        ret  

Answer 2

Why do you need unchecked conversion there? Why not

   function UC_Bool_To_U8 (Value : Boolean) return T_U8
     is (Boolean'Pos (Value));

Creating unchecked conversion for types with different sizes is unsafe/non-portable, so I think, compiler warning is fair. I suggest you to fix the code instead of deactivating the warning.

Answer 3

While it is easy to just hide a warning, I suggest you different way.

Create a numeric type with the same size, convert to it and then convert to the type you need:

type T_U1 is range 0 .. 1 with Size => 1;
function UC_Bool_To_U1 (Value : Boolean) return T_U1 is new Ada.Unchecked_Conversion
 (Source => Boolean, Target => T_U1);
function UC_Bool_To_U8 (Value : Boolean) return T_U8 is (
    T_U8 (UC_Bool_To_U1 (Value))
);