Re: Strict aliasing, is it OK?

[Victor Porton <porton@narod.ru>]

Victor Porton wrote:

> I've started an ambitious open source project:
> https://en.wikiversity.org/wiki/Automatic_transformation_of_XML_namespaces
> 
> I am going to implement it in Ada.
> 
> Note the file
> 
> https://github.com/vporton/redland-bindings/blob/ada2012/ada/src/rdf-raptor-parser.adb
> 
> Here is an extract from my code:
> 
>    type My_Dummy_Access is access constant RDF.Auxiliary.Dummy_Record;
> 
>    function C_Raptor_Parser_Get_Description (Parser: Handle_Type) return
>    My_Dummy_Access
>       with Import, Convention=>C,
>       External_Name=>"raptor_parser_get_description";
> 
>    function Get_Description (Parser: Parser_Type) return
>    RDF.Raptor.Syntaxes.Syntax_Description_Type is
>       function Conv is new Ada.Unchecked_Conversion(My_Dummy_Access,
>       RDF.Raptor.Syntaxes.Syntax_Description_Type);
>    begin
>       return Conv( C_Raptor_Parser_Get_Description(Get_Handle(Parser)) );
>    end;
> 
> 
> When I compile this:
> 
> $ (cd src && gnatgcc -gnatf -c -fPIC -g -O2 -gnat2012
> rdf-raptor-parser.adb) rdf-raptor-parser.adb:132:07: warning: possible
> aliasing problem for type "Syntax_Description_Type"
> rdf-raptor-parser.adb:132:07: warning: use -fno-strict-aliasing switch for
> references rdf-raptor-parser.adb:132:07: warning: or use "pragma
> No_Strict_Aliasing (Syntax_Description_Type);"
> 
> I am not sure if it is OK to insert pragma No_Strict_Aliasing into my
> code.
> 
> GNAT Users Guide is cryptic. The only thing I understood for sure is that
> sometimes strict aliasing may cause a trouble. I am not sure if this is a
> case with my code. My code looks quite innocent, so I feel my idea will
> not be broken by the compiler, but I am not sure. Please help.

Note that "handles" in my code a "C" conventions accesses to C records.

-- 
Victor Porton - http://portonvictor.org