Re: Strict aliasing, is it OK?

[Niklas Holsti <niklas.holsti@tidorum.invalid>]

On 17-07-04 21:30 , Victor Porton wrote:
> Niklas Holsti wrote:
>
>> On 17-07-04 20:49 , 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.
>>
>> To me the GNAT UG
>> (https://gcc.gnu.org/onlinedocs/gcc-7.1.0/gnat_ugn.pdf#f3) seems very
>> clear.
>>
>>> The only thing I understood for sure is that sometimes strict
>>> aliasing may cause a trouble.
>>
>> The question is whether your program uses Unchecked_Conversion (or some
>> other unchecked tricks) to make a variable of type "access SomeType"
>> point to an object that is not of the type "SomeType".
>>
>> Your instances of Unchecked_Conversion make GNAT suspect that your
>> program might do such things. GNAT is warning you that your program
>> would then violate the assumptions that underlie some of GNAT's code
>> optimizations when the "strict aliasing" option is on, which means that
>> the optimized code might not behave in the way you expect. The User
>> Guide has an example of such unexpected behaviour.
>>
>>> 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.
>>
>> Just ask yourself if, in your program, an "access SomeType" always
>> points to "SomeType". If so, you should be ok. If not, or if you are
>> unsure, it is safer to use the pragma or compiler option that GNAT
>> suggests; your program is then more likely to work as you want it to.
>
> I posted because I need help.

I tried to help.

> Please you answer the question:
>
> if, in your program, an "access SomeType" always points to "SomeType"?

It is impossible to answer based on the code you show, because it 
depends on what your program _does_ with the Unchecked_Conversion instances.

Note that your imported "C" functions that return access values can also 
lead to violation of the strict aliasing assumption, but again depending 
on how they are used.

> Well, I do understand an example of wrong behavior in UG.

Good.

> but I don't know
> whether my code is also an example of wrong behavior.

The code you show does not have wrong behaviour, because it is just 
function declarations. Wrong behaviour (with respect to the strict 
aliasing rule) _may_ happen when you use those functions, but it depends 
on _how_ you use them.

As I said, if you cannot answer the question, just follow GNAT's suggestion.

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
       .      @       .