[fpc-devel] TypeInfo RTTI / possible inconsistency

[Sven Barth]

Am 18.05.2021 um 15:24 schrieb Martin Frb via fpc-devel:
> I was looking at TypeInfo (based on 3.2.2rc)
>
> line 632  unit TypInfo
>>             tkMethod:
>>               (MethodKind : TMethodKind;
>>                ParamCount : Byte;
>>                ParamList : array[0..1023] of Char
>>              {in reality ParamList is a array[1..ParamCount] of:
>>                   record
>>                     Flags : TParamFlags;
>>                     ParamName : ShortString;
>>                     TypeName : ShortString;
>>                   end;
>>               followed by
>>                   ResultType : ShortString     // for mkFunction, 
>> mkClassFunction only
>>                   ResultTypeRef : PPTypeInfo;  // for mkFunction, 
>> mkClassFunction only
>>                   CC : TCallConv;
>>                   ParamTypeRefs : array[1..ParamCount] of PPTypeInfo;}
>>               );
>
> To find out about alignment (for any target which needs alignment), I 
> looked through other code:
>
> fpc_3.2.2\source\tests\webtbs\tw12038.pp
>    line 194    procedure DisplayDetails(Informations : TPropInfo; 
> const expectedresult: ansistring);
> appears to access the data (except for the ParamTypeRefs array)
> The testcase does not seem to make any adjustments for alignment.

It does at line 285.

>
> fpc_3.2.2\source\packages\rtl-objpas\src\inc\rtti.pp
>    line 3341    TRttiMethodType.GetParameters
> also appears to access the same typeinfo.
> It makes alignments in 3 different places
>    ptr := AlignTParamFlags(ptr);                // foreach   Flags : 
> TParamFlags;   in the ParamList
>    ptr := AlignToPtr(PByte(ptr) + ptr^ + SizeOf(Byte));    // 
> ResultTypeRef
>    paramtypes := PPPTypeInfo(AlignTypeData(ptr));       // ParamTypeRefs
>
> Which is correct?
> Or do they access different structures?

They are for different parts of the structure, cause they have different 
alignment requirements (e.g. the param names and flags are aligned to 
the TParamFlags (which is 2 Byte) while the ResultType is aligned to a 
pointer. Though right now I don't know why the last one uses 
AlignTypeData which would align to the QWord fields instead of Pointer 
ones...

> A few other notes:
> FPC_REQUIRES_PROPER_ALIGNMENT
> does not seem to be listed on 
> https://www.freepascal.org/docs-html/prog/progap7.html

There are a few from TOption.TargetOptions (in options.pas) that are not 
yet documented.

> TypInfo line 1461
> in Procedure GetPropInfos(TypeInfo : PTypeInfo;PropList : PPropList);
> TP:=aligntoptr(PPropInfo(aligntoptr((Pointer(@TD^.UnitName)+Length(TD^.UnitName)+1)))); 
>
> Why call aligntoptr twice?
> The TypeCast in between does not change the alignment. (all the math 
> is done within the innermost call)

That one is already fixed in trunk.

Regards,
Sven