[Dwarf-Discuss] What to do with Pascal properties?
Michael Eager
eager@eagercon.com
Fri May 28 15:06:20 GMT 2021
In 5/28/21 7:11 AM, Joost van der Sluis via Dwarf-Discuss wrote:
> Hi all,
>
> I am one of the Free Pascal developers and also work on a debugger,
> aimed towards Free Pascal.
>
> Now in Pascal there are 'properties'. Maybe you know these from c# which
> has something alike. Basically a property is an alias in a structure
> that links to other members of the same structure for reading, writing
> and/or storage-information.
>
> Example:
>
> type
> ? TMyClass=class
> ? private
> ??? FProp: Integer;
> ??? FPropIsStored: Boolean;
> ? protected
> ??? function GetProp: Integer;
> ??? function GetItem(const Index: Integer): string;
> ? public
> ??? property IndividualItem[Index: Integer]: string read GetItem;
> ? published
> ??? property Prop: Integer read GetProp stored FPropIsStored;
> ??? property OtherProp: Integer read FProp write FProp;
> ??? property Item2: string index 2 read GetItem;
> ? end;
>
> var
> ? MyClass = TMyClass;
>
> Reading MyClass.Prop effectively calls GetProp.
>
> MyClass.Prop is read-only, and during streaming the information in
> FPropIsStored is being used.
>
> MyClass.OtherProp is read/write, and is more or less an alias for the
> private FProp field.
>
> MyClass.IndividualItem[6] is accessible like it is an array. And Item2
> has a fixed index.
>
> I want to encode this propery into the Dwarf debug-information. At this
> moment we only generate debug-information for cases similar to
> MyClass.OtherProp by duplicating the debug-information of FProp with
> another visibility-class.
>
> I've added the following attributes:
> ? DW_AT_FPC_property_read (0x3230)
> ? DW_AT_FPC_property_write (0x3231)
> ? DW_AT_FPC_property_stored (0x3232)
>
> And then those attributes contain a link to the corresponding
> DW_TAG_members. This to keep the debug-information as compact as
> possible. Furthermore I've added the tag DW_TAG_FPC_property (0x4230) or
> else other debuggers may be confused when they encounter a DW_TAG_member
> with only one or more of these specific fpc-attributes.
>
> I'll also have to add something like DW_AT_FPC_property_index to store
> the value of a fixed index.
>
> I have two questions:
> 1. Am I on the good track, or did I miss something?
Generally, it looks like you are heading in the right direction. I'd
have to look into the details much more to make any suggestions.
> 2. Maybe implementation specific, but maybe someone might have an idea:
> to generate the proposed debug-info, I need to add a label to every
> DW_TAG_member. Using global labels inside these structures though, may
> lead to all kinds of troubles, for example when a linker regards these
> labels as the start of a new section. Or on Darwin/macOS. Calculating
> the location of these tags is in principle possible I think, but
> difficult. Any tips here?
The general DWARF philosophy is to minimize relocations where ever
possible. It isn't clear whether you are pointing the TAG
with a relocatable address or not. Use offsets from the start of the
section where ever possible.
If the question is about local vs global symbols, many assemblers
will treat symbols which start with a period (e.g., .start) as
local and not export them to the object file symbol table.
>
> This is a dwarfdump of the class-definition above with my current
> additions:
>
> < 1><0x00000123>??? DW_TAG_class_type
> ????????????????????? DW_AT_name????????????????? TMyClass
> ????????????????????? DW_AT_byte_size???????????? 0x00000010
> < 2><0x0000012e>????? DW_TAG_inheritance
> ??????????????????????? DW_AT_accessibility???????? DW_ACCESS_public
> ??????????????????????? DW_AT_data_member_location? DW_OP_plus_uconst 0
> ??????????????????????? DW_AT_type????????????????? <GOFF=0x00005122>
> < 2><0x00000137>????? DW_TAG_member
> ??????????????????????? DW_AT_name????????????????? FProp
> ??????????????????????? DW_AT_data_member_location? DW_OP_plus_uconst 8
> ??????????????????????? DW_AT_accessibility???????? DW_ACCESS_private
> ??????????????????????? DW_AT_type????????????????? <GOFF=0x000000f3>
> < 2><0x00000146>????? DW_TAG_member
> ??????????????????????? DW_AT_name????????????????? FPropIsStored
> ??????????????????????? DW_AT_data_member_location? DW_OP_plus_uconst 12
> ??????????????????????? DW_AT_accessibility???????? DW_ACCESS_private
> ??????????????????????? DW_AT_type????????????????? <GOFF=0x000057a7>
> < 2><0x0000015d>????? <Unknown TAG value 0x4230>
> ??????????????????????? DW_AT_name????????????????? IndividualItem
> ??????????????????????? <Unknown AT value 0x3230>?? <GOFF=0x0000020c>
> ??????????????????????? DW_AT_type????????????????? <GOFF=0x000057c4>
> < 2><0x00000175>????? <Unknown TAG value 0x4230>
> ??????????????????????? DW_AT_name????????????????? Prop
> ??????????????????????? <Unknown AT value 0x3230>?? <GOFF=0x000001ad>
> ??????????????????????? <Unknown AT value 0x3232>?? <GOFF=0x00000146>
> ??????????????????????? DW_AT_type????????????????? <GOFF=0x000000f3>
> < 2><0x00000187>????? <Unknown TAG value 0x4230>
> ??????????????????????? DW_AT_name????????????????? OtherProp
> ??????????????????????? <Unknown AT value 0x3230>?? <GOFF=0x00000137>
> ??????????????????????? <Unknown AT value 0x3231>?? <GOFF=0x00000137>
> ??????????????????????? DW_AT_type????????????????? <GOFF=0x000000f3>
> < 2><0x0000019e>????? <Unknown TAG value 0x4230>
> ??????????????????????? DW_AT_name????????????????? Item2
> ??????????????????????? <Unknown AT value 0x3230>?? <GOFF=0x0000020c>
> ??????????????????????? DW_AT_type????????????????? <GOFF=0x000057c4>
> < 2><0x000001ad>????? DW_TAG_subprogram
> ??????????????????????? DW_AT_name????????????????? GetProp
> ??????????????????????? DW_AT_prototyped??????????? yes(1)
> ??????????????????????? DW_AT_calling_convention
> DW_CC_GNU_borland_fastcall_i386
> ??????????????????????? DW_AT_external????????????? yes(1)
> ??????????????????????? DW_AT_accessibility???????? DW_ACCESS_protected
> ??????????????????????? DW_AT_type????????????????? <GOFF=0x000000f3>
> ??????????????????????? DW_AT_low_pc??????????????? 0x00404840
> ??????????????????????? DW_AT_high_pc?????????????? 0x00404867
> < 3><0x000001ce>??????? DW_TAG_formal_parameter
> ????????????????????????? DW_AT_name????????????????? this
> ????????????????????????? DW_AT_location????????????? 0x7678 DW_OP_breg6-8
> ????????????????????????? DW_AT_artificial??????????? yes(1)
> ????????????????????????? DW_AT_type????????????????? <GOFF=0x00000110>
> < 3><0x000001dc>??????? DW_TAG_variable
> ????????????????????????? DW_AT_name????????????????? $result
> ????????????????????????? DW_AT_location????????????? 0x7674
> DW_OP_breg6-12
> ????????????????????????? DW_AT_type????????????????? <GOFF=0x000000f3>
> < 3><0x000001ec>??????? DW_TAG_variable
> ????????????????????????? DW_AT_name????????????????? GETPROP
> ????????????????????????? DW_AT_location????????????? 0x7674
> DW_OP_breg6-12
> ????????????????????????? DW_AT_type????????????????? <GOFF=0x000000f3>
> < 3><0x000001fc>??????? DW_TAG_variable
> ????????????????????????? DW_AT_name????????????????? RESULT
> ????????????????????????? DW_AT_location????????????? 0x7674
> DW_OP_breg6-12
> ????????????????????????? DW_AT_type????????????????? <GOFF=0x000000f3>
> < 2><0x0000020c>????? DW_TAG_subprogram
> ??????????????????????? DW_AT_name????????????????? GetItem
> ??????????????????????? DW_AT_prototyped??????????? yes(1)
> ??????????????????????? DW_AT_calling_convention
> DW_CC_GNU_borland_fastcall_i386
> ??????????????????????? DW_AT_external????????????? yes(1)
> ??????????????????????? DW_AT_accessibility???????? DW_ACCESS_protected
> ??????????????????????? DW_AT_type????????????????? <GOFF=0x000057c4>
> ??????????????????????? DW_AT_low_pc??????????????? 0x00404870
> ??????????????????????? DW_AT_high_pc?????????????? 0x00404889
> < 3><0x0000022d>??????? DW_TAG_formal_parameter
> ????????????????????????? DW_AT_name????????????????? this
> ????????????????????????? DW_AT_location????????????? 0x7670
> DW_OP_breg6-16
> ????????????????????????? DW_AT_artificial??????????? yes(1)
> ????????????????????????? DW_AT_type????????????????? <GOFF=0x00000110>
> < 3><0x0000023b>??????? DW_TAG_variable
> ????????????????????????? DW_AT_name????????????????? $result
> ????????????????????????? DW_AT_location????????????? 0x766806
> DW_OP_breg6-24 DW_OP_deref
> ????????????????????????? DW_AT_type????????????????? <GOFF=0x000057c4>
> < 3><0x0000024c>??????? DW_TAG_formal_parameter
> ????????????????????????? DW_AT_name????????????????? Index
> ????????????????????????? DW_AT_location????????????? 0x7678 DW_OP_breg6-8
> ????????????????????????? DW_AT_type????????????????? <GOFF=0x000000f3>
> < 3><0x0000025a>??????? DW_TAG_variable
> ????????????????????????? DW_AT_name????????????????? GETITEM
> ????????????????????????? DW_AT_location????????????? 0x766806
> DW_OP_breg6-24 DW_OP_deref
> ????????????????????????? DW_AT_type????????????????? <GOFF=0x000057c4>
> < 3><0x0000026b>??????? DW_TAG_variable
> ????????????????????????? DW_AT_name????????????????? RESULT
> ????????????????????????? DW_AT_location????????????? 0x766806
> DW_OP_breg6-24 DW_OP_deref
> ????????????????????????? DW_AT_type????????????????? <GOFF=0x000057c4>
>
>
>
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Michael Eager
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