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digitalmars.D.debugger - DWARF output, type of char[]?

reply Cristian Vlasceanu <cristian zerobugs.org> writes:
What is the DWARF type of char[], as output by the DMD backend?

I have compiled this sample:

import std.stdio;

int main()
{
    char[] greet = "Hello";

    writefln(greet);
    return 0;
} 

Then ran 
readelf -w hello

As shown in this snippet, the type of greet is unsigned long long (entry at
offset <c2>):

<1><c2>: Abbrev Number: 2 (DW_TAG_base_type)
     DW_AT_name        : unsigned long long
     DW_AT_byte_size   : 8
     DW_AT_encoding    : 7  (unsigned)
 <1><d8>: Abbrev Number: 4 (DW_TAG_subprogram)
     DW_AT_sibling     : <104>
     DW_AT_name        : _Dmain
     DW_AT_decl_file   : 1
     DW_AT_decl_line   : 3
     DW_AT_type        : <bb>
     DW_AT_low_pc      : 0x804a358
     DW_AT_high_pc     : 0x804a383
     DW_AT_frame_base  : 1 byte block: 55   (DW_OP_reg5)
 <2><f5>: Abbrev Number: 3 (DW_TAG_variable)
     DW_AT_name        : greet
     DW_AT_type        : <c2>
     DW_AT_location    : 2 byte block: 91 78    (DW_OP_fbreg: -8)
Apr 02 2007
parent reply BCS <ao pathlink.com> writes:
Reply to Cristian,


 What is the DWARF type of char[], as output by the DMD backend?


I've wondered at this kind of thing to. Why isn't it some sort of ptr/length 
pair struct? Name it something remotely readable and it will be a ton better 
than unsigned long long. The expression that gets the content of a char[] 
as something gdb can read is several inches long.
Apr 02 2007
parent reply Cristian Vlasceanu <cristian zerobugs.org> writes:
BCS Wrote:


 Reply to Cristian,
 

 What is the DWARF type of char[], as output by the DMD backend?


 
 The expression that gets the content of a char[] 
 as something gdb can read is several inches long.
 


I am not sure that I understand this? Is there is a way of transforming the
unsigned long long into something meaningful?

Zero treats all types as if the source were C/C++.

I am working on allowing the user to customize the way variables are being
displayed (via Python scripts). I am doing it mainly for being able to display
C++/STL containers in a sane way, but I think that with a little work it may do
the trick for D types as well.

   Cristian
Apr 02 2007
parent reply BCS <BCS pathlink.com> writes:
Cristian Vlasceanu wrote:

 BCS Wrote:
 
 

Reply to Cristian,



What is the DWARF type of char[], as output by the DMD backend?


The expression that gets the content of a char[] 
as something gdb can read is several inches long.


 
 I am not sure that I understand this? Is there is a way of transforming the
unsigned long long into something meaningful?
 


yes

given T[] arr;

the contents are:  *(cast(T*)(cast(void*)(&arr)[1])
the length is:  *cast(size_t*)(&arr)

and that's if I remember correctly. It's ugly and messy and a total pain 
in the whatever. But it works.

However if an array of type T[] were to claim to be

struct
{
	T* ptr;
	size_t length;
}

or something close to that, then you could access it almost as normal.



 Zero treats all types as if the source were C/C++.
 
 I am working on allowing the user to customize the way variables are being
displayed (via Python scripts). I am doing it mainly for being able to display
C++/STL containers in a sane way, but I think that with a little work it may do
the trick for D types as well.
 
    Cristian
Apr 03 2007
parent reply Cristian Vlasceanu <cristian zero-bugs.com> writes:
BCS wrote:

 Cristian Vlasceanu wrote:

 BCS Wrote:



 Reply to Cristian,



 What is the DWARF type of char[], as output by the DMD backend?


 The expression that gets the content of a char[] as something gdb can 
 read is several inches long.


 I am not sure that I understand this? Is there is a way of 
 transforming the unsigned long long into something meaningful?


 
 yes
 
 given T[] arr;
 
 the contents are:  *(cast(T*)(cast(void*)(&arr)[1])
 the length is:  *cast(size_t*)(&arr)
 
 and that's if I remember correctly. It's ugly and messy and a total pain 
 in the whatever. But it works.
 
 However if an array of type T[] were to claim to be
 
 struct
 {
     T* ptr;
     size_t length;
 }
 
 or something close to that, then you could access it almost as normal.
 


I do not know enough D, but is there "unsigned long long" an otherwise 
legal D type? Because if (as I supsect) it isn't, and it can be 
unequivocally determined that it stands for  a char[] whenever the 
translation unit's language is D, I could easily make it work in Zero. 
Out of the box.
Apr 03 2007
parent reply Frits van Bommel <fvbommel REMwOVExCAPSs.nl> writes:
Cristian Vlasceanu wrote:

 I do not know enough D, but is there "unsigned long long" an otherwise 
 legal D type? Because if (as I supsect) it isn't, and it can be 
 unequivocally determined that it stands for  a char[] whenever the 
 translation unit's language is D, I could easily make it work in Zero. 
 Out of the box.


You may want to check what type "ulong" is marked as. Since that's a 
64-bit unsigned integer (like unsigned long long typically is) it may 
also be marked as "unsigned long long"...
Apr 03 2007
parent reply BCS <BCS pathlink.com> writes:
Frits van Bommel wrote:

 Cristian Vlasceanu wrote:
 

 I do not know enough D, but is there "unsigned long long" an otherwise 
 legal D type? Because if (as I supsect) it isn't, and it can be 
 unequivocally determined that it stands for  a char[] whenever the 
 translation unit's language is D, I could easily make it work in Zero. 
 Out of the box.


 
 
 You may want to check what type "ulong" is marked as. Since that's a 
 64-bit unsigned integer (like unsigned long long typically is) it may 
 also be marked as "unsigned long long"...


What is needed is for T[] to be marked as something special. Then you 
could treat it as described. FWIW D has it's own DWARF "language number" 
so we should be able to add new types. Then the debugger could treat it 
as said struct.

A short term hack could be to have the compiler do the magic and mark it 
as some sort of auto generated struct type.

Disclaimer: I know slightly more than 0 about how DWARF works under the 
hood.
Apr 03 2007
parent reply Cristian Vlasceanu <cristian zerobugs.org> writes:
BCS Wrote:


 Frits van Bommel wrote:

 Cristian Vlasceanu wrote:
 

 I do not know enough D, but is there "unsigned long long" an otherwise 
 legal D type? Because if (as I supsect) it isn't, and it can be 
 unequivocally determined that it stands for  a char[] whenever the 
 translation unit's language is D, I could easily make it work in Zero. 
 Out of the box.


 
 
 You may want to check what type "ulong" is marked as. Since that's a 
 64-bit unsigned integer (like unsigned long long typically is) it may 
 also be marked as "unsigned long long"...


 
 What is needed is for T[] to be marked as something special. Then you 
 could treat it as described. FWIW D has it's own DWARF "language number" 
 so we should be able to add new types. Then the debugger could treat it 
 as said struct.
 
 A short term hack could be to have the compiler do the magic and mark it 
 as some sort of auto generated struct type.
 
 Disclaimer: I know slightly more than 0 about how DWARF works under the 
 hood.



I have verified this, with 1.010, ulong and char[] are indistinguishable in the
DWARF.

My personal preference is to have it represented as a struct.
Apr 03 2007
parent reply Frits van Bommel <fvbommel REMwOVExCAPSs.nl> writes:
Cristian Vlasceanu wrote:

 I have verified this, with 1.010, ulong and char[] are indistinguishable in
the DWARF.
 
 My personal preference is to have it represented as a struct.


A struct doesn't tell you how much of the data at .ptr is valid.
IMHO, for '-g' (native-D debugging info) it should be a special type. 
(Assuming DWARF supports language-specific types, is that the case?) 
That way, D-aware debuggers can for example pretty-print char[] strings 
as regular strings instead of (size_t, char*) pairs.
For '-gc' (pretends-to-be-C debugging info) a struct should probably be 
used though.
Apr 03 2007
next sibling parent BCS <ao pathlink.com> writes:
Reply to Frits,


 Cristian Vlasceanu wrote:
 

 I have verified this, with 1.010, ulong and char[] are
 indistinguishable in the DWARF.
 
 My personal preference is to have it represented as a struct.
 


 A struct doesn't tell you how much of the data at .ptr is valid.
 IMHO, for '-g' (native-D debugging info) it should be a special type.
 (Assuming DWARF supports language-specific types, is that the case?)


I think so based on this: http://dwarfstd.org/Dwarf3.pdf


 That way, D-aware debuggers can for example pretty-print char[]
 strings
 as regular strings instead of (size_t, char*) pairs.
 For '-gc' (pretends-to-be-C debugging info) a struct should probably
 be used though.


That's about what I'm thinking.
Apr 03 2007
prev sibling parent reply Cristian Vlasceanu <cristian zerobugs.org> writes:
Frits van Bommel Wrote:


 Cristian Vlasceanu wrote:

 I have verified this, with 1.010, ulong and char[] are indistinguishable in
the DWARF.
 
 My personal preference is to have it represented as a struct.


 
 A struct doesn't tell you how much of the data at .ptr is valid.
 IMHO, for '-g' (native-D debugging info) it should be a special type. 
 (Assuming DWARF supports language-specific types, is that the case?) 
 That way, D-aware debuggers can for example pretty-print char[] strings 
 as regular strings instead of (size_t, char*) pairs.
 For '-gc' (pretends-to-be-C debugging info) a struct should probably be 
 used though.



How about adding a "DWARF vendor extension":

add a new tag, DW_TAG_array.
Array "classes" will have a DW_TAG_array corresponding entry in the debug info,
each with children entries such as:
    DW_AT_type // DWARF std, points to the element type
    ...                 // TBD    


array instances will be represented by entries that have children of the
following types:
    DW_AT_capacity    // D extension
    DW_AT_size          // DWARF standard
    DW_AT_value        // address of 1st elem, DWARF standard
    DW_AT_type          // DWARF std, would point to an entry of DW_TAG_array
type

thoughts?
Apr 03 2007
parent BCS <ao pathlink.com> writes:
Reply to Cristian,


 Frits van Bommel Wrote:
 

 Cristian Vlasceanu wrote:
 

 I have verified this, with 1.010, ulong and char[] are
 indistinguishable in the DWARF.
 
 My personal preference is to have it represented as a struct.
 


 A struct doesn't tell you how much of the data at .ptr is valid.
 IMHO, for '-g' (native-D debugging info) it should be a special type.
 (Assuming DWARF supports language-specific types, is that the case?)
 That way, D-aware debuggers can for example pretty-print char[]
 strings
 as regular strings instead of (size_t, char*) pairs.
 For '-gc' (pretends-to-be-C debugging info) a struct should probably
 be
 used though.


 How about adding a "DWARF vendor extension":
 
 add a new tag, DW_TAG_array.
 Array "classes" will have a DW_TAG_array corresponding entry in the
 debug info,
 each with children entries such as:
 DW_AT_type // DWARF std, points to the element type
 ...                 // TBD
 array instances will be represented by entries that have children of
 the following types:
 DW_AT_capacity    // D extension
 DW_AT_size          // DWARF standard
 DW_AT_value        // address of 1st elem, DWARF standard
 DW_AT_type          // DWARF std, would point to an entry of
 DW_TAG_array type
 thoughts?
 


Sounds good, but I'm no DWARF expert.

Walter?
Apr 03 2007