[Issue 9800] Numerous issues with DWARF debug output

Sun Nov 17 05:24:20 PST 2013

https://d.puremagic.com/issues/show_bug.cgi?id=9800

--- Comment #2 from Iain Buclaw <ibuclaw at ubuntu.com> 2013-11-17 05:24:14 PST ---
The long response:

1. DW_TAG_enumeration_type is only valid for integral types, and the gcc
backend assumes this is the case (you could blame C/C++).  GDC implements this
for *named* enums only (this is an improvement made in the last 3 months).

Code:

module debug;
enum A : int { a = 42 }
---

Printing values in GDB:

(gdb) p (int)'debug.A.a'
$1 = 42
(gdb) p ('debug.A') 42
$2 = debug.A.a

2. Anonymous enums and manifest constants have their symbolic values removed by
the frontend.

Code:

enum Foo = 42;
auto foo = Foo;

enum { Bar = 1 }
auto bar = Bar;
---

What is sent to the glue/codegen by the frontend:

auto foo = 42;
auto bar = 1;

3. GDC emits DW_TAG_array_type - though I guess this is subjective, as a fixed
array of eg: int[4] is treated like a struct with 4 int fields.

4. DW_TAG_string_type is only valid for Fortran.  I also see no reason for it
either.  D strings are already specially handled in gdb so that all dynamic
arrays are formatted correctly in the printer command.

5. We could do this, and although DW_TAG_shared_type is known to GCC, it is not
emitted anywhere (time to send patches to GCC-proper :o)

6. In D, static != private.  Static decls can be used outside the declared
module, ergo DW_AT_external is set.

7. DW_TAG_module is only valid for Fortran/Modula-2, but I'd argue that this is
a bug.  It would be nice to represent statics as being part of a module, my
hope would be that adding support would mean that we'd no longer have to use
'quotations' or the full mangled name to get/set global decls in the debugger. 
Though it is worth noting that DWARF2 does not have DW_TAG_module.

8. If you are referring to what I think you are, this will be fixed when #7 is
addressed.

module a
int foo;
---
module b;
import a;
int bar;
---

Halting the program at a breakpoint in module b:

# This works if current module scope is 'module b'.
(gdb) print bar

# 'foo' is outside our current module scope at this breakpoint.
(gdb) print a.foo

9. Yeah - that's a bug - I fixed it just now (because it was quick and easy).
You also forgot about DW_TAG_interface_type too, which I've also added in for
GDC.

10. Another bug, the frontend really needs to record this information like it
does for VarDeclarations fields (there should be a FuncDeclarations methods).

11. GDC already does this, but it is worth pointing out that DWARF3 does not
have DW_AT_linkage_name.

12. GDC sets input/source location on all decls, types, statements,
expressions, etc...  Of course, any artificial or builtin symbol gets a magical
builtin location that is tied to DW_TAG_decl_file, but it is only in special
circumstances (eg: you are a D compiler developer) that you would be setting
breakpoints in artificial functions.

13. GDC emits reference types (classes and ref parameters/foreach declarations)
as DW_TAG_reference_type - if you spot any cases where this is wrong, do let
this be known with code examples.

14. C type names are all aliases defined in druntime (see #17).

15. According to the ABI page for DMD, this is as per design.  For GDC, we emit
these as two field structs, represented as string, immutable(wchar)[], and
immutable(dchar)[] accordingly.

*BUG* - D types whose internal layout is a structure are represented as
DW_TAG_structure_type.  This includes D arrays, delegates, and associative
arrays.

16. Anonymous unions/structs are flattened by the front-end when adding fields
to AggregateDeclaration::fields - I have raised this before as an
implementation issue.  I have ran into ICEs in GCC backend in the past because
it did not expect two fields to be at the same location in a struct, but I
haven't managed to reproduce it at least in the last 2 years (something must be
hiding the bug somewhere).

struct A
{
  int a;
  union { int b, c }
}
---

What is sent to the glue/codegen by the frontend:

AggregateDeclaration::fields
 => dim  = 3;
 => data = [a, b, c];
---

What would be more ideal information to be sent:

AggregateDeclaration::fields
 => dim  = 2;
 => data = [a, __anonymous];
   => __anonymous::fields
    => dim = 2;
    => data = [b, c];

17. Likewise to #2 - the frontend removes all symbolic values to aliases.  If
you require symbolic debugging information, use typedef (which is, er,
deprecated).

alias int MyInt;
MyInt foo = 42;
---

What is sent to the glue/codegen by the frontend:

int foo = 42;

18. GDC does not do this, but then again GDC does not reverse all non-variadic
arguments in its ABI.

19. DW_AT_pure is only valid for Fortran.  GCC currently does not emit it (even
in fortran programs as far as I can see).  The ability to warn about calling
impure functions from the debugger will need to be implemented separately in
gdb.

20. DW_AT_elemental is only valid for Fortran, and does not describe @property
at all.  We would need a new flag such as DW_AT_property.

21. DW_AT_main_subprogram is only valid for Fortran.  This could be seen as a
reasonable suggestion.

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