[RFC] Debuginfo: Add default value support for subprogram arguments

[David Blaikie]

On Thu, Aug 21, 2014 at 9:36 AM, Frédéric Riss <friss at apple.com> wrote:
>
>> On Aug 16, 2014, at 12:10 AM, David Blaikie <dblaikie at gmail.com> wrote:
>>
>> On Thu, Aug 14, 2014 at 10:22 AM, David Blaikie <dblaikie at gmail.com> wrote:
>>> On Thu, Aug 14, 2014 at 9:17 AM, Frédéric Riss <friss at apple.com> wrote:
>>>> Hi,
>>>>
>>>> This is an initial RFC regarding adding support for emitting default value
>>>> debug information for function/method arguments. Although I’d appreciate
>>>> initial review comments (patch attached), this is more about getting your
>>>> feedback on the approach, as it diverges a bit from the standard.
>>>
>>> Cool! I hadn't realized there was DWARF support for this (& even GCC
>>> doesn't seem to implement it - though I don't have a GCC ToT build
>>> around to test, just 4.8) - always annoying having to type "dump(0)"
>>> instead of just "dump()" when dumping some LLVM types that have
>>> optional "dump to this specified stream" support.
>>>
>>>> The Dwarf standard reads:
>>>>
>>>> A DW_AT_default_value attribute for a formal parameter entry. The value of
>>>> this attribute is a reference to the debugging information entry for a
>>>> variable or subroutine, or the value may be a constant. If the attribute
>>>> form is of class reference, the default value of the parameter is the value
>>>> of the referenced variable (which may be constant) or the value returned by
>>>> the referenced subroutine; a reference value of 0 means that no default
>>>> value has been specified. If the value is of class constant, that constant
>>>> is interpreted as a default value of the type of the formal parameter.
>>>>
>>>> For a constant form there is no way to express the absence of a default
>>>> value.
>>>>
>>>> As you can see, this text limits us to emitting information about default
>>>> values being:
>>>> - simple constants (DW_FORM_data*)
>>>> - global variable (DW_FORM_ref*)
>>>> - calls to functions without arguments (DW_FORM_ref*)
>>>>
>>>> I tried to go a bit further and implement a more complete solution that
>>>> adds:
>>>> - any link time constant (DW_FORM_block*): in this form, the block of data
>>>> represents the default value as it would be stored in the target memory
>>>
>>> How does this differ from the "simple constant" case above? What sort
>>> of constants can't be expressed with DW_FORM_data* that can be
>>> expressed with DW_FORM_block*?
>>>
>>>> - arbitrary complex expressions (DW_FORM_str*): when no other of the above
>>>> forms fits, this dumps the default value expression as textual form. The
>>>> debugger can then evaluate this expression in the context of the declaration
>>>> if it has this capacity
>>>
>>> Sounds plausible.
>>>
>>>> First of all, would these kind of extensions be welcome? If you agree that
>>>> they are worthwhile, I’ll submit a request to augment the standard wording.
>>>
>>> I suspect the better place to start is on the DWARF committee, but I
>>> agree there's some chicken-and-egg problems there & it's helpful to
>>> have implementation buy-in, etc. (& I haven't participated in the
>>> DWARF committee - so I might be totally off there)
>>>
>>>> The attached patches seems to get at least the basics right, but 2 issues
>>>> are still bothering me:
>>>>
>>>> - Forward declaration:
>>>>
>>>> int foo();
>>>>
>>>> void bar(int i = foo()) {return i;}
>>>>
>>>> int foo() {return 1;}
>>>>
>>>>
>>>> the default value references a forward decl of the function. When emitting
>>>> the debuginfo metadata in CGDebugInfo.cpp:EmitDeclare(), the init function
>>>> decl isn’t yet in the DeclCache.
>>>
>>> This ones my fault - I implemented DeclCache as a half-hearted way to
>>> support using declarations in C++, but you've found one of the
>>> limitations:
>>>
>>> namespace X {
>>> void foo();
>>> void bar() {
>>> }
>>> }
>>>
>>> using X::foo; // no debug info for this
>>> using X::bar; // debug info for this
>>>
>>> Currently clang has no smarts for producing debug info for function
>>> declarations on demand (and replacing them with definitions when a
>>> definition appears later) - except for the special case of
>>> declarations of member functions.
>>>
>>> If we add that functionality (to lazily create declarations, replace
>>> them with definitions when the latter follows the former, etc) we can
>>> fix/improve the using declaration support, and use it for features
>>> like this.
>>>
>>> Probably the right way to approach it is as I described (and means
>>> that the improvements can be made now/orthogonally to whenever the
>>> default argument support goes in) - by refactoring the clang support
>>> for emitting declarations (of global variables and non-member
>>> functions) under the goal of improving using declaration support, then
>>> it'll be there/ready/improved when the default argument support needs
>>> it.
>>
>> I don't think Clang has infrastructure for emitting declarations of
>> global variables into the debug info metadata either. If/when you
>> plumb this through, it should be most of the work to fix
>> http://llvm.org/bugs/show_bug.cgi?id=14501 as well.
>
> I wanted to start looking into this again, and I’m pretty sure the bug you’re referring to isn’t the one intended. So you need to either provide me the right reference, or explain me how this is related :-) (Maybe it’s the 14500, then one about labels ?)

Indeed I've been juggling a few bug numbers of late. I thought we had
a particular bug on the limitations of DW_TAG_imported_declaration
support, but it's possible the only lead is my (perhaps rather vague)
comment when I closed http://llvm.org/bugs/show_bug.cgi?id=14606

Essentially here's an example:

$ cat using.cpp
namespace ns {
  int var;
  extern int var_decl;
  void func() {
  }
  void func_decl();
}

using ns::var;
using ns::var_decl;
using ns::func;
using ns::func_decl;
blaikie at blaikie-linux:/tmp/dbginfo$ clang++-tot using.cpp -g -c &&
llvm-dwarfdump-tot -debug-dump=info using.o | grep DW_TAG_imported
0x00000071:   DW_TAG_imported_declaration [7]
0x00000078:   DW_TAG_imported_declaration [7]
blaikie at blaikie-linux:/tmp/dbginfo$ g++-4.8 using.cpp -g -c &&
llvm-dwarfdump-tot -debug-dump=info using.o | grep DW_TAG_imported
0x00000078:   DW_TAG_imported_declaration [8]
0x0000007f:   DW_TAG_imported_declaration [8]
0x00000086:   DW_TAG_imported_declaration [8]
0x0000008d:   DW_TAG_imported_declaration [8]

We should have four DW_TAG_imported_declarations, but we only have
two. (for the things where the definition preceeded the using decl) If
we could emit declarations for functions and variables, we could then
use that to correctly emit the debug info here.

(this is also a pain for debug info size optimization - ideally if
these using declarations aren't used, we might want to avoid emitting
them - otherwise it makes debug info for a TU that, say, just includes
iostream and does nothing with it, a bit bigger - but we already have
that problem with my initial implementation... & I'm not sure how much
% it matters in practice - just something that might be worth cehcking
along the way)

- David