[Dwarf-Discuss] DW_AT_default_value reference to "subroutine"

Ron Brender ron.brender at gmail.com
Tue Sep 16 14:07:38 PDT 2014


Looking back through old email and DWARF drafts, I find:

The constant/variable null reference implies no default verbiage existed in
V2 (as others have noted). Al Grant (by way of David Anderson) first raised
the issue of why a reference to an otherwise unnecessary constant should be
needed to handle the simple case of a constant in about Sept 2004. This
eventually became Issue 040909.1 (Matthew Gretton-Dann was the champion).
The allowance of constant forms first shows up in DW3 draft 9.5 in July
2005.

The matter of calling a function was discussed only once that I can find.
One suggestion was to allow a form string to specify text that the debugger
was to evaluate (including making a function call). This achieved no
traction (issues of scope and closures abound). Someone else noted that in
one implementation the compiler creates an anonymous function that just
contains the full call, with arguments as needed. This was thought "too
complicated". And there the matter was dropped.

The summary appears to be: Back in 2005, the reference to subroutine
problem was noted and discussed with no action taken. And here we are nine
years later...

Ron,
Your editor and historian

On Tue, Sep 16, 2014 at 3:32 PM, Robinson, Paul <
Paul_Robinson at playstation.sony.com> wrote:

> The DW_AT_default_value attribute on a formal_parameter DIE can be used to
> express a default argument for the parameter; C++ allows this, for example.
>     int foo(int a, int b = 2);
>     int bar(int x = foo(3)); // default for x is foo(3,2)
> The description of the attribute in DWARF 4 (section 4.1 item 9, p.70)
> says it can be a reference to "a variable or subroutine" as well as a
> constant. What it means for a variable or constant is pretty obvious; but
> it's not really clear about subroutines.  DWARF 4 says it's "the value
> returned by the referenced subroutine."
>
> Is this "subroutine" actually a DW_TAG_subprogram DIE?  That's not
> expressive enough for the full glory of C++ default arguments.  It's
> not even expressive enough to allow the debugger to call a single
> arbitrary compiled function, as there's no way to express what
> parameters to pass to the compiled function.
>
> Is this "subroutine" actually a DWARF procedure?  That suggests it
> should be evaluated like DW_OP_call_ref, which allows executing an
> arbitrary DWARF expression, but a DWARF expression can't actually
> describe calling a compiled function.
>
> Any old-timers out there who can shed light on the intent of this
> terminology?  I see similar verbiage all the way back to DWARF 2.
> Thanks,
> --paulr
>
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