Issue 141117.1: Arbitrary expressions as formal parameter default values

Author: Adrian Prantl
Champion: Adrian Prantl
Date submitted: 2014-11-17
Date revised: 2022-10-21
Date closed: 2022-10-21
Type: Enhancement
Status: Accepted with modifications
DWARF Version: 6

Section 4, pg

Background for discussion

Languages such as C++ allow for arbitrary expressions to be provided as default arguments. In particular, C++11 allows any initializer expression as a default argument as the following example from the language specification illustrates::

int a = 1;
int f(int);
int g(int x = f(a));
void h() {
  a = 2;
  int a = 3;
  g(); // g(f(::a)) --> g(f(2))

from (8.3.6) <>

It would be useful to encode default arguments in DWARF because developers may want to evaluate expressions in the context of the program. Currently, it is possible to encode constant-value default arguments in DWARF. While at first it may seem tempting to try and encode default arguments using DWARF expressions, this is actually not possible, because DWARF expressions cannot call into user code; i.e., it would be impossible to express the above default argument of ::f(::a) as a DWARF expression.

We therefore suggest encoding complex default arguments as strings containing a fragment in the programming language specified by the compile unit. A consumer that is sophisticated enough to make use of a default expression should also be able to evaluate a source language fragment.


Section 4.1 Data Object Entries


A DW_AT_default_value attribute for a formal parameter entry. The value of this attribute may be a constant, or a reference to the debugging information entry for a variable, or a reference to a debugging information entry containing a DWARF procedure.


, or a string containing a source language fragment.
If the attribute form is of class string, that string is interpreted as an expression in the source language defined by the compilation unit's DW_AT_language attribute that is to be evaluated according to the rules defined by the source language.

Table 7.5: Attribute encodings

DW_AT_default_value    0x1e   constant, reference, flag, string

Appendix D

[add new section]

Default value examples

The default expression for parameter "x" in the C++ function declaration in Figure [1] can be described in DWARF as illustrated in Figure [2].

Figure 1.

void g (int x = 13;
        int y = f());

Figure 2.

    DW_AT_name ("g")

       DW_AT_name ("x")
       DW_AT_type (reference to type "int")
       DW_AT_default_value@DW_FORM_sdata (13)

       DW_AT_name ("y")
       DW_AT_type (reference to type "int")
       DW_AT_default_value@DW_FORM_string ("f()")

Notes for Figure 2:

  1. This figure explicitly shows the form used by certain attributes (indicated by a trailing @DW_FORM_xxx) when it is critical, while the form is most often left implicit.
  2. The string value for 'y' is three characters in length and does not include any quotes. (The quotes are an artifact of the presumed dumper tool that created this interpretation.)
  3. The default value for x could also be encoded as DW_AT_default_value@DW_FORM_string("13"); however. this is generally a less convenient form for consumers to process.

A string FORM in DW_AT_default_value always represents a source code fragment, even in languages that have a native string type. For example, the default string parameter of the Ada function in Figure [3] is encoded in DWARF as a string containing the Ada string literal including the quotation marks (see Figure [4]).

Figure 3.

procedure s (x : string := "abc";
             y : string := "abcd"+10) is
end s;

Figure 4.

[NOTE TO THE EDITOR: Ideally we can use two different kinds of quotes here to distinguish the literal double-quotes in the default value from the ones that dwarfdump adds.] [NOTE FROM EDITOR: Agreed. LaTeX can surely help here...]

   DW_AT_name (“s”)
      DW_AT_name (“x”)
      DW_AT_type (reference to type “string”)
      DW_AT_default_value@DW_FORM_data4 (0x61626364)    \\ Big-endian
      DW_AT_name (“y”)
      DW_AT_type (reference to type “string”)
      DW_AT_default_value@DW_FORM_string (“"abcd"+10”)

2014-11-19 -- Deferred to version 6.
2022-09-19 -- Updated examples.
2022-10-21 -- Updated examples.
2022-10-21 -- Accepted with modifications (non-normative text TBD)