Implement support for .debug_macinfo

[fitzgen]

Section 6.3 of the standard.

Some languages, such as C and C++, provide a way to replace text in the source program with macros defined either in the source file itself, or in another file included by the source file. Because these macros are not themselves defined in the target language, it is difficult to represent their definitions using the standard language constructs of DWARF. The debugging information therefore reflects the state of the source after the macro definition has been expanded, rather than as the programmer wrote it. The macro information table provides a way of preserving the original source in the debugging information.

As described in Section 3.1.1, the macro information for a given compilation unit is represented in the .debug_macinfo section of an object file. The macro information for each compilation unit is represented as a series of “macinfo” entries. Each macinfo entry consists of a “type code” and up to two additional operands. The series of entries for a given compilation unit ends with an entry containing a type code of 0.

[tromey]

There's a GCC extension, .debug_macro, that encodes the information in a more compact way. It's been accepted into DWARF 5: http://www.dwarfstd.org/ShowIssue.php?issue=110722.1

It's reasonably easy to read both (or eventually, all 3) forms using the same reader; this is what gdb does.