gdb/dwarf: write offset to parent entry for DW_IDX_parent
New in v2:
- add doc
- fix computation of offset in entry pool
Due to a mistake in the DWARF 5 spec, the way that GDB interprets
DW_IDX_parent when generating and reading .debug_names is not correct.
In Section 6.1.1.2, the parent index entry attribute is described as:
Parent debugging information entry, a reference to the index entry for
the parent. This is represented as the offset of the entry relative to
the start of the entry pool.
But in Table 6.1, DW_IDX_parent is described as:
Index of name table entry for parent
These two contradict each other. The former is the correct one and the
latter is an unfortunate leftover from an earlier version of the
proposal, according to [1]. It does make sense, because pointing to a
name table entry is ambiguous, while poiting to an index entry directly
is not. Unfortunately, GDB implemented pointing to a name table entry.
Changes on the writer side:
- For each written pool entry, remember the offset within the pool.
- Change the DW_IDX_parent form to DW_FORM_data4.
Using DW_FORM_udata isn't an option, because we don't know the actual
value when doing the first pass of writing the pool (see next point),
so we wouldn't know how many bytes to reserve, if we used a
variable-size encoding.
Using a fixed 4 bytes encoding would be an issue if the entry pool
was larger than 4 GiB, but that seems unlikely.
Note that clang uses DW_FORM_ref4 for this, but I'm not sure it is
appropriate, since forms of the reference class are specified as
referring "to one of the debugging information entries that describe
the program". Since we're not referring to a DIE, I decided to stay
with a form of the "constant" class. I think that readers will be
able to understand either way.
- Write a dummy 4 byte number when writing the pool, then patch those
values later. This is needed because parents can appear before their
children in the pool (there's no way to ensure that parents always
appear before their children), so we might now know at first what
value to put in.
- Add a `write_uint` method to `class data_buf` to support that use
case of patching a value in the middle of the data buffer.
- Simplify the type of `m_name_to_value_set`, we no longer need to
track the index at which a name will be written at.
- Produce a new augmentation string, "GDB3", to be able to distinguish
"old" and "new" indexes. It would be possible for a reader to
distinguish the two semantics of DW_IDX_parent using the form.
However, current versions of GDB don't do that, so they would be
confused trying to read a new index. I think it is preferable to use
a new augmentation string so that they will reject a new index
instead.
Changes on the reader side:
- Track the GDB augmentation version, in addition to whether the
augmentation string indicates the index was produced by GDB.
- When reading index entries, maintain a "pool offset" -> "cooked index
entry" mapping, to be able to find parents by pool offset.
- When resolving parents, keep the existing behavior of finding parents
by name table index if the augmentation string is "GDB2. Otherwise,
look up parents by pool offset. This assumes that .debug_names from
other producers (if/when we add support for reading them) use pool
offsets for DW_IDX_parent. This at least what clang does.
- Simplify augmentation string comparison a bit by using array views.
Update the "Extensions to ‘.debug_names’" section of the documentation
to reflect the new augmentation string version.
Tested by:
- manually producing executables with "GDB2" and "GDB3" .debug_names
sections and reading them.
- running the testsuite with the cc-with-debug-names board
projects / thirdparty / binutils-gdb.git / commit
