Hello all,
I am a compiler engineer at AMD, working on tools for debugging
online-compiled
programs. The problem I am attempting to solve was brought up previously
in the
DWARF Standard issue 161018.1 titled "DWARF-embedded source for
online-compiled
programs", and is the result of runtimes like OpenCL doing online
compilation
in an environment where it is not desireable (or even feasible) to write
sources to disk. In these cases, it would be useful to support embedding
the
source directly in the resulting DWARF. I would like to propose a
similar
solution to the one outlined in the above issue, but without structural
changes
to the specification.
====
Add two new optional fields to the file_names prologue of the line
table.
Section 6.2.4.1:
Add two bullets after "5. DW_LNCT_MD5"
6. DW_LNCT_has_source
DW_LNCT_has_source indicates that the value is a boolean which
affects the
interpretation of an accompanying DW_LNCT_source value. When present
there
must be an accompanying DW_LNCT_source value. When true, consumers
may use
the embedded source instead of attempting to discover the source on
disk.
When false, consumers will ignore the DW_LNCT_source value. This
code point
is always paired with a flag form (e.g. DW_FORM_flag or
DW_FORM_flag_present).
7. DW_LNCT_source
DW_LNCT_source indicates that the value is a null-terminated string
which
is the original source text of the file. When present there must be
an
accompanying DW_LNCT_has_source value. The string will contain the
UTF-8
encoded source text with '\n' line endings. When the accompanying
DW_LNCT_has_source value is false, the value of DW_LNCT_source will
be the
empty string. This code point is always paired with a string form
(e.g.
DW_FORM_string, DW_FORM_line_strp, DW_FORM_strp).
New type codes can be allocated for them in a backwards-compatible way,
or
codes for these new content types can be added in the range of
[DW_LNCT_lo_user, DW_LNCT_hi_user] to avoid changing the spec itself.
Table 7.27:
Add DW_LNCT_has_source 0x6
Add DW_LNCT_source 0x7
Any DWARFv5 consumer which is unaware of this extension would continue
to
operate as before, ignoring the new fields. Any consumer which is aware
of the
extension would know to check DW_LNCT_has_source for each file_name
entry in
order to determine whether the embedded source field (DW_LNCT_source)
contains
the source text of the corresponding file.
====
My team and I believe this simplifies the design by removing the need
for
changes to the compile unit sections, and by avoiding the addition of
multiple
file_name_entry_formats in a single program, all without sacrificing any
information. We have a preliminary implementation in LLVM/Clang, which
supports
embedding source (clang -gdwarf-5 -gembed-source) and inspecting it via
llvm-dwarfdump and llvm-objdump (with the -source flag). The patches are
available at https://reviews.llvm.org/D42765 (LLVM) and
https://reviews.llvm.org/D42766 (Clang).
I would like any and all feedback on the design, and want to see about
the
possibility of adding the new content type codes outside of the "user"
range
(i.e. adding new entries for them in Table 7.27) in the next version of
the
specification.
Regards,
Scott Linder
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