[Dwarf-Discuss] DWARF and source text embedding

[John DelSignore]

Hi Scott,

Question: What does the DW_TAG_compile_unit look like for an embedded source file? For example, what does the DW_AT_name and DW_AT_comp_dir look like?

Cheers, John D.


On 01/31/18 17:05, scott at scottlinder.com wrote:
> 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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