[Dwarf-Discuss] DWARF and source text embedding

[Michael Eager]

On 02/01/2018 12:01 PM, scott at scottlinder.com wrote:
> Hi Paul,
> 
> My intention was to support an empty source string; I want to be 
> explicit about the presence of embedded source for each file.

I'm not fond of the belt and suspenders approach.  If there is one
specifier for an attribute, there's no need for a second to say that
it's valid.  There's always the issue of what it means when the two
attributes disagree, such as when you have a flag saying that there
is embedded source, but the source string is empty.  Is that an error?

> When reading the spec I did notice places where an empty string can 
> indicate the absence of the attribute (e.g. DW_AT_name), but I would 
> prefer to be explicit here.
> 
> Scott
> 
> On 2018-02-01 11:19, paul.robinson at sony.com wrote:
>>> -----Original Message-----
>>> From: Dwarf-Discuss [mailto:dwarf-discuss-bounces at lists.dwarfstd.org] On
>>> Behalf Of scott at scottlinder.com
>>> Sent: Wednesday, January 31, 2018 2:05 PM
>>> To: dwarf-discuss at lists.dwarfstd.org
>>> Subject: [Dwarf-Discuss] DWARF and source text embedding
>>>
>>> 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).
>>
>> Would a zero-length string indicate something other than 
>> "has_source=false"?
>> If not, then a separate has_source flag seems redundant.
>> --paulr
>>
>>>
>>> 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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>>> Dwarf-Discuss at lists.dwarfstd.org
>>> http://lists.dwarfstd.org/listinfo.cgi/dwarf-discuss-dwarfstd.org
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-- 
Michael Eager    eager at eagercon.com
1960 Park Blvd., Palo Alto, CA 94306