[Dwarf-Discuss] DWARF and source text embedding

[scott@scottlinder.com]

Hi John,

In the case where the files are actually available on disk, and the 
source is simply being "cached", the attributes are exactly the same. In 
the case where sources are generated, and so have no true path on disk, 
I would suggest we might just leave the exact meaning to be 
implementation defined; the producer can still provide valuable 
information which will aid in locating where sources originate, such as 
indicating the OpenCL kernel name. Consumers which are unaware of this 
extension will simply fail to find the source (as before), while new 
consumers can at least provide an identifier to distinguish sources.

The remaining attributes (DW_AT_language, DW_AT_producer, etc.) seem 
pretty naturally orthogonal.

Regards,
Scott

On 2018-01-31 14:40, John DelSignore wrote:
> 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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