Michael,
In the case of this proposal, then, I suggest the CU fields
(AT_{name,comp_dir}) retain their exact current definitions. Language
implementations, regardless of whether they might want to support
embedding
source, currently use the filesystem. This extension is essentially just
cacheing source which may become unavailable to the consumer by the time
the
program is debugged. This means the producer can put standard values in
each CU
field, and also embed source in the line table. If in the future there
is a
need to add CU fields or modify existing ones to capture some other
attribute,
that can be done in a different proposal.
Scott
On 2018-02-01 17:32, Michael Eager wrote:
On 02/01/2018 08:07 AM, sc...@scottlinder.com wrote:
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.
Implementation-defined generally means that different implementations
will be incompatible. Incompatible implementations are the antithesis
of a standard.
As a general DWARF principle, there should be no secret understandings
between producer and consumer. There should be no "secret handshake"
such as the one you describe where a producer provides "valuable
information" in some undefined manner usable only by a consumer which
is "in on the secret". It's not that a different consumer doesn't
implement the extension, it's that a different consumer cannot
implement
the extension.
Attributes which have a defined meaning, such as AT_name or
AT_comp_dir,
should have a well defined meaning in all circumstances.
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, sc...@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
_______________________________________________
Dwarf-Discuss mailing list
Dwarf-Discuss@lists.dwarfstd.org
http://lists.dwarfstd.org/listinfo.cgi/dwarf-discuss-dwarfstd.org
_______________________________________________
Dwarf-Discuss mailing list
Dwarf-Discuss@lists.dwarfstd.org
http://lists.dwarfstd.org/listinfo.cgi/dwarf-discuss-dwarfstd.org
_______________________________________________
Dwarf-Discuss mailing list
Dwarf-Discuss@lists.dwarfstd.org
http://lists.dwarfstd.org/listinfo.cgi/dwarf-discuss-dwarfstd.org
