Hi all,

I was the author of the original proposal for DWARF source text embedding, and my use case was OpenCL kernel code, which is often generated at runtime. For debugging, this generated source needs to be embedded in the binary in many cases since you may not have a writeable file system, or a file system at all (this may seem like a niche case, but is actually common in embedded and mobile OpenCL environments).


FWIW I'm happy to get behind this new proposal since there's already an LLVM implementation and it seems to cover my use-cases.

Simon


On 14/02/18 00:45, Michael Eager wrote:
On 02/13/18 09:37, sc...@scottlinder.com wrote:
Michael, Paul,

In the current proposal, it is not an error to have any value (including an empty string) in the _source attribute when the _has_source flag is true, which
allows for embedding an empty source string.

After seeing more feedback on this point, I think you are right that the extra flag is unnecessary. Looking at similar attributes like MD5 and how they are handled I think it would be best to modify the proposal to remove the flag and
require the source be present on all files in the same line table if the
attribute is present in the prologue. I still think we should have wording which indicates an empty string is still a valid value for embedded source, and should not be interpreted as indicating the absence of embedded source for that file. This is analogous to the current MD5 attribute, as even 16 null
bytes is a valid MD5. What are your thoughts on this approach?

Scott

Are you saying that if any source file in embedded, that all need to be?
Including both ephemeral generated source as well as fixed include
files?

What does it mean to have embedded source which is an empty string?
How is that different from saying that embedded source is absent?

I can imagine situations where embedding ephemeral generated source in a
DWARF debug entry can make sense.  But I have the feeling that there is
more to this in your environment than what I imagine.  Can you give a description of the use case in which this might be used?


On 2018-02-01 17:20, Michael Eager wrote:
On 02/01/2018 12:01 PM, sc...@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.robin...@sony.com wrote:
-----Original Message-----
From: Dwarf-Discuss [mailto:dwarf-discuss-boun...@lists.dwarfstd.org] On
Behalf Of sc...@scottlinder.com
Sent: Wednesday, January 31, 2018 2:05 PM
To: dwarf-discuss@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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