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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