[llvm-dev] [DWARFv5] Assembler syntax for new line-table features

[via llvm-dev]

To pass the MD5 checksum to the assembler, I added a new optional clause to the .file directive:
md5 "checksum"
where checksum is the 16-byte checksum in hex.  It's quoted because the assembler doesn't have a way to parse a 16-byte integer.  Also this is the same syntax Reid invented for the CodeView equivalent.

To convey the root source filename, I allow the file number on the .file directive to have file number 0.  There is special handling in the AsmParser to allow accepting ".file 0" when we're not actually emitting DWARF 5, the root source file is kept in a separate field and not in the normal file table.  If MC does emit a v5 .debug_line section, then it dumps that file entry first before the rest of the file table.

I've addressed the latest can't-build-Linux revert of my patch by suppressing both the md5 clause and '.file 0' for pre-v5.  That way the feature is there for people experimenting with v5, but should not interfere with anybody else.  I'll commit that later this morning.

Regarding the discussion, it might be that dwarf-discuss is a better venue, because GCC people will be on that list who care about DWARF.  Let me know what you think.
--paulr

From: David Blaikie [mailto:dblaikie at gmail.com]
Sent: Thursday, March 29, 2018 10:59 AM
To: Robinson, Paul; Eric Christopher; Adrian Prantl; Jonas Devlieghere
Cc: llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] [DWARFv5] Assembler syntax for new line-table features

+Eric Christopher<mailto:echristo at gmail.com>  +Adrian Prantl<mailto:aprantl at apple.com> +Jonas Devlieghere<mailto:jdevlieghere at apple.com> (seems Jonas is doing a bunch of debug info work - guessing he's working with you, Adrian?)

I'm guessing Eric's the most likely to have contacts over in GCC land to maybe bridge the gap when talking about assembly syntax across the two. Eric - any ideas how best to negotiate this pseudo-standard? (there's another feature or two I'd like to propose too - at least to standardize what the syntax /should/ be, even if gas doesn't support it immediately)

Paul - perhaps a brief description of the proposed syntax would be helpful to get the ball rolling (even if it's just discussing it amongst ourselves before it ends up in a cross-project discussion).

- Dave
On Wed, Mar 28, 2018 at 11:16 AM via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
TL;DR: If I'm trying to define new assembler directive syntax to
support DWARF v5, it seems like a good idea for all the various
assemblers out there in the world to support the same syntax.
How would I go about negotiating that syntax with other assembler
providers?  Is GNU as the only really relevant one?

Long version:

DWARF v5 introduces a couple of new features in the .debug_line section
that require assembler syntax, because the information relates to the
files read by the compiler and there's no other way to inform the
assembler.

The two bits of information are:
(1) the MD5 checksum of each source file; and
(2) the primary source filename.

The primary source filename is given in the .debug_info section. In
DWARF v5 this is repeated in the .debug_line section; prior to DWARF v5
it is not.  In both cases, file number 0 refers to this file.  Because
the compiler emits the .debug_info section directly, the assembler is
not aware of the name of the primary source file without some new syntax
to provide that information.  And, it needs the MD5 checksum as well, so
relying on the old-format '.file' directive is insufficient.

I've added support in LLVM for both of these features, but somewhat
arbitrarily defined assembler syntax to support them.  Obviously if
implementers of other assemblers also want to support DWARF v5, the same
information will have to be represented with assembler syntax somehow,
and of course it would be best if all assemblers supporting DWARF v5
used the same syntax.  But I don't know how to go about doing that.

Any advice would be welcome.
Thanks,
--paulr

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