llvm dev - Mar 2018 - [DWARFv5] Assembler syntax for new line-table features

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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On Thu, Mar 29, 2018 at 8:27 AM <paul.robinson at sony.com> wrote:
> 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.
>
I'd guess, long-term, that's probably not a great motivation for
choosing
pseudo-standardized syntax.

>   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.
>
So .file 0 is accepted and ignored pre-5? & that's to support some
weird/old assembly?

>  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.
>
Yeah, I'm guessing that might be useful - could see how this conversation
goes for a little bit.

- Dave

> --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 <echristo at gmail.com>  +Adrian Prantl <aprantl
at apple.com>
>  +Jonas Devlieghere <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>
> 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
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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>> 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.
>
> I'd guess, long-term, that's probably not a great motivation for
> choosing pseudo-standardized syntax.
? anyone compiling source to asm needs to inform the assembler about the
checksum, because the assembler might not have access to the original
source when it runs.  This will be a common problem across all assemblers
that speak DWARF v5.

 
>> 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.
>
> So .file 0 is accepted and ignored pre-5? & that's to support some
> weird/old assembly?
No, that was just expressed badly.  File #0 is stashed in a separate
field, which used to be just the compilation dir.  The '.file 0'
parsing stuffs the info there without bothering to check the DWARF
version.  When the assembler finally emits the line table, if it's a
v5 line table then the root source file comes out first.  If it's a
v4 line table, the root source file is not emitted.  (Note the root
source file is still available from the .debug_info section.)

It would be feasible to reject '.file 0' unless the user requested
DWARF v5 specifically.  Similarly we could reject the md5 clause.
On the other hand, accepting and ignoring means an assembler file
generated for DWARF v5 could be re-assembled for DWARF v4 without
having to hand-modify the assembler source, so it seemed better to
silently accept the syntax regardless of DWARF version.

--paulr