llvm dev - Jul 2016 - Gauging interest in generating PDBs from LLVM-backed languages

Hi all,

I've recently been doing some work on a novel language that uses LLVM for
optimization and machine codegen. The language is self-hosted, I'm building
my third iteration of its garbage collector, and also writing a thin IDE to
stretch the language a bit. Needless to say, debugging is a major concern
for me.

My primary experience (and primary development focus) is Windows-centric,
so my go-to debuggers are Visual Studio and WinDbg. I know of the lldb/VS
integration efforts, and of course with appropriate setup I could also
leverage gdb (IIRC). But these aren't quite what I was looking for,
personally.

Long story short I set out to build a PDB emitter that could generate debug
information for my language based on the existing CodeView emission support
as of LLVM 3.8. I'm happy to report success as of this afternoon. More
details about the effort and its status can be found at [0].


The high-level overview of my strategy is to crack the CodeView blob from
LLVM (.data$S COFF section) and reassemble it, plus some augmentation, then
feed that to the API exposed by MSPDB140.dll (Visual Studio 2015's
version). This works and I can debug programs in both VS and WinDbg
assuming the front-end supplies sane metadata to the LLVM layer.


My question to the list - is this work valuable for anyone else? Would
there be general interest in documentation or even example code that
assembles what I've learned throughout this effort?


Thanks,



 - Mike


[0] -
https://github.com/apoch/epoch-language/wiki/Knowledge-Dump---Debugging-Epoch-Programs
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Michael Lewis via llvm-dev <llvm-dev <at> lists.llvm.org> writes:
> My question to the list - is this work valuable for anyone else? Would
there be general interest in documentation or even example code that
assembles what I've learned throughout this effort?

I imagine Julia, Rust, and others would appreciate that kind of
resource (and improving the PDB support that's available upstream in
LLVM) for better integration with PDB-based Windows debuggers, so yes!

-Tony

We've been pursuing the direction of writing PDBs from scratch in
llvm/lib/DebugInfo/PDB/Raw and related directories. It might be interesting
to have code that talks to MSPDB140.dll in LLVM, but we really want LLD to
be able to produce its output on any platform.

On Sun, Jul 24, 2016 at 9:18 PM, Michael Lewis via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hi all,
>
> I've recently been doing some work on a novel language that uses LLVM
for
> optimization and machine codegen. The language is self-hosted, I'm
building
> my third iteration of its garbage collector, and also writing a thin IDE to
> stretch the language a bit. Needless to say, debugging is a major concern
> for me.
>
> My primary experience (and primary development focus) is Windows-centric,
> so my go-to debuggers are Visual Studio and WinDbg. I know of the lldb/VS
> integration efforts, and of course with appropriate setup I could also
> leverage gdb (IIRC). But these aren't quite what I was looking for,
> personally.
>
> Long story short I set out to build a PDB emitter that could generate
> debug information for my language based on the existing CodeView emission
> support as of LLVM 3.8. I'm happy to report success as of this
afternoon.
> More details about the effort and its status can be found at [0].
>
>
> The high-level overview of my strategy is to crack the CodeView blob from
> LLVM (.data$S COFF section) and reassemble it, plus some augmentation, then
> feed that to the API exposed by MSPDB140.dll (Visual Studio 2015's
> version). This works and I can debug programs in both VS and WinDbg
> assuming the front-end supplies sane metadata to the LLVM layer.
>
>
> My question to the list - is this work valuable for anyone else? Would
> there be general interest in documentation or even example code that
> assembles what I've learned throughout this effort?
>
>
> Thanks,
>
>
>
>  - Mike
>
>
> [0] -
>
https://github.com/apoch/epoch-language/wiki/Knowledge-Dump---Debugging-Epoch-Programs
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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On Tue, Jul 26, 2016 at 8:02 AM, Reid Kleckner <rnk at google.com> wrote:
> We've been pursuing the direction of writing PDBs from scratch in
> llvm/lib/DebugInfo/PDB/Raw and related directories. It might be interesting
> to have code that talks to MSPDB140.dll in LLVM, but we really want LLD to
> be able to produce its output on any platform.
>


I completely agree about independence from arbitrary platform-specific
binaries. (After all, escaping LINK.EXE just to rely on MSPDB140.dll isn't
that huge of a win by itself.)

I forgot to mention it in my original mail, but my plan is to transition
away from using MSPDB140.dll and fabricate "whole cloth" PDB/MSF files
in
the near future. Based on the microsoft-pdb GitHub repo and the code I've
seen in LLVM, I think this is a totally reachable goal.

Most of what I've found has little to do with the mechanics of the MSF
format (yet!) and more to do with the semantic content of PDBs and how to
coax the debuggers in question to yield the desired results. I don't know
how much of that stuff is mysterious to the larger community and what was
just mysterious to me when I set out on this little venture :-)

But in any case I'd be glad to compare notes with anyone who's working
in
this space.



 - Mike
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