[llvm-dev] [llvm-pdbutil] : merge not working properly

Vivien Millet via llvm-dev llvm-dev at lists.llvm.org
Wed Jan 16 13:38:22 PST 2019

I would be up to improve pdbutil but I doubt I have enough knowledge or
time to provide the complete merge feature, it would still be a very
specific kind of merge as you describe it. Anyway I could start trying to
do it in my jit compiler and then, once I get something working (if that
happens :)), i can come back to you with the piece of code and see if it is
worth integrating it to pdbutil and how ?

Le mer. 16 janv. 2019 à 22:12, Zachary Turner <zturner at google.com> a écrit :

> Well, that’s certainly possible, but improving llvm-pdbutil is another
> possibility. Doing it directly in your jit compiler will probably save you
> time though, since you won’t have to worry about writing tests and going
> through code review
> On Wed, Jan 16, 2019 at 1:01 PM Vivien Millet <vivien.millet at gmail.com>
> wrote:
>> Thanks for the tips !
>> When you talk about doing all of this I suppose you think about using
>> llvm/debuginfo/pdb, pick code here and there to generate the pdb in memory,
>> read the executable one and perform the merge directly in my jit compiler,
>> right ? Not using pdbutil ?
>> Le mar. 15 janv. 2019 à 22:49, Zachary Turner <zturner at google.com> a
>> écrit :
>>> On Tue, Jan 15, 2019 at 2:50 AM Vivien Millet <vivien.millet at gmail.com>
>>> wrote:
>>>> Hello Zachary !
>>>> Thanks for your time !
>>>> So you are one of the happy guys who suffered from the lack of PDB
>>>> format information :)
>>> Yes, that would be me :)
>>>> To be honest I'm really a beginner in the PDB stuff, I just read some
>>>> llvm documentation to understand what went wrong when merging my PDBs.
>>>> In my case, what I do with my team and try to achieve is this :
>>>> - Run our application under a visual studio debugger
>>>> - Generate JIT code ( using llvm MCJIT  )
>>>> - Then, either :
>>>>    - export as COFF obj file with dwarf information and then convert it
>>>> with cv2pdb to obtain a pdb of my JIT symbols (what I do now)
>>>>    - export directly to PDB my JIT debug info (what i would like to do,
>>>> if you have an idea how..)
>>>> - Detach the visual studio debugger
>>>> - Merge my JIT pdb into a copy of the executable pdb (where things
>>>> start to go bad..)
>>>> - Replace original executable by the copy (creating a backup of
>>>> original)
>>>> - Reattach  the visual studio debugger to my executable (loading the
>>>> new pdb version)
>>>> - Debug JIT code with visual studio.
>>>> - On each JIT rebuild, restart these steps from the original native
>>>> executable PDB to avoid merge conflict between the multiple JIT iterations
>>> Yea, it's an interesting use case.  It makes me think it would be nice
>>> if the PDB format supported some way of having a symbol which simply refers
>>> to another PDB file, that way you could re-write that PDB file at runtime
>>> once all your code is jitted, and when the debugger tries to look up that
>>> symbol, it finds a record that tells it to go check the other PDB file.
>>> So, here are the things I think you would need to do:
>>> 1) Create a JIT module in the module list with a unique name.  All
>>> symbols will go here.  llvm-pdbutil dump -modules shows you the list.  Be
>>> careful about putting it at the end though, because there's already one at
>>> the end called * LINKER * that is kind of special.  On the other hand, you
>>> don't want to put it first because it means you will have to do lots of
>>> fixups on the EXE PDB.  It's probably best to add it right before the
>>> linker module, this has the least chance of breaking anything.
>>> 2) In the debug stream for this module, add all symbols.  You will need
>>> to fix up their type indices.  As you noticed, llvm-pdbutil already merges
>>> type information from the JIT PDB, so after merging the type indices in the
>>> EXE PDB will be different than they were in the JIT PDB, but the symbol
>>> records will refer to the JIT PDB type indices.  So these need to be fixed
>>> up.  LLD already has code to do this, you can probably borrow a similar
>>> algorithm with some slight modifications (lldb/COFF/PDB.cpp, search for
>>> mergeSymbolRecords)
>>> 3) Merge in the new section contributions and section map.  See LLD
>>> again for how to modify these.  Hopefully the object file you exported
>>> contains relocated symbol addresses so you don't have to do any fixups here.
>>> 4) Merge in the publics and globals.  This shouldn't be too hard, I
>>> think you can just iterate over them in the JIT PDB and add them to the new
>>> EXE PDB.
>>> You're kind of in uncharted territory here, so this is just a rough idea
>>> of what needs to be done.  There may be other issues that you don't
>>> encounter until you actually try it out.
>>> Unfortunately I don't personally have the time to work on this, but it
>>> sounds neat, and I'm happy to help if you run into questions or problems
>>> along the way.
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