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

[Zachary Turner via llvm-dev]

Sure. Along the way I’m happy to answer any specific questions you might
have too even if it’s for your downstream project
On Wed, Jan 16, 2019 at 1:38 PM Vivien Millet <vivien.millet at gmail.com>
wrote:

> 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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