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

Thu Jan 17 10:03:25 PST 2019

Hi Zachary !
If there a way to easily create a new PDBFileBuilder from an existing
PDBFile or can/should I do the translation myself ?
I would like to start from a builder filled with the EXE PDB data and then
complete its DBI stream with the JIT module/symbols.

Thanks !

Le mer. 16 janv. 2019 à 23:41, Vivien Millet <vivien.millet at gmail.com> a
écrit :

> Thank you Zachary !
> I will have some soon I think ..
> I first need to explore the llvmpdb-util code more because I don't even
> know where to start with the PDB api..
>
> Le mer. 16 janv. 2019 à 22:51, Zachary Turner <zturner at google.com> a
> écrit :
>
>> 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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