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

Vivien Millet via llvm-dev llvm-dev at lists.llvm.org
Wed Jan 16 14:41:15 PST 2019

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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