[llvm-dev] [MCJIT] messy call stack debug on x64 code in VisualStudio (original) (raw)

Jameson Nash via llvm-dev llvm-dev at lists.llvm.org
Sat Feb 29 20:54:40 PST 2020

I've always just hacked support for this in to the various JITs (for JuliaLang, in our debuginfo.cpp file), by setting the no-frame-pointer-optim flag in the IR, then creating and populating a dummy unwind description object in the .text section, and registering that dynamically. Some day I hope to actually just register the .pdata/.xdata sections with the unwinder.

PDBs are a bit different though, since the above steps work well for gdb, but generally I find that WinDbg is less willing or able to be given JIT-frame information from LLVM. (I assume somehow it can be done, for dotNET. I just don't know how.)

On Sat, Feb 29, 2020 at 11:07 PM Reid Kleckner via llvm-dev < llvm-dev at lists.llvm.org> wrote:

Yes, I think https://bugs.llvm.org/showbug.cgi?id=24233 needs to be implemented to fix this.

The Windows x64 unwinder doesn't generally look at frame pointers. We would need to register unwind info to make this work. What you see is fairly typical of attempting to unwind the stack when unwind info is missing. PDBs shouldn't generally enter into the picture. On Sat, Feb 29, 2020 at 8:14 AM Vivien Millet via llvm-dev <_ _llvm-dev at lists.llvm.org> wrote:

Hi,

I'm using IR and MCJIT to compile a script language. I debug it with on the fly generated .pdb files. During debugging, almost each time I step into a function, I loose information about calling function inside the visual studio callstack view or I have a bunch of pure addresses in the callstack in between the current function and the calling function, for example : MyJit.dll!MyCurrentFunction() [0x1234567887654321] [0x8765432112345678] MyJit.dll!MyCallingFunction() ... It looks like visual studio get lost while walking up stack. Does anyone know where it could come from ? I have disabled all optimisations (among them is the omit-frame-pointer). I have seen this bug here : https://bugs.llvm.org/showbug.cgi?id=24233 which is quite similar but it is quite old now, and since the proposed patch has been posted, the code in RuntimeDyldCOFFX8664.h has changed and it is difficult for me to know if it has really been fixed since or not. Could it be related to the way IR CreateAlloca are used to build local variables ? Could it be related to missing informations inside the PDB ? (I don't know if there is stack related information inside PDB files to ensure good stack walking). Thanks. Vivien

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