[Dwarf] Register a local variable in DIBuilder and locate it later with a DwarfContext

Hi,

First, considering I’m using an IRBuilder and a DIBuilder to build my program, how can I automatically bind the CreateAlloca with my named local variable inside the DIBuilder ? Is it automatic with the Twine name of CreateAlloca ? And/Or should I use DIBuilder::createAutoVariable and how ?

Then, I’m wondering how to locate back my local variable in memory (register or stack) once i have a DwarfContext ready.

I’m a bit lost with all this pipeline, sorry If my question seem trivial.

Thanks !

Vivien

Binding the alloca to the debug-info metadata is not automatic. You need to emit an intrinsic function call to llvm.dbg.declare to bind the two together.

If you are simply trying to emit DWARF for your program, the rest of the processing should already be in place for that. If you are trying to do something else, you would need to describe that so we understand what you need.

–paulr

Hi Paul,
How can i make this call to intrinsic from the c++ code ?
I’m not working with the IR language, but directly in C++ with IRBuilder::CreateAlloca.
My goal is that one :

  • Generate machine code with an instance of the class ‘IRBuilder’
  • Emit ‘ObjFile’ class instance with MCJIT
  • Create a DwarfContext instance directly from the emitted ObjFile object (DwarfContextInMemory) (on JitEventListener::NotifyObjectEmitted)
  • Use this DwarfContext to step into my home made runtime debugger.

Now I can perform step over/into/out in my debugger using the DwarfContext instance without problem, but I’m still stuck on how
to watch my local variables in my debugger (i can’t figure out how to locate them in registers/stack from that DwarfContext)

Thanks for your help

“IR” often refers to the general concept/semantics, not only the textual format in .ll files (there are 3 main forms - bitcode, textual IR, and the in-memory representation (llvm::Module, etc - constructed using IRBuilder)).

A great place to start is to look at what Clang does to produce debug info - it uses IRBuilder, for instance. So you could look at how Clang uses the IRBuilder when compiling a small source file containing a local variable. You’ll find it calls into DIBuilder (the debug info equivalent of IRBuilder) for constructing types and the location descriptions (see “insertDeclare”/insertDbgValueIntrinsic", etc.).

Thanks I will try that, it seems to be exactly what I need for the IR<->DI binding part !

Once i have done this, do you have an idea on how I can extract the information back from my DwarfContext ?
I assume it should start with a call to DwarfContext::getDebugFrame(), but I’m not really sure, I’m really lost with that format…
What i would like is to be able to provide my function address and get, from a string “myLocalVar”, the information of the register/stack where the variable is.
I m still digging the llvm source code to find…
Thanks !