[LLVMdev] llvm-gcc: missing dbg.declare/dbg.stoppoint at optimization level > O0

[Martijn]

Hi Devang,

>> I use llvm and llvm-gcc as a C-to-C transformation tool using a
>> modified version of the c backend, and rely on llvm debug instructions
>> to link back to the original source code.
>>
>> Does anyone know how to get detailed line number and variable debug
>> information at optimization levels beyond O0?
>>
>> Currently, I extract this debug information by compiling with -g.
>> This works fine with "llvm-gcc -g -O0 --emit-llvm", i.e. no
>> optimizations, but when I enable optimizations, e.g. "llvm-gcc -g -O2
>> --emit-llvm",
>> most of the dbg.stoppoint and dbg.declare instructions dissapear.
>>
>> I tried to enable at least debug.declare instructions by removing the
>> "if(optimize) return" statement in gcc/llvm-debug.cpp,
>> DebugInfo::EmitDeclare(), but I still do not get any dbg.declare
>> instructions.
>
> This will enable dbg.declare, but they'll be removed by mem2reg pass
> immediately.
>
>> Is there a way to keep all debug information with optimizations
>> enabled in llvm-gcc?
>> If I have to disable some specific optimizations to keep the debug
>> info correct that is fine.
>
> Without mem2reg pass, you're unlikely to get much optimization done on your IR.

Clear, thanks! I presume the mem2reg pass is also essential in the
CLang frontend, so CLang is not an alternative to llvm-gcc to keep
more debug information?

For the dbg.stoppoints: many of the stoppoint instructions dissapear
at higher optimization levels. We need these to link back instructions
to original C statements. I guess this is not related to the mem2reg
pass. Is there a way to at least keep the stoppoint instructions?

>
> We're working on the required support to enable variable debug info at -O1+.
> http://nondot.org/~sabre/LLVMNotes/DebugInfoVariableInfo.txt
> Let us know if you'd like to contribute in this area!

The llvm.dbg.value proposal looks very interesting. We would like to
contribute if possible. I estimate we do not have sufficient llvm
experience to contribute ourselves, but may take up a part of the work
using a contractor. Can you identify a clear part for which we could
hire a contractor?

Martijn