llvm dev - Oct 2012 - [LLVMdev] Debugging LLVM IR with GDB

Duncan Sands wrote:
> Hi Pablo,
> 
> > Has anybody debugged LLVM IR with GDB? I'm using dragonegg to
transform C into
> > IR, then applying my optimizations. Passing "-g" to
dragonegg doesn't seem to
> > work since it generates debug info for the C code, not the IR. I
really need GDB
> > (lli doesn't solve my problems) in order to debug multi-threaded
and
> > multi-process MPI code.
> >
> > More clearly, if I have a file hello.ll and I execute:
> >
> > $> llc hello.ll -march=x86-64 -o hello.s
> > $> mpicc -O0 hello.s -o hello
> >
> > Can I debug "hello" with GDB at the IR level? If so, how? If
not, would it be
> > possible to add debug info generation at the IR level (and how much
effort
> > required)? I read something about DwarfDebug but it's not clear to
me how to use
> > it or even if it does what I need. I hope I didn't miss any
obvious solution.
> 
> it would be great to have a utility that, given an IR file, adds location
debug
> info to each line of it, the location being the line number within the IR
file.
> Then in the debugger you would get to see yourself moving around in the IR
file
> as you step through the program built from it.  Unfortunately I have no
idea how
> to implement this in a reasonable way.
> 
> Ciao, Duncan.
There's always what the integrated assembler does if you say "-g"
and
there aren't any debug-info directives in the assembler source.  It
does something based on input line number, not that I find it very 
useful, but it might be the most appropriate tactic for LLC.
--paulr

On 18/10/12 03:58, Robinson, Paul wrote:
> There's always what the integrated assembler does if you say
"-g" and
> there aren't any debug-info directives in the assembler source. 
Do you mean passing "-g" to llc or to llvm-as? Both return with an 
"unknown command line option: -g" error.

Thanks,
Pablo

> On 18/10/12 03:58, Robinson, Paul wrote:
> > There's always what the integrated assembler does if you say
"-g" and
> > there aren't any debug-info directives in the assembler source.
>
> Do you mean passing "-g" to llc or to llvm-as? Both return with
an
> "unknown command line option: -g" error.
Sorry, I wasn't clear.  My thought would be along the lines of teaching
llc to understand "-g".  This option would mean, if there is no debug
metadata, then pretend that each IR instruction in the input source file
had metadata, which pointed to the source line for that instruction in
the .ll file.

My inspiration was the case where you write a target-machine assembler
file with no debug-info directives (.loc etcetera) and then do
  clang -g foo.s
This will put debug info into the foo.o file, and that info will point
to the foo.s source lines for each machine instruction.
--paulr