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

[Devang Patel]

Hi Martijn,

On Tue, Nov 17, 2009 at 3:25 AM, Martijn <martijn at martijnrutten.com> wrote:
>>
>> 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?

Yes.

> 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?

Good news. In mainline svn sources, llvm, llvm-gcc and clang, are not
using dbg.stoppoints. Now location info (line, column and scope) is
attached directly with llvm instructions. This way, the info is more
likely to survive through optimization passes. The optimizer may lose
info when it replaces one set of instructions with another set,
however that is now easier to track and fix. For example, inliner is
now appropriately coping info when it copies function body. This is a
recent development.

>>
>> 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?

This work can be largely divided into 3 phases.

1) Emit and preserve llvm.dbg.var intrinsic through target independent
optimization passes. This requires experience dealing with llvm
transformation passes. Victor Hernandez has started working on this
phase.
2) Lower llvm.dbg.var and preserve variable info through code
generation passes. This requires codegen familiarity.
3) Emit appropriate DWARF entries based on the variable info that
survives through optimization and codegen passes. DWARF knowledge is
useful here.

-
Devang