[LLVMdev] Implementing minimal debug info (-g1?) for Clang

[Eric Christopher]

On Apr 5, 2012, at 10:24 AM, Alexey Samsonov <samsonov at google.com> wrote:

> Hi!
> 
> Currently Clang "-g" flag emits full debug info, which is fine for debugging, but increases the binary size significantly.
> It may be useful to produce less debug info, that is still enough for collecting nice stack traces with file names and line numbers,
> but would introduce less overhead. Cary Coutant made a patch which does this for GCC (it didn't hit trunk yet) -
> reduces debug info to only descriptions of functions, extern variables, line number tables and inlined subroutine info
> by setting "-gmlt" ("minimum line table") or "-g1" flags.
> (See this patch at http://old.nabble.com/-patch--Add-new--gmlt-option-for-min.-debug-info-with-line-tables-(issue4440072)-td31482851.html
> or http://codereview.appspot.com/4440072). This patch is used in Google for about 2 years already.
> 
> I get the following binary sizes of 483.xalancbmk benchmark from SPEC 2006 (clang from trunk vs. gcc 4.6.x with Google patches):
> 11026073 Xalan_base.clang_O0
> 45882529 Xalan_base.clang_O0_g
> 11079688 Xalan_base.gcc_O0
> 16437776 Xalan_base.gcc_O0_gmlt
> 54221056 Xalan_base.gcc_O0_g
> 
> WDYT of implementing similar option in Clang? Clearly, there are two options:
> 1. Don't modify Clang codegen, but instead erase all the extra debug info by LLVM (pro - there actually is StripDebugInfo pass already, but it's very short and easy,
> and would have to be patched a lot).
> 2. Emit less information in Clang. We're also quite interested if this may reduce the compilation time as well.
> Which approach looks better in your opinion? I'd like to start working on that enhancement, but certainly would be happy to hear some advice beforehand.

I was holding off trying to come up with ways to give advice and I've only come up with a couple. Unfortunately a lot of how things work is tied into the recursive route that we take. I think it may require a bit of an overhaul for how we end up limiting debug information, but I'm very interested in seeing it. If there's anything I can do to help let me know.

-eric