[llvm-dev] DebugInfo proposal: Emit an explicit empty address range on CUs with no code addresses

[Adrian Prantl via llvm-dev]

> On Nov 28, 2018, at 9:40 AM, David Blaikie <dblaikie at gmail.com> wrote:
> 
> So I've been looking at a particular performance problem with LLVM's symbolizer due to the use of ThinLTO, split DWARF, and split DWARF inlining info.
> 
> This combination has a couple of problems:
> 
> 1) it means multiple CUs in a single DWO, which isn't well defined/specified, and best avoided - so I'm working on fixing that here (won't fix split DWARF+Full LTO) because we already don't use cross-CU references in the split units (because there's no supported way to express that in DWARF), so we clone/move any DIEs (like subprograms) referenced cross-CU into the CU that references them (eg: cross-CU inlining places the abstract subprogram definition for the inlined subroutine into the CU that has the inlining - rather than cross-CU referencing into the other CU)) - and in ThinLTO the only reason other units exist is to cross-CU optimize/inline, no code for imported CUs is ever emitted (except where it's been inlined) - so a ThinLTO compile has one primary unit, and some other units it inlines from - so those other units never emit anything in the split unit, just a few DIEs in the skeleton unit if you're using split DWARF inlining (or no unit at all if you aren't using that feature) - so I'm working on making it so those units are non-split (rather than having a degenerate/empty split unit)
> 
> 2) symbolizer performance is hurt because whenever it sees a unit without ranges at the unit DIE, it assumes the producer just skipped those - and goes searching through the implementation DIEs (which may mean going over to the .dwo, or loading a whole .dwp) to see where their addresses are.
> 
> It's this second step that's a bit painfully unnecessary, especially for a large DWP on a remote filesystem, etc. 
> 
> So, anyone have opinions on whether we should
> 
> a) decide that a unit without ranges covers no ranges - and don't do the search

Are there compilers that do this ("forget" to emit ranges) that we care to support with llvm-symbolizer?

-- adrian

> 
> b) emit zero-length ranges on any unit that has no code ranges (low/high pc zero? Could pick anything, but that seems the most obvious)
> 
> Thanks,
> - Dave