[PATCH] Have clang list the imported modules in the debug info

[Adrian Prantl]

> On Mar 17, 2015, at 6:44 PM, David Blaikie <dblaikie at gmail.com> wrote:
> 
> 
> 
> On Tue, Mar 17, 2015 at 3:47 PM, Adrian Prantl <aprantl at apple.com <mailto:aprantl at apple.com>> wrote:
> 
> > On Mar 17, 2015, at 10:03 AM, Greg Clayton <gclayton at apple.com <mailto:gclayton at apple.com>> wrote:
> >
> >
> >> On Mar 17, 2015, at 9:46 AM, David Blaikie <dblaikie at gmail.com <mailto:dblaikie at gmail.com>> wrote:
> >>
> >>
> >>
> >> On Tue, Mar 17, 2015 at 9:42 AM, Greg Clayton <gclayton at apple.com <mailto:gclayton at apple.com>> wrote:
> >>
> >>> On Mar 16, 2015, at 6:47 PM, David Blaikie <dblaikie at gmail.com <mailto:dblaikie at gmail.com>> wrote:
> >>>
> >>>
> >>>
> >>> On Mon, Mar 16, 2015 at 5:14 PM, Adrian Prantl <aprantl at apple.com <mailto:aprantl at apple.com>> wrote:
> >>>
> >>> Thanks for the explanation David, I missed that it is entirely the linker's (or some dwarf post-processor's) responsibility to find the module files and link in the debug info from the .pcm files, so debugger doesn’t notice a difference.
> >>>
> >>> I think there's still some confusion here. Sorry if I'm rehashing something, but I'll try to explain how this all works.
> >>>
> >>> Normal split DWARF:
> >>>
> >>> Compiler generates two files: .o and .dwo.
> >>> .dwo has static, non-relocatable debug info.
> >>> .o has a skeleton compile_unit that has the name of the .dwo file and a hash to verify that the .dwo file isn't stale when the debugger reads it.
> >>> The .o files are all linked together, the .dwo files stay where they are.
> >>> The debugger reads the linked executable, finds the skeleton compile_units contained therein, and find/loads the .dwo files
> >>>
> >>> The scenario I have in mind for module debug info is this:
> >>> Module is compiled as an object file with debug info (this file is actually a .dwo file, even if it has some other extension - it has the non-relocatable debug info in it)
> >>> .o file has a comdat'd skeleton compile_unit describing the .dwo/module file
> >>> <from here on no extra work is required, the linker and debugger just act as normal>
> >>> The .o files are linked together, the skeleton compile_units get deduplicated by the linker (comdat sections)
> >>
> >> One issue I can think of is we will need to figure out a way to make COMDAT work with mach-o. COMDAT requires large number of sections and mach-o can only have 255.
> >>
> >> Ah, fair enough - how does MachO handle inline functions (the most common use of comdat) currently, then?
> >
> > Currently mach-o relies on symbols in the symbol table being marked as weak and I believe the data for these symbols are in special sections that are marked as containing items that can be coalesced.
> >
> That’s not necessarily an issue that needs to be solved on Darwin, or am I maybe missing something? The linker leaves all debug info in the .o (as it currently does) and llvm-dsymutil is resolving all the external module type references while creating the .dSYM bundle.
> 
> Yeah, with a debug aware linker (or in the case of dsymutil, a debug-only linker) you would just know that since you're looking at object files, module references will be redundant across objects and should be deduplicated (by the dwo hash, most likely).
> 
> If you're not teaching your debugger to read modules, and want to link the debug info in from the .dwos - at that point you can probably drop the skeleton stuff entirely (you'd still need to teach your debugger about .dwo sections and some of the esoteric things there - like str_index and the extra/special line table just for file names (decl_file, etc, uses this)) and just put the contents of the module debug info straight in the dsym. It'd be a bit weird, but do-able without too much work, I'd imagine. You could move them back into the original sections, if you wanted to avoid the weird .dwo +non-.dwo sections together... *shrug* not sure what exactly you'd want there.

My plan was to have -gmodules to behave like the latter variant unless -gsplit-dwarf is also present; this way there wouldn't be any weird Darwin-specific code paths.

-- adrian

> 
> - David
>  
> 
> -- adrian
> 

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