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

[Adrian Prantl]

> On Mar 17, 2015, at 10:03 AM, Greg Clayton <gclayton at apple.com> wrote:
> 
> 
>> On Mar 17, 2015, at 9:46 AM, David Blaikie <dblaikie at gmail.com> wrote:
>> 
>> 
>> 
>> On Tue, Mar 17, 2015 at 9:42 AM, Greg Clayton <gclayton at apple.com> wrote:
>> 
>>> On Mar 16, 2015, at 6:47 PM, David Blaikie <dblaikie at gmail.com> wrote:
>>> 
>>> 
>>> 
>>> On Mon, Mar 16, 2015 at 5:14 PM, Adrian Prantl <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.

-- adrian