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

[Adrian Prantl]

> On Mar 16, 2015, at 2:55 PM, David Blaikie <dblaikie at gmail.com> wrote:
> 
> 
> 
>> On Mon, Mar 16, 2015 at 2:45 PM, Robinson, Paul <Paul_Robinson at playstation.sony.com> wrote:
> Beyond the above (that using a new tag would mean this would go from 'free' to 'not free' for GDB) having a new top level tag is pretty substantial (we only have two at the moment, and with our talk of modules being a "bag of dwarf" might go back to having one top level tag? (it's not clear to me from DWARF4 whether DW_TAG_module is currently a top-level tag, I don't think it is?)
> 
>> The .debug_info section contains one or more compilation units, partial units, or in DWARF 5, type units.  DW_TAG_module isn't a unit, if you want it to be handled independently then it would need to be wrapped in a DW_TAG_partial_unit.  You would probably then use DW_TAG_imported_unit to refer to it, rather than DW_TAG_imported_module.
>> 
> 
> This makes a fair bit of sense - though the terminology's never going to quite line up with modules, I suspect, and this would still require modifying existing consumers (well, GDB) that can handle split-dwarf today, I suspect (not sure how it'd handle partial_unit - maybe that does work? - and still don't know how existing consumers would handle imported_unit either - could be worth some testing, as it sounds sort of right out of several less right options).

Thanks for all the input so far!
To concretize this end of the discussion up let’s sketch some dwarf of how this could look like in practice.

ELF (no imports)
----------------

On ELF or COFF a foo.c referencing types from the module Foundation looks like this:

.debug_info:
  DW_TAG_compile_unit
    DW_AT_name(“foo.c”)

.debug_info.dwo (on ELF: group 0x1234ABCDE, comdat)
  DW_TAG_partial_unit
    DW_AT_dwo_name(“/tmp/org.llvm.clang/ModuleCache/1234ABCDE/Foundation.pcm”)
    DW_AT_dwo_id(“0x1234ABCDE”)


Side question: Is .debug_info.dwo the right section to put the module skeleton in, or should it be a .debug_info section like normal fission skeletons?


Mach-O (no comdat, no imports)
------------------------------

Mach-O doesn’t do comdat, so with -split-dwarf=Disable (not sure if that option is the best discriminator) this could look like:

.debug_info:
  DW_TAG_compile_unit
    DW_AT_name(“foo.c”)
  DW_TAG_partial_unit
    DW_AT_dwo_name(“/tmp/org.llvm.clang/ModuleCache/1234ABCDE/Foundation.pcm”)
    DW_AT_dwo_id(“0x1234ABCDE”)


Mach-O (no comdat, with imports)
------------------------------

If we add the module import information to this, we get:

.debug_info:
  DW_TAG_compile_unit
    DW_AT_name(“foo.c”)
    DW_TAG_imported_module
      DW_AT_import(DW_FORM_ref_addr 0x10) 

  DW_TAG_partial_unit
    DW_AT_dwo_name(“/tmp/org.llvm.clang/ModuleCache/1234ABCDE/Foundation.pcm”)
    DW_AT_dwo_id(“0x1234ABCDE”)

0x10:
    DW_TAG_module
      DW_AT_name(“Foundation”)
      DW_AT_LLVM_sysroot(“/“)
      DW_AT_LLVM_include_dir(“”)
      DW_AT_LLVM_macros(“-DNDEBUG”)
      ...


ELF (comdat, with imports)
--------------------------

But now let’s go back to ELF. Since the skeleton with the partial unit is comdat'd, I assume that this breaks the FORM_ref_addr used in the DW_AT_import. We could reuse the module hash as a signature for the module:

.debug_info:
  DW_TAG_compile_unit
    DW_AT_name(“foo.c”)
    DW_TAG_imported_module
      DW_AT_import(DW_FORM_ref_addr 0x1234ABCDE) 

.debug_info.dwo (group 0x1234ABCDE, comdat)
  DW_TAG_partial_unit
    DW_AT_dwo_name(“/tmp/org.llvm.clang/ModuleCache/1234ABCDE/Foundation.pcm”)
    DW_AT_dwo_id(“0x1234ABCDE”)

    DW_TAG_module
      DW_AT_signature(“0x1234ABCDE”)
      DW_AT_name(“Foundation”)
        
This is bending the definition of DW_AT_signature, but I guess it could be made to work. Or we could say that for now, users have to choose between the comdat optimization and having the module imports recorded in Dwarf, since GDB wouldn’t know what to do with that information anyway.

Thoughts?

-- adrian