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

[Adrian Prantl]

> On Feb 23, 2015, at 3:37 PM, David Blaikie <dblaikie at gmail.com> wrote:
> 
> 
> 
> On Mon, Feb 23, 2015 at 3:32 PM, Adrian Prantl <aprantl at apple.com <mailto:aprantl at apple.com>> wrote:
> 
>> On Feb 23, 2015, at 3:14 PM, David Blaikie <dblaikie at gmail.com <mailto:dblaikie at gmail.com>> wrote:
>> 
>> 
>> 
>> On Mon, Feb 23, 2015 at 3:08 PM, Adrian Prantl <aprantl at apple.com <mailto:aprantl at apple.com>> wrote:
>> 
>>> On Feb 23, 2015, at 2:59 PM, David Blaikie <dblaikie at gmail.com <mailto:dblaikie at gmail.com>> wrote:
>>> 
>>> 
>>> 
>>> On Mon, Feb 23, 2015 at 2:51 PM, Adrian Prantl <aprantl at apple.com <mailto:aprantl at apple.com>> wrote:
>>> 
>>> > On Jan 20, 2015, at 11:07 AM, David Blaikie <dblaikie at gmail.com <mailto:dblaikie at gmail.com>> wrote:
>>> >
>>> > My vague recollection from the previous design discussions was that these module references would be their own 'unit' COMDAT'd so that we don't end up with the duplication of every module reference in every unit linked together when linking debug info?
>>> >
>>> > I think in my brain I'd been picturing this module reference as being an extended fission reference (fission skeleton CU + extra fields for users who want to load the Clang AST module directly and skip the split CU).
>>> 
>>> Apologies for letting this rest for so long.
>>> 
>>> Your memory was of course correct and I didn’t follow up on this because I had convinced myself that the fission reference would be completely sufficient. Now that I’ve been thinking some more about it, I don’t think that it is sufficient in the LTO case.
>>> 
>>> Here is the example from the http://lists.cs.uiuc.edu/pipermail/cfe-dev/2014-November/040076.html <http://lists.cs.uiuc.edu/pipermail/cfe-dev/2014-November/040076.html>:
>>> 
>>> foo.o:
>>> .debug_info.dwo
>>>   DW_TAG_compile_unit
>>>      // For DWARF consumers
>>>      DW_AT_dwo_name ("/path/to/module-cache/MyModule.pcm")
>>>      DW_AT_dwo_id   ([unique AST signature])
>>> 
>>> .debug_info
>>>   DW_TAG_compile_unit
>>>     DW_TAG_variable
>>>       DW_AT_name "x"
>>>       DW_AT_type (DW_FORM_ref_sig8) ([hash for MyStruct])
>>> 
>>> In this example it is clear that foo.o imported MyModule because its DWO skeleton is there in the same object file. But if we deal with the result of an LTO compilation we will end up with many compile units in the same .debug_info section, plus a bunch of skeleton compile units for _all_ imported modules in the entire project. We thus loose the ability to determine which of the compile units imported which module.
>>> 
>>> Why would we need to know which CU imported which modules? (I can imagine some possible reasons, but wondering what you have in mind)
>> 
>> When the debugger is stopped at a breakpoint and the user wants to evaluate an expression, it should import the modules that are available at this location, so the user can write the expression from within the context of the breakpoint (e.g., without having to fully qualify each type, etc).
>> 
>> I'm not sure how much current debuggers actually worry about that - (& this may differ from lldb to gdb to other things, of course). I'm pretty sure at least for GDB, a context in one CU is as good as one in another (at least without split-dwarf, type units, etc - with those sometimes things end up overly restrictive as the debugger won't search everything properly).
>> 
>> eg: if you have a.cpp: int main() { }, b.cpp: void func() { } and you run 'start' in gdb (which breaks at the beginning of main) you can still run 'p func()' to call the func, even though there's no declaration of it in a.cpp, etc.
> 
> LLDB would definitely care (as it is using clang for the expression evaluation supporting these kinds of features is really straightforward there). By importing the modules (rather than searching through the DWARF), the expression evaluator gains access to additional declarations that are not there in the DWARF, such as templates. But since clang modules are not namespaces, we can’t generally "import the world” as a debugger would usually do.
> 
> Sorry, not sure I understand this last sentence - could you explain further?
> 
> I imagine it would be rather limiting for the user if they could only use expressions that are valid in this file from the file - it wouldn't be uncommon to want to call a function from another module/file/etc to aid in debugging.

Usually LLDB’s expression evaluator works by creating a clang AST type out of a DWARF type and inserting it into its AST context. We could pre-polulate it with the definitions from the imported modules (with all sorts of benefits as described above), but that only works if no two modules conflict. If the declaration can’t be found in any imported module, LLDB would still import it from DWARF in the “traditional” fashion.

-- adrian

>  
> 
> -- adrian
>>>  
>>> I think it really is necessary to put the info about the module imported into the compile unit that imported it. Or is there a way to do this using the fission capabilities that I’m not aware of?
>>> 
>>> -- adrian
>>> 
>>> >
>>> > [rambling a bit more along those lines:
>>> > This would work fine in the case of the module (now an object file) containing all the static debug info
>>> > The future step, when we put IR/object code in a module to be linked into the final binary, we could put the skeleton CU in that object file that's being linked in (then we wouldn't need to COMDAT it) or, optionally, link in the debug info itself (skipping the indirection through the external file) if a standalone debug info executable was desired]
>>> 
>>> 
>>> 
>>> >
>>> > On Tue, Jan 20, 2015 at 9:39 AM, Adrian Prantl <aprantl at apple.com <mailto:aprantl at apple.com>> wrote:
>>> > As a complementary part of the module debugging story, here is a proposal to list the imported modules in the debug info. This patch is not about efficiency, but rather enables a cool debugging feature:
>>> >
>>> > Record the clang modules imported by the current compile unit in the debug info. This allows a module-aware debugger (such as LLDB) to @import all modules visible in the current context before evaluating an expression, thus making available all declarations in the current context (that originate from a module) and not just the ones that were actually used by the program.
>>> >
>>> > This implementation uses existing DWARF mechanisms as much as possible by emitting a DW_TAG_imported_module that references a DW_TAG_module, which contains the information necessary for the debugger to rebuild the module. This is similar to how C++ using declarations are encoded in DWARF, with the difference that we're importing a module instead of a namespace.
>>> > The information stored for a module includes the umbrella directory, any config macros passed in via the command line that affect the module, and the filename of the raw .pcm file. Why include all these parameters when we have the .pcm file? Apart from module chache volatility, there is no guarantee that the debugger was linked against the same version of clang that generated the .pcm, so it may need to regenerate the module while importing it.
>>> >
>>> > Let me know what you think!
>>> > -- adrian
>>> >
>>> >
>>> >
>>> 
>>> 
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