> On Mar 18, 2015, at 5:03 PM, David Blaikie <[email protected]> wrote: > > > > On Wed, Mar 18, 2015 at 4:53 PM, Adrian Prantl <[email protected] > <mailto:[email protected]>> wrote: > >> On Mar 18, 2015, at 4:41 PM, David Blaikie <[email protected] >> <mailto:[email protected]>> wrote: >> >> >> >> On Wed, Mar 18, 2015 at 4:31 PM, Adrian Prantl <[email protected] >> <mailto:[email protected]>> wrote: >> >>> On Mar 18, 2015, at 4:02 PM, David Blaikie <[email protected] >>> <mailto:[email protected]>> wrote: >>> >>> >>> >>> On Wed, Mar 18, 2015 at 3:50 PM, Adrian Prantl <[email protected] >>> <mailto:[email protected]>> wrote: >>> >>>> On Mar 17, 2015, at 6:44 PM, David Blaikie <[email protected] >>>> <mailto:[email protected]>> wrote: >>>> >>>> >>>> >>>> On Tue, Mar 17, 2015 at 3:47 PM, Adrian Prantl <[email protected] >>>> <mailto:[email protected]>> wrote: >>>> >>>> > On Mar 17, 2015, at 10:03 AM, Greg Clayton <[email protected] >>>> > <mailto:[email protected]>> wrote: >>>> > >>>> > >>>> >> On Mar 17, 2015, at 9:46 AM, David Blaikie <[email protected] >>>> >> <mailto:[email protected]>> wrote: >>>> >> >>>> >> >>>> >> >>>> >> On Tue, Mar 17, 2015 at 9:42 AM, Greg Clayton <[email protected] >>>> >> <mailto:[email protected]>> wrote: >>>> >> >>>> >>> On Mar 16, 2015, at 6:47 PM, David Blaikie <[email protected] >>>> >>> <mailto:[email protected]>> wrote: >>>> >>> >>>> >>> >>>> >>> >>>> >>> On Mon, Mar 16, 2015 at 5:14 PM, Adrian Prantl <[email protected] >>>> >>> <mailto:[email protected]>> 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. >>> >>> Not sure I quite follow (mostly my fault given the rambling paragraph up >>> there) - given the lack of a dsymutil-like tool on other platforms as part >>> of the common tool path for debug info, I'm not sure module debug info >>> without split dwarf is viable in that world. There's no tool to read these >>> extra files at any point. >> >> In theory someone could port llvm-dsymutil to a different platform, but that >> scenario is a little far-fetched. I’m not sure what will happen if LLDB is >> presented with linked, non-split debug info that contains module references. >> >> Linked non-split debug info should come out for free - all the debug info >> would be is a bunch of TUs in a single comdat - no skeleton CU, nothing >> else. It would look just like normal DWARF, except with one comdat instead >> of multiple, for each set of types from a module. (& there would be no real >> size gains - since you'd be redundantly including all the type information >> in every object file) >> >> >>> >>> I suppose we could be creating one giant comdat for the module's debug info >>> (no skeleton unit, no distinct type unit comdats, just one big comdat). But >>> we'd probably want/need a tool to do the merging at compile time (like the >>> objcopy feature for split-dwarf, but in reverse - we'd compile, then run a >>> tool to smoosh all the comdats from the modules onto the object we just >>> generated). It wouldn't provide much in the way of space savings, a little >>> less stress on the linker (fewer comdats to handle), etc. Not sure if >>> there's a default mode of objcopy that would cope with this straight out, >>> or whether we'd need a new feature there (which wouldn't be a priority for >>> Google to implement, since we use fission, nor a priority for you to >>> implement since you have dsymutil, etc - so I'm not sure anyone would >>> bother) >>> >>> Long story short: maybe just error on -gmodules if -gsplit-dwarf isn't >>> specified or the platform isn't darwin? (& if it's darwin, dsymutil could >>> read the module skeletons to find which modules to link into the .dSYM?) >> >> That’s reasonable, too :-) >> The plan is for llvm-dsymutil to follow the references in the module >> skeletons, copy the module CUs >> >> TUs for now >> >> into the .dSYM, and fixup the external type references to become >> DW_FORM_ref_addrs. >> >> Sounds good for you guys - the fixup work will be a bit non-trivial, since >> it'll need to remove the type skeletons in the CUs, move all the extra >> members from the skeletons into the type unit (& resolve any duplicates), >> etc... - does that make sense? (otherwise I can provide some DWARF snippets >> to explain better) > > Or we use a weird Darwin-specific code path to not emit the modules with > -generate-type-units in the first place (bag of DWARF+index mapping hash to > DIE), > > bag-o-dwarf still doesn't address all the issues with type member merging I > described above. Certain things can't go in the type in the module because > they depend on context - most importantly/obviously, implicit special members > and member function template instatiations. > > I suppose you could still have type references reference the type in the > bag-o-dwarf/type unit directly (DW_AT_type with DW_FORM_ref_sig8) while > having the partial type (the type declaration with its extra CU-specific > members) which would simplify the dwarf in the easy cases.
Yes, something along these lines would make a good first iteration. > > which would make dsymutil's job really easy. As much as I’d like to get rid > of platform-specific behavior, due to the automatic way that modules are > generated on Darwin I don’t see an elegant way of making this switchable by > the user. > > Not sure I quite follow here how implicit modules impact this functionality. > We can still have a flag that you pass to the compiler that dictates how > debug info in modules is created/what schema we use. The problem is the combination of implicit generation and a global module cache. I guess we could treat a module with the wrong kind of debug info as out of date, but I’m not excited. -- adrian > > - David > > > -- adrian >> >> >> -- adrian >> > >
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