On Wed, Mar 18, 2015 at 4:53 PM, Adrian Prantl <[email protected]> wrote:
> > On Mar 18, 2015, at 4:41 PM, David Blaikie <[email protected]> wrote: > > > > On Wed, Mar 18, 2015 at 4:31 PM, Adrian Prantl <[email protected]> wrote: > >> >> On Mar 18, 2015, at 4:02 PM, David Blaikie <[email protected]> wrote: >> >> >> >> On Wed, Mar 18, 2015 at 3:50 PM, Adrian Prantl <[email protected]> wrote: >> >>> >>> On Mar 17, 2015, at 6:44 PM, David Blaikie <[email protected]> wrote: >>> >>> >>> >>> On Tue, Mar 17, 2015 at 3:47 PM, Adrian Prantl <[email protected]> >>> wrote: >>> >>>> >>>> > On Mar 17, 2015, at 10:03 AM, Greg Clayton <[email protected]> >>>> wrote: >>>> > >>>> > >>>> >> On Mar 17, 2015, at 9:46 AM, David Blaikie <[email protected]> >>>> wrote: >>>> >> >>>> >> >>>> >> >>>> >> On Tue, Mar 17, 2015 at 9:42 AM, Greg Clayton <[email protected]> >>>> wrote: >>>> >> >>>> >>> On Mar 16, 2015, at 6:47 PM, David Blaikie <[email protected]> >>>> wrote: >>>> >>> >>>> >>> >>>> >>> >>>> >>> On Mon, Mar 16, 2015 at 5:14 PM, Adrian Prantl <[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. > 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. - David > > -- adrian > > > >> >> -- adrian >> > > >
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