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) > > -- adrian >
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