On Fri, May 1, 2015 at 5:19 PM, Adrian Prantl <[email protected]> wrote:
> > On May 1, 2015, at 4:55 PM, David Blaikie <[email protected]> wrote: > > > > On Fri, May 1, 2015 at 4:39 PM, Adrian Prantl <[email protected]> wrote: > >> >> > On May 1, 2015, at 10:01 AM, David Blaikie <[email protected]> wrote: >> > >> > >> > >> > On Fri, May 1, 2015 at 9:52 AM, Adrian Prantl <[email protected]> >> wrote: >> >> >> >>> On May 1, 2015, at 9:23 AM, David Blaikie <[email protected]> wrote: >> >>> >> >>> >> >>> >> >>> On Thu, Apr 30, 2015 at 5:21 PM, Adrian Prantl <[email protected]> >> wrote: >> >>> >> >>> > On Apr 30, 2015, at 4:55 PM, David Blaikie <[email protected]> >> wrote: >> >>> > >> >>> > >> >>> > >> >>> > On Thu, Apr 30, 2015 at 4:31 PM, Adrian Prantl <[email protected]> >> wrote: >> >>> >> >> >>> >> > On Mar 19, 2015, at 5:37 PM, David Blaikie <[email protected]> >> wrote: >> >>> >> > >> >>> >> > >> >>> >> > >> >>> >> > On Thu, Mar 19, 2015 at 5:24 PM, Adrian Prantl < >> [email protected]> wrote: >> >>> >> >> >> >>> >> >> > On Mar 16, 2015, at 2:55 PM, David Blaikie < >> [email protected]> wrote: >> >>> >> >> > >> >>> >> >> > >> >>> >> >> > >> >>> >> >> >> On Mon, Mar 16, 2015 at 2:45 PM, Robinson, Paul < >> [email protected]> 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 >> >>> >> > >> >>> >> > For now I'd suggest we use compile_unit - that way it'll just >> work with existing split-dwarf consumers. We can see about standardizing a >> top-level DW_TAG_module or using DW_TAG_partial_unit here later, perhaps? >> I'm not sure. >> >>> >> > >> >>> >> >> >> 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? >> >>> >> > >> >>> >> > Skeletons go in .debug_info, the dwo sections are just for the >> .dwo file (or the module file, in our new case - the extension isn't >> actually important). >> >>> >> > >> >>> >> > It might be worth you compiling an example or two of split-dwarf >> to see how this all works hands-on. >> >>> >> > >> >>> >> >> 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) >> >>> >> > >> >>> >> > Since we got went down the tangent of explaining split-dwarf >> many emails ago, I've forgotten (& can't readily find) what we were >> discussing about what ways the imported_module could work. >> >>> >> > >> >>> >> > The simplest representation I can think of would be to have it >> reference, by signature, the module unit (whatever tag it uses) - >> DW_FORM_ref_sig8, seems the simplest thing to do. >> >>> >> > >> >>> >> >> >> >>> >> >> DW_TAG_partial_unit >> >>> >> >> >> DW_AT_dwo_name(“/tmp/org.llvm.clang/ModuleCache/1234ABCDE/Foundation.pcm”) >> >>> >> >> DW_AT_dwo_id(“0x1234ABCDE”) >> >>> >> >> >> >>> >> >> 0x10: >> >>> >> > >> >>> >> > This is inside the partial unit? I figured we'd just put these >> attributes on the top level (compile_unit, or whatever it might be later) - >> potentially conditionalized on platform, sure. >> >>> >> > >> >>> >> >> 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) >> >>> >> > >> >>> >> > Still only really need these imported_modules for lldb, right? >> I'd consider having them off-by-default for non-darwin, but I'm not >> strictly wedded to that notion. Wouldn't mind seeing size impact numbers of >> some kind - if it's really fractional % increase & GDB doesn't fall over >> when it sees them (in whatever FORM/tag/etc we decide on) then that's not >> the end of the world. >> >>> >> > >> >>> >> > Just seems nice if the default mode is the nice, standard, >> split-dwarf output. Doesn't need anything fancy. >> >>> >> > >> >>> >> > >> >>> >> >> .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”) >> >>> >> > >> >>> >> > >> >>> >> > The thing you haven't covered is the actual .dwo sections >> (.debug_info.dwo (we'll probably need a simple stub compile_unit to make >> this correct split-dwarf) and .debug_types.dwo being important - but all >> the supporting .dwo sections will be necessary) that go in the module file. >> >>> >> > >> >>> >> >> 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. >> >>> >> >> >>> >> Sorry for the long delay. Here’s a more complete example that >> should include all the suggestions made so far. For context I also included >> external type references in the example although admittedly this is a bit >> out of scope for this thread: >> >>> >> >> >>> >> ELF (typeunits, comdats, with imports) >> >>> >> -------------------------------------- >> >>> >> >> >>> >> On ELF or COFF a bar.c referencing type Foo from the module FooLib >> looks like this: >> >>> >> >> >>> >> bar.o >> >>> >> ~~~~~ >> >>> >> >> >>> >> // To keep this example focussed/readable, I'm assuming that bar.o >> itself was not compiled with fission. >> >>> >> .debug_info: >> >>> >> DW_TAG_compile_unit >> >>> >> DW_AT_name(“bar.c”) >> >>> >> ... >> >>> >> >> >>> >> DW_TAG_imported_module // <- This could be optional on ELF. >> >>> >> DW_AT_import [DW_FORM_ref_sig8] (0xABCD1234) >> >>> >> >> >>> >> DW_TAG_variable >> >>> >> DW_AT_name(“MyFoo”) >> >>> >> DW_AT_type [DW_FORM_ref4] 0x20 >> >>> >> 0x20: >> >>> >> DW_TAG_structure_type >> >>> >> DW_AT_declaration (true) >> >>> >> DW_AT_signature [DW_FORM_ref_sig8] (0xF00) >> >>> >> >> >>> >> >> >>> >> // Split DWARF skeleton CU for the module Foo. >> >>> >> DW_TAG_compile_unit >> >>> >> >> DW_AT_dwo_name(“/tmp/org.llvm.clang/ModuleCache/1234ABCDE/FooLib-XYZ.pcm”) >> >>> >> DW_AT_dwo_id(“0xFEDB9876”) >> >>> >> ... >> >>> >> >> >>> >> // Comdat’d partial unit containing the optional module descriptor. >> >>> >> .debug_info, group 0xABCD1234, comdat >> >>> >> DW_TAG_partial_unit >> >>> >> DW_TAG_module >> >>> >> DW_AT_name(“FooLib”) >> >>> >> DW_AT_LLVM_sysroot(“/“) >> >>> >> DW_AT_LLVM_include_dirs(“-I/path”) >> >>> >> DW_AT_LLVM_macros(“-DNDEBUG”) >> >>> >> ... >> >>> >> >> >>> >> FooLib-XYZ.pcm >> >>> >> ~~~~~~~~~~~~~~ >> >>> >> >> >>> >> .debug_info.dwo >> >>> >> DW_TAG_compile_unit >> >>> >> DW_AT_dwo_id(“0xFEDB9876”) >> >>> >> ... >> >>> >> >> >>> >> // Type unit for the type Foo. >> >>> >> .debug_types.dwo, group 0xF00, comdat >> >>> >> DW_TAG_type_unit >> >>> >> DW_TAG_structure_type >> >>> >> DW_AT_name (“Foo”) >> >>> >> ... >> >>> >> >> >>> >> >> >>> >> I think it awkward to have both the skeleton compile_unit in >> .debug_info and the partial_unit containing the TAG_module. Personally I’d >> prefer putting the TAG_module into the skeleton CU and then just refer to >> it via a FORM_ref_addr; but if we want to put the TAG_module into a comdat >> section, it looks like that’s what’s necessary. >> >>> > >> >>> > It's been a while & I've probably lost all the context, but I think >> my original theory was to have the skeleton compile_unit be comdat'd so >> they'd deduplicate on linking (so we'd only have one reference to the >> module.dwo in the linked binary). I don't recall there being a need for a >> separate partial_unit - I imagine we'd just put the LLDB/LLVM extension >> attributes on the skeleton compile_unit and expect debuggers that didn't >> understand them, to ignore them. >> >>> > >> >>> > Was there some reason this didn't work/make sense? Because you need >> a DW_TAG_module to import with DW_TAG_imported_module? >> >>> Using DW_TAG_module was the best practice that was recommended on >> dwarf-discuss. >> >>> >> >>> Did they have any ideas on how to reference it without duplicating it >> in every CU? >> >> >> >> We didn’t touch the deduplication issue. >> >> >> >>> Once we've got the "Bag O Dwarf" stuff (rather than the narrower type >> units) this would be easier - (I suppose we could do a partial >> solution/abuse of type units - use a type unit header (perhaps with Eric's >> merged type/compile unit work) and a DW_FORM_ref_sig8 value for the >> DW_AT_module in the DW_TAG_imported_module. >> >>> >> >>> Though I suppose if we're going to have DW_TAG_imported_module in >> every CU that references a module, it might not be that big of a deal to >> include the DW_TAG_module itself there too... while I don't care about this >> scheme immediately, Google's growing LLDB investment in various platforms, >> so I am vaguely concerned about getting this right & it's not immediately >> obvious to me what that right answer is. >> >> >> >> Maybe the best path forward is to stage this by initially putting the >> DW_TAG_module into the main CU and leave the deduplication as an >> optimization to be implemented once the bag’o dwarf is more fleshed out. >> This way we won’t do anything that would confuse consumers (assuming they >> ignore unknown tags) and the extra overhead is likely not even going to be >> noticeable, since all the string attributes inside the TAG_module can >> already be deduplicated by traditional means. >> > >> > Perhaps. I'd still like to think through/document what this looks like >> a bit more. Where the data ends up, what it's used for, etc. Sorry to draw >> this out. >> > >> > :/ *ponders* >> >> >> Let’s construct this: >> >> The most straightforward representation is to not unique the TAG_module >> and place it into the main CU. >> >> bar.o >> ~~~~~ >> >> .debug_info: >> DW_TAG_compile_unit >> ... >> DW_TAG_imported_module >> DW_AT_import [DW_FORM_ref4] (0x20) >> 0x20: >> DW_TAG_module >> DW_AT_name(“FooLib”) >> DW_AT_LLVM_sysroot(“/“) >> DW_AT_LLVM_include_dirs(“-I/path”) >> DW_AT_LLVM_macros(“-DNDEBUG”) >> > > Might as well put all these LLVM attributes on the skeleton CU, though - > so they can be deduplicated (& just put the dwo_id in this module > somewhere, perhaps just using the DW_AT_dwo_id attribute - possibly that's > the only attribute the DW_TAG_module would need, ideally). Unless we need > to consider the submodule issue (in which case the skeleton unit would > reference the whole module but the submodules would reference/describe the > respective submodules?)? > > > We cannot put them into the skeleton CU if the skeleton CU is going to be > comdat’d, because we’d then have to refer to it via a signature and that > leads us directly to the can of worms discussed in the next paragraph :-) > > > >> ... >> >> // Split DWARF skeleton, comdat'd. >> .debug_info, group 0xFEDB9876, comdat >> DW_TAG_compile_unit >> >> DW_AT_dwo_name(“/tmp/org.llvm.clang/ModuleCache/1234ABCDE/FooLib-XYZ.pcm”) >> DW_AT_dwo_id(“0xFEDB9876”) >> ... >> >> On Mach-O the split DWARF skeleton would not be a comdat’d, but >> llvm-dsymutil can just ignore it. >> >> >> If we want to dedup the TAG_module we need to refer to it via signature. >> This means we need to wrap it in a type_unit or a DWARF5 TAG_type_unit. We >> might as well throw it in with the skeleton CU. >> >> .debug_info: >> DW_TAG_compile_unit >> ... >> DW_TAG_imported_module >> DW_AT_import [DW_FORM_ref_sig8] (0xABCD1234) >> >> // Split DWARF skeleton, comdat'd. >> .debug_info, group 0xFEDB9876, comdat >> DW_TAG_compile_unit >> >> DW_AT_dwo_name(“/tmp/org.llvm.clang/ModuleCache/1234ABCDE/FooLib-XYZ.pcm”) >> DW_AT_dwo_id(“0xFEDB9876”) >> ... >> DW_TAG_type_unit (signature: 0xABCD1234) >> > > Can't really put a type_unit inside a compile_unit - it'd need to be > top-level with an appropriate type unit header, etc. & then we'd need two > different units/headers, could still comdat them, but it's a weird abuse of > type units & would probably confuse consumers. I don't know whether that's > worth the effort. > > Oh right. > > > >> DW_TAG_module >> DW_AT_name(“FooLib”) >> DW_AT_LLVM_sysroot(“/“) >> DW_AT_LLVM_include_dirs(“-I/path”) >> DW_AT_LLVM_macros(“-DNDEBUG”) >> ... >> >> Now that raises the question about what happens with multiple modules >> within one PCM. > > > Is the right term "submodule"? it's sort of confusing to talk about > multiple modules within a pcm. > > > Yes, a module with nested submodules. > http://clang.llvm.org/docs/Modules.html#submodule-declaration > > > >> Assuming that the ELF linker is linking and deduping all the non-.dwo >> sections, we may loose some of the TAG_modules (if not every CU imports all >> submodules) in the binary, but that wouldn’t matter because the consumer >> would find all TAG_modules by signature in the .pcm > > > Is there any reason we need to reference the submodules individually, > rather than just reference the whole module > > > My assumption is that an AST-aware debugger will want to import the exact > submodules that were imported by the CU before dropping into the expression > evaluator to replicate the environment of the CU as much as possible. > I'm just not picturing that. It seems pretty likely that a debugger user is more likely to treat the whole set of names in the program, not just those syntactically valid at that point in the source file. A simple example would be if I'm debugging LLVM and I'm in some generic optimization pass, but I want to cast my Instruction pointer to some specific instruction type to examine it in more detail - even though this pass doesn't care about that specific Instruction type nor include the header in which it's declared. > > (& have just a single, whole module in the pcm)? > > > That’s probably not what you meant, but just to be sure: The pcm will > always have the entire module with all submodules in it. But the debugger > may choose to import only a subset of those. > > > >> file referred to by whichever skeleton CU makes it into the binary: >> >> FooLib-XYZ.pcm >> ~~~~~~~~~~~~~~ >> >> .debug_info.dwo >> DW_TAG_compile_unit >> DW_AT_dwo_id(“0xFEDB9876”) >> ... >> >> DW_TAG_type_unit (signature: 0xABCD1234) >> DW_TAG_module >> DW_AT_name(“FooLib”) >> ... >> DW_TAG_type_unit (signature: 0xCDEF3456) >> DW_TAG_module >> DW_AT_name(“FooLib”) >> DW_TAG_module >> DW_AT_name(“SubFoo”) >> ... >> >> So.. this should work as long as nobody points out that a module isn’t >> really a type. >> > > Yeah, probably worth waiting for "Bag O DWARF". > > For now, as you mentioned earlier, maybe just putting the imported_module > and the module into the compile_unit when tuning for LLDB (so Darwin by > default, and anywhere else where someone tunes for LLDB in the future) & > leave them out otherwise. > > > Sounds prefectly reasonable. > > > Could you remind me why LLDB wants to know which modules are referenced > from a CU? (rather than just all the modules used by a program overall?) > > > LLDB uses clang for the expression evaluation. Traditionally it would look > up a type in DWARF, build a clang AST out of it and then import it. With > this it could directly import the clang modules and have access to > everything in the module. But, clang modules are not namespaces, so modules > can conflict (and that would probably manifest as a crash in libclang). > What's an example of such a conflict? Is that valid (or is it just in ODR violations) - as mentioned above, it seems to me that only importing the things lexically available in this source file isn't what a debugger user would really want. I certainly think I'd trip over that a lot. > It therefore needs to know which modules are imported in the current CU > before dropping into the expression evaluator. > > - adrian > > > >> >> >> >> On Macho-O, in the absence of comdats, we have: >> >> bar.o >> ~~~~~ >> >> .debug_info: >> DW_TAG_compile_unit >> ... >> DW_TAG_imported_module >> DW_AT_import [DW_FORM_ref4] (0x20) >> >> DW_TAG_module // uniqued by dsymutil. >> DW_AT_name(“FooLib”) >> DW_AT_LLVM_sysroot(“/“) >> DW_AT_LLVM_include_dirs(“-I/path”) >> DW_AT_LLVM_macros(“-DNDEBUG”) >> ... >> >> // Split DWARF skeleton, thrown out by dsymutil. >> > > Thrown out? Because it's going to read everything in from the module and > merge it in to a single linked debug info blob, I take it? > > >> .debug_info, group 0xFEDB9876, comdat >> DW_TAG_compile_unit >> >> DW_AT_dwo_name(“/tmp/org.llvm.clang/ModuleCache/1234ABCDE/FooLib-XYZ.pcm”) >> DW_AT_dwo_id(“0xFEDB9876”) >> ... >> >> FooLib-XYZ.pcm >> ~~~~~~~~~~~~~~ >> >> .debug_info: >> DW_TAG_compile_unit >> DW_AT_dwo_id(“0xFEDB9876”) >> ... >> >> DW_TAG_module >> DW_AT_name(“FooLib”) >> DW_TAG_module >> DW_AT_name(“SubFoo”) >> ... >> >> -- adrian >> >> > >> >> >> >>> >> >>> > If it turns out that's the right way to get a target for the >> imported_module, we could put both the skeleton CU and the partial unit in >> the same comdat and dedup them both together. >> >>> >> >>> I think this works as long as we only have one TAG_module per .pcm >> file (because we need to refer to it via signature). >> >>> >> >>> Not quite following here - why would we have more than one module per >> pcm - a pcm is a module, right? >> >> >> >> Clang modules may have submodules and a compile unit could import two >> submodules that live in the same .pcm file. For example on Darwin there is >> a module Darwin.pcm that contains a submodule “C" that contains the >> submodule “stdio". >> > >> > OK, so this bit's relevant to your use case in LLDB of loading the >> right things for the right context, but not relevant to the context-less >> debuggers like GDB that will just treat everything as one big namespace >> (except for file-local things, etc). So it's important for your imported >> modules but not for the basic Fission style debug reference. >> > >> > Well, maybe - I'm not sure what you're picturing in terms of the DWARF >> in the module for submodules? If you want that granularity we'll have to >> talk about how to split the DWARF in the module into chunks per submodule? >> > >> >> >> >>> >> >>> But if we don’t mind having duplicate dwo_* references in the same .o >> file this would also work with more than one TAG_module (or submodules). >> >>> >> >>> >> >>> .debug_info: >> >>> DW_TAG_compile_unit >> >>> DW_AT_name(“bar.c”) >> >>> ... >> >>> >> >>> DW_TAG_imported_module // <- This could be optional on ELF. >> >>> DW_AT_import [DW_FORM_ref_sig8] (0xFEDB9876) >> >>> >> >>> ... >> >>> >> >>> // Comdat’d split DWARF skeleton CU for the module Foo. >> >>> .debug_info, group 0xFEDB9876, comdat >> >>> DW_TAG_compile_unit >> >>> >> DW_AT_dwo_name(“/tmp/org.llvm.clang/ModuleCache/1234ABCDE/FooLib-XYZ.pcm”) >> >>> DW_AT_dwo_id(“0xFEDB9876”) >> >>> ... >> >>> >> >>> DW_TAG_module >> >>> DW_AT_name(“FooLib”) >> >>> DW_AT_LLVM_sysroot(“/“) >> >>> DW_AT_LLVM_include_dirs(“-I/path”) >> >>> DW_AT_LLVM_macros(“-DNDEBUG”) >> >>> ... >> >>> >> >>> >> >>> > >> >>> > But this gets into complicated territory when the original binary >> is built with fission... which will be relevant for modules on ELF with >> LLDB. Hmm, maybe it's not too complicated - the partial_unit would end up >> in the .dwo file (maybe we'd have to teach the .dwo file to deduplicate >> these too - the same way it does for type units... - might require a new >> header to include the hash, etc :/)... would be tricky to have the dwp tool >> resolve the relocations to these things. Cross-unit references as you've >> got there aren't something that every DWARF consumer is totally cool with, >> I don't think? >> >>> >> >>> Ah. I thought the deduplication happens because all ELF sections >> sharing the same group are uniqued based on the group id. >> >>> >> >>> COMDAT groups deduplicate for a normal non-fission build, but fission >> linking doesn't require the .dwo file to use/contain COMDATs as it uses a >> DWARF-aware tool (so you don't bother putting the type units in COMDAT >> groups, for example - the fission linker knows how to parse debug_types, >> find the type unit headers and their hashes and deduplicates them that way). >> >> >> >> Ok that makes sense. >> >> >> >> -- adrian >> >> >> >>> >> >>> It certainly would be nice if we could avoid introducing a new >> .debug_info header... >> >>> >> >>> > >> >>> > Sort of inclined to have the imported module stuff just for LLDB, >> but I've lost some of the context for that in the ensuing weeks. >> >>> >> >>> -- adrian >> >>> >> >>> > >> >>> >> >> >>> >> >> >>> >> >> >>> >> >> >>> >> MachO (no typeunits, no comdats, with imports) >> >>> >> ---------------------------------------------- >> >>> >> >> >>> >> Since we don’t have comdat sections in Mach-O and we don’t have >> the tool support for type units, the way that external types can be >> referenced necessarily needs to be a bit different. The design that Greg >> and I came up with for Mach-O relies on llvm-dsymutil to fix up the DWARF >> for non-module-aware consumers. Just as ELF DWARF consumers need not be >> able to tell the difference between module debugging an split DWARF, on >> Mach-O the .dSYM bundle generated by llvm-dsymutil looks like traditional >> DWARF. >> >>> >> >> >>> >> There are three differences in the DWARF output that make this >> possible: >> >>> >> - Refer to external types by UID rather than by type signature. >> >>> >> (This doubles as the key that allows a debugger to look import >> the type >> >>> >> directly from the AST and protects us against hash collisions) >> >>> >> - Add an index to the .o file that maps UID -> module file. >> >>> >> (Fast lookup + UIDs for C and ObjC are only unique within a >> module) >> >>> >> - Add an entry for each type’s UID to the types accelerator >> table. >> >>> >> (Fast lookup) >> >>> >> >> >>> >> bar.o >> >>> >> ~~~~~ >> >>> >> >> >>> >> .debug_info: >> >>> >> DW_TAG_compile_unit >> >>> >> DW_AT_name(“bar.c”) >> >>> >> DW_TAG_imported_module >> >>> >> DW_AT_import(DW_FORM_ref_addr 0x40) >> >>> >> >> >>> >> DW_TAG_variable >> >>> >> DW_AT_name(“MyFoo”) >> >>> >> DW_AT_type [DW_FORM_strp] (“_ZTS3Foo”) // We could use a >> custom FORM here >> >>> >> >> >>> >> // Skeleton unit. >> >>> >> DW_TAG_compile_unit >> >>> >> >> DW_AT_dwo_name(“/tmp/org.llvm.clang/ModuleCache/1234ABCDE/FooLib-XYZ.pcm”) >> >>> >> DW_AT_dwo_id(“0xFEDB9876”) >> >>> >> ... >> >>> >> 0x40: >> >>> >> DW_TAG_module >> >>> >> DW_AT_name(“FooLib”) >> >>> >> DW_AT_LLVM_sysroot(“/“) >> >>> >> DW_AT_LLVM_include_dirs(“-I/path”) >> >>> >> DW_AT_LLVM_macros(“-DNDEBUG”) >> >>> >> >> >>> >> // This index uses the usual accelerator table format. >> >>> >> .apple_exttypes: >> >>> >> { “_ZTS3Foo” => debug_str offset of >> ”/tmp/org.llvm.clang/ModuleCache/1234ABCDE/FooLib-XYZ.pcm” } >> >>> >> >> >>> >> FooLib-XYZ.pcm >> >>> >> ~~~~~~~~~~~~~~ >> >>> >> >> >>> >> .debug_info >> >>> >> DW_TAG_compile_unit >> >>> >> DW_AT_dwo_id(“0xFEDB9876”) >> >>> >> >> >>> >> 0x80: >> >>> >> DW_TAG_structure_type >> >>> >> DW_AT_name (“Foo”) >> >>> >> DW_AT_signature >> >>> >> ... >> >>> >> >> >>> >> // In addition to the entry for “Foo”, there is also an entry for >> the type’s UID “_ZTS3Foo” pointing to the type definition DIE. >> >>> >> .apple_types >> >>> >> { “Foo” => 0x80 } >> >>> >> { “_ZTS3Foo” => 0x80 } >> >>> >> >> >>> >> >> >>> >> >> >>> >> When the debug info linker (llvm-dsymutil) is run, it first pulls >> in the .debug_info section from the clang module and fixes up all the >> DW_FORM_strp external type references by turning them into a >> DW_FORM_ref_addr that references the type in the DW_TAG_compile_unit pulled >> in from the module. To find the correct type DIE it looks up the UID in the >> .apple_exttypes index, finds the module, looks up the UID in the regular >> .apple_types accelerator table and replaces the temporary DW_FROM_strp with >> a DW_FORM_ref_addr (which incidentally takes up the same amount of space in >> the DIE). >> >>> >> >> >>> >> >> >>> >> Thoughts? >> >>> >> -- >> >>> >> adrian >> >>> >> >> >>> > >> >>> >> >> >> >> >> > >> > > >
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