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.
> 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] > <mailto:[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). The standard specifically recommends DW_TAG_partial_unit for #include directives so that sounds like a comparatively good match. Partial units were already introduced in DWARF3 so maybe GDB supports them. But even if it doesn’t this shouldn’t necessarily be a problem (unless it crashes). The DW_TAG_imported_unit since this is primarily useful for AST-based debuggers that know how to import a module before expression evaluation. -- adrian > - David > (Sorry about the top-quoting but Outlook can't handle HTML editing properly.) > Unfortunately the gmail client somewhat forces a thread to HTML — gmail quotation markers mysteriously disappear in the plain text version displayed by other mail clients. > --paulr > > > > From: David Blaikie [mailto:[email protected] <mailto:[email protected]>] > Sent: Monday, March 16, 2015 1:36 PM > To: Adrian Prantl > Cc: Richard Smith; Eric Christopher; llvm cfe; Greg Clayton; Robinson, Paul > Subject: Re: [PATCH] Have clang list the imported modules in the debug info > > > > > > > > On Mon, Mar 16, 2015 at 1:24 PM, Adrian Prantl <[email protected] > <mailto:[email protected]>> wrote: > > > > On Mar 10, 2015, at 12:10 PM, David Blaikie <[email protected] > <mailto:[email protected]>> wrote: > > > > > > > > On Tue, Mar 10, 2015 at 12:05 PM, Adrian Prantl <[email protected] > <mailto:[email protected]>> wrote: > > > > On Mar 9, 2015, at 5:16 PM, David Blaikie <[email protected] > <mailto:[email protected]>> wrote: > > > > > > > > On Mon, Mar 9, 2015 at 5:07 PM, Adrian Prantl <[email protected] > <mailto:[email protected]>> wrote: > > > > On Mar 9, 2015, at 2:14 PM, David Blaikie <[email protected] > <mailto:[email protected]>> wrote: > > > > > > > > On Mon, Mar 9, 2015 at 1:52 PM, Adrian Prantl <[email protected] > <mailto:[email protected]>> wrote: > > > > On Feb 24, 2015, at 3:06 PM, David Blaikie <[email protected] > <mailto:[email protected]>> wrote: > > On Tue, Feb 24, 2015 at 2:56 PM, Adrian Prantl <[email protected] > <mailto:[email protected]>> wrote: > > On Feb 24, 2015, at 2:36 PM, David Blaikie <[email protected] > <mailto:[email protected]>> wrote: > > On Mon, Feb 23, 2015 at 3:45 PM, Adrian Prantl <[email protected] > <mailto:[email protected]>> wrote: > > On Feb 23, 2015, at 3:37 PM, David Blaikie <[email protected] > <mailto:[email protected]>> wrote: > > On Mon, Feb 23, 2015 at 3:32 PM, Adrian Prantl <[email protected] > <mailto:[email protected]>> wrote: > > On Feb 23, 2015, at 3:14 PM, David Blaikie <[email protected] > <mailto:[email protected]>> wrote: > > On Mon, Feb 23, 2015 at 3:08 PM, Adrian Prantl <[email protected] > <mailto:[email protected]>> wrote: > > On Feb 23, 2015, at 2:59 PM, David Blaikie <[email protected] > <mailto:[email protected]>> wrote: > > On Mon, Feb 23, 2015 at 2:51 PM, Adrian Prantl <[email protected] > <mailto:[email protected]>> wrote: > > > > On Jan 20, 2015, at 11:07 AM, David Blaikie <[email protected] > > <mailto:[email protected]>> 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 > thehttp://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. > > > But it would import it from DWARF in other TUs rather than use the module > info just because the module wasn't directly referenced from this TU? That > would seem strange to me. (you would lose debug info fidelity (by falling > back to DWARF even though there are modules with the full fidelity info) > unnecessarily, it sounds like) > > > > I think it’s reasonable to expect full fidelity for everything that is > available in the current TU, and having the normal DWARF-based debugging > capabilities for everything beyond that. But we can only ever provide full > fidelity if we have the list of imports for the current TU. > > > > Would it be reasonable to use the accelerator table/index to lookup the > types, then if the type is in the module you could use the module rather than > the DWARF stashed alongside it? (so the comdat'd split-dwarf skeleton CU for > the module would have an index to tell you what names are inside it, but if > you got an index hit you'd just look at the module instead of loading the > split-dwarf debug info in the referenced file) > > > > I don’t think this approach would work for templates and enumerator values; > > > Not sure why enumerator values are an issue - but templates (& all manner of > other things that don't make it into the index, unfortunately), sure. > > > they aren’t in the accelerator tables to begin with. It would also be slower > if the declaration is available in a module. > > > Though you're rapidly going to end up loading a lot of modules in (as you go > up & down a stack printing various things you'll cross into other TUs & load > more modules). > > For a standard DWARF consumer, it seems fine to just have a comdat'd skeleton > CU for a module without the need for other CUs to mention which module CUs > they reference (but I could be wrong here) & that's the design we originally > discussed. > > It would seem unfortunate to bloat every CU with a non-deduplicable list of > every module it references, but if that's necessary for a serialized AST > aware debugger, it might be fine to have it as an option (so long as it can > be turned off) & may still benefit from that list not being the authoritative > module reference, but a /very/ terse reference to it so all the extra flags & > stuff can be in the deduplicable comdat (& to keep it as consistent as > possible between the flag (on/off) codepaths for this extra data). Maybe a > FORM_block (?) of fixed-size hashes of all the modules back-to-back, so it's > as small as possible? > > But I wouldn't mind spending some more time discussing whether there's a > better way to keep these things streamlined/symmetric/the same between > modular and non-modular debug info. > > Sure! > > Now that we established that recording the list of imported modules for every > CU is useful for an AST-based debugger, > > > +Richard, just to see if he's got some ideas about how a debugger might > efficiently use modules to support debugger scenarios and whether or not > having a list of which modules are referenced from which contexts is valuable > in that. > > It still concerns me that this would create something of a > regression/oddity/difference between AST-based debug info (you wouldn't be > able to handle expressions referencing things in other TUs) and non-AST based > debug info (where I think the average user is used to not worrying about what > headers are included in the current file they're debugging when they try to > use a type or other identifier) > > > > If I understood you correctly, this is not actually the case. The list of > imported modules allows the AST-based debugger to import all the modules that > were imported by the CU that the current frame is in. This enables the user > to, e.g., type "p myVector->size()" even though std::vector<MyClass>::size() > was not used by the CU and is thus not available in DWARF. > > If the user types “p foo” even though foo was not defined in any imported > module the debugger can — after failing to import foo via clang — still fall > back to looking up foo in DWARF and do what it always did. > > > If you do the DWARF fallback then you'll get a pretty clear inconsistency > between templates and non-templates. If I have a function foo and a function > template foo_tmpl in one file, and I'm debugging in another file I'll be able > to call 'foo' (normal DWARF fallback/search) but not foo_tmpl (if I'm calling > a new instantiation of foo_tmpl - if I'm calling an existing instantiation > presumably the fallback would catch me). Seems unfortunate/confusing, perhaps. > > > > Good point, but it my guess is that this wouldn’t be any worse than the “why > can’t I print the size() of this vector!?”-situations we have at the moment. > > > Sure - it's strictly better in the sense that there are strictly more > expressions that can be evaluated, but seems incomplete is my point, and > maybe worth considering alternative designs that might be more-betterer. > > > In certain situations (i.e., non-templates) the debugger could use the DWARF > in the modules to print a message about which module to import. > > > > > > > > > > let’s talk about how to most efficiently represent this information. > > > > In the CU, using DW_TAG_imported_module appears to be the most appropriate > choice, even though there is some room for confusion since C++ using > declarations are also represented this way. Inside the > DW_TAG_imported_module, we could use > > (1) a DW_AT_import that references the skeleton (I hope that is the right > terminology) CU for the module, the idea being that the skeleton CU would > contain all the details (flags, name, include dirs, hash, ...) and be in a > comdat'ed section. > > > I'd be concerned about overloading the terminology & confusing other > debuggers - they might try to follow the DW_AT_import and be surprised that > it doesn't refer to a DW_TAG_namespace tag. > > > > That’s a valid concern, and we probably should not be emitting this if we > have any evidence of, e.g., gdb crashes when encountering such a construct. > Then again, we would be using a DW_TAG_imported_module to express what it is > meant to express according to the DWARF spec (namely importing a module)... > but I admit that the tag also does have a very specific meaning for C++, > which we maybe shouldn’t overload. > > > That's my concern, yes. > > > The right thing here is probably to put aside my personal sense of aesthetics > and use a private _LLVM_ namespace for all new additions, and then attempt to > standardize an official DWARF version once we know what is really needed and > what isn't. > > > I'd prefer this, yes. I mean the usual bar we use for language features is > that they're at least proposed for standardization before we adopt them in > clang - I wouldn't mind a similar bar here. If you want to bring up this use > of DW_TAG_imported_module with the DWARF committee & see if it sounds > reasonable (& test/inquire about GDB's behavior here). > > > > I started a thread on dwarf-discuss to this end > (http://lists.dwarfstd.org/listinfo.cgi/dwarf-discuss-dwarfstd.org > <http://lists.dwarfstd.org/listinfo.cgi/dwarf-discuss-dwarfstd.org>, the list > archives are only visible to subscribers, but anyone can subscribe). > > > Cool cool > > > > To paraphrase the replies that my question solicited: We are, perhaps not > very surprising, encouraged to follow the standard and use a > DW_TAG_imported_module that references a DW_TAG_module. If we, however, > choose to describe the module by using a skeleton DW_TAG_compile_unit, we > should be careful (my own words) about using a DW_TAG_imported_module until > that use is sanctioned by the standard. > > > > I see two possible ways to proceed in this spirit: > > a) Rename the module skeleton DW_TAG_compile_units to DW_TAG_module, but keep > all the comdat/split dwarf goodness from the original proposal [1]. My > understanding is that even though we are making clever use of the split DWARF > features, GDB would still need to be taught to follow references to external > files, > > > Not sure what you're referring to here, perhaps a misunderstanding about how > split DWARF works. > > To the best of my knowledge, what we've talked about for module DWARF debug > info is actually just split-dwarf, no extra work required by DWARF consumers*. > > * It's, admittedly, a little tricksy to include type unit references in an > object file that doesn't include the type unit at all - relying on it being > linked into the final executable. But DWARF doesn't really talk about objects > versus executables, etc - so, so long as the type unit is there in the end, > it's valid DWARF no matter how it got there (& should work fine for existing > consumers - they can't tell if the type unit was in every object file that > referenced the type or not once it's been linked and deduplicated). > > > so having it recognize a new tag in this context doesn’t appear to be much > additional effort (but others may provide more insight here). > > > 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?) > > > b) Emit an LLVM-specific DW_AT_LLVM_import attribute inside the > DW_TAG_imported_module (or vice versa) that refers to the skeleton > DW_TAG_compile_unit. > > > > I think that option (a) is a bit more elegant and it is bending the dwarf > standard not quite as much and will make the dwarf output a bit more readable. > > > > -- adrian > > > > [1] Module debugging proposal for reference: > http://lists.cs.uiuc.edu/pipermail/cfe-dev/2014-November/040076.html > <http://lists.cs.uiuc.edu/pipermail/cfe-dev/2014-November/040076.html> > > > > - David > > > > > -- adrian > > > > > > But extension tags seems like the conservatively correct option (not sure > what GDB does on tags it doesn't recognize - I forget if it warns or just > completely ignores them, hopefully the latter) > > > > (2) David’s suggestion of using a custom form that records the module hash > directly is quite space-efficient, but it has the drawback of not being > resilient against small changes to the imported module > > > That's going to be true of the normal fission info here (the skeleton CU and > the full CU in the .dwo file (or module) are associated by hash) - granted, > in the "loading an AST" mode, you can ignore those hashes and rely on your > custom attributes instead. > > > , since clang’s module hash changes each time the module is being rebuilt. > > > Clang's module hash only changes if the DWARF contents change - it doesn't > use a timestamp or anything. It seems like actually you're going to want to > fail to load even more aggressively - there are ways the AST might've changed > that the debug info doesn't reflect but are still important (a type > unreferenced in this module, but built into some other code that is not built > with debug info changes - no hash changes because the debug info for that > type is unreferenced here, but if you try to use it you could have an > incompatible layout, etc). > > > > Agreed: If the module contents changed the debugger needs to display a big > flashing "here be dragons" warning. > > > > > This is less of an issue if the hash is referring to a skeleton CU in the > same file, which contains all the detailed information. > > > > Personally I’d prefer option 1 because mostly uses the existing mechanisms > from DWARF. Here’s a visual guide to the options on the table: > > > > (1) > > foo.o (compiled with, let’s call it .. "-gmodule-imports”) > > ----- > > .debug_info: > > DW_TAG_compile_unit > > DW_AT_name(“foo.c”) > > DW_TAG_imported_module > > DW_AT_import(DW_FORM_ref_addr 0x123) // Could be a FORM_ref_sig8 > 0x1234ABCDE as well. > > DW_TAG_imported_module > > DW_AT_import(...) > > > > .debug_info.dwo: > > // Skeleton CUs for modules imported by foo.o. > > 0x123: > > DW_TAG_compile_unit > > // Used by split-dwarf debuggers to find external type definitions. > > DW_AT_dwo_name(“/tmp/org.llvm.clang/ModuleCache/1234ABCDE/Foundation.pcm”) > > DW_AT_dwo_id(“0x1234ABCDE”) > > > > // Used by AST-based debuggers to import the module. > > DW_AT_name(“Foundation”) > > > (side notes: the mixed indentation here makes it a bit hard to read this > example, and I'd make sure /all/ the extended attributes (including the name > here) use custom attribute names, not standard ones) > > > > Agreed. > > > > > > > DW_AT_LLVM_sysroot(“/“) > > DW_AT_LLVM_include_dir(“”) > > DW_AT_LLVM_macros(“-DNDEBUG”) > > > > (2) > > .debug_info.dwo: > > (As above.) > > > > .debug_info: > > DW_TAG_compile_unit > > DW_AT_name(“foo.c”) > > DW_AT_LLVM_imported_modules(DW_FORM_block 0x1234ABCDE 0xDEADBEEF 0x....) > > > > Now I’m curious what option (3) will look like; the one that we’ll actually > implement! > > > ;) > > > > > -- adrian > > > > > > > > > > > > > >
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