[PATCH] Have clang list the imported modules in the debug info

[David Blaikie]

On Mon, Mar 16, 2015 at 2:45 PM, Robinson, Paul <
Paul_Robinson at playstation.sony.com> 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).

- David


>
>
> (Sorry about the top-quoting but Outlook can't handle HTML editing
> properly.)
>
> --paulr
>
>
>
> *From:* David Blaikie [mailto:dblaikie at gmail.com]
> *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 <aprantl at apple.com> wrote:
>
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>  On Mar 10, 2015, at 12:10 PM, David Blaikie <dblaikie at gmail.com> wrote:
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> On Tue, Mar 10, 2015 at 12:05 PM, Adrian Prantl <aprantl at apple.com> wrote:
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>  On Mar 9, 2015, at 5:16 PM, David Blaikie <dblaikie at gmail.com> wrote:
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> On Mon, Mar 9, 2015 at 5:07 PM, Adrian Prantl <aprantl at apple.com> wrote:
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>  On Mar 9, 2015, at 2:14 PM, David Blaikie <dblaikie at gmail.com> wrote:
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> On Mon, Mar 9, 2015 at 1:52 PM, Adrian Prantl <aprantl at apple.com> wrote:
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>  On Feb 24, 2015, at 3:06 PM, David Blaikie <dblaikie at gmail.com> wrote:
>
> On Tue, Feb 24, 2015 at 2:56 PM, Adrian Prantl <aprantl at apple.com> wrote:
>
>  On Feb 24, 2015, at 2:36 PM, David Blaikie <dblaikie at gmail.com> wrote:
>
>  On Mon, Feb 23, 2015 at 3:45 PM, Adrian Prantl <aprantl at apple.com> wrote:
>
>  On Feb 23, 2015, at 3:37 PM, David Blaikie <dblaikie at gmail.com> wrote:
>
>  On Mon, Feb 23, 2015 at 3:32 PM, Adrian Prantl <aprantl at apple.com> wrote:
>
>  On Feb 23, 2015, at 3:14 PM, David Blaikie <dblaikie at gmail.com> wrote:
>
> On Mon, Feb 23, 2015 at 3:08 PM, Adrian Prantl <aprantl at apple.com> wrote:
>
>  On Feb 23, 2015, at 2:59 PM, David Blaikie <dblaikie at gmail.com> wrote:
>
> On Mon, Feb 23, 2015 at 2:51 PM, Adrian Prantl <aprantl at apple.com> wrote:
>
>
> > On Jan 20, 2015, at 11:07 AM, David Blaikie <dblaikie at gmail.com> 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 the
> 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, 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
>
>
>
>
> - 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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