[lldb-dev] [cfe-dev] Plans for module debugging

[Eric Christopher]

On Mon Dec 01 2014 at 11:02:00 AM Ben Langmuir <blangmuir at apple.com> wrote:

> On Dec 1, 2014, at 10:41 AM, Adrian Prantl <aprantl at apple.com> wrote:
>
>
> On Dec 1, 2014, at 10:32 AM, Adrian Prantl <aprantl at apple.com> wrote:
>
>
> On Dec 1, 2014, at 10:27 AM, Ben Langmuir <blangmuir at apple.com> wrote:
>
>
> On Nov 25, 2014, at 5:25 PM, Adrian Prantl <aprantl at apple.com> wrote:
>
>
> On Nov 24, 2014, at 4:55 PM, Richard Smith <richard at metafoo.co.uk> wrote:
>
> On Fri, Nov 21, 2014 at 5:52 PM, Adrian Prantl <aprantl at apple.com> wrote:
>
>> Plans for module debugging
>> ==========================
>>
>> I recently had a chat with Eric Christopher and David Blaikie to discuss
>> ideas for debug info for Clang modules and this is what we came up with.
>>
>> Goals
>> -----
>>
>> Clang modules [1], (and their siblings C++ modules and precompiled header
>> files) are a method for improving compile time by making the serialized AST
>> for commonly-used headers files directly available to the compiler.
>>
>> Currently debug info is totally oblivious to this, when the developer
>> compiles a file that uses a type from a module, clang simply emits a copy
>> of the full definition (some exceptions apply for C++) of this type in
>> DWARF into the debug info section of the resulting object file. That's a
>> lot of copies.
>>
>> The key idea is to emit DWARF for types defined in modules only once, and
>> then only emit references to these types in all the individual compile
>> units that import this module. We are going to build on the split DWARF and
>> type unit facilities provided by DWARF for this. DWARF consumers can follow
>> the type references into module debug info section quite similar to how
>> they resolve types in external type units today. Additionally, the format
>> will allow consumers that support clang modules natively (such as LLDB) to
>> directly look up types in the module, without having to go through the
>> usual translation from AST to DWARF and back to AST.
>>
>> The primary benefit from doing all this is performance. This change is
>> expected to reduce the size of the debug info in object files significantly
>> by
>> - emitting only references to the full types and thus
>> - implicitly uniquing types that are defined in modules.
>> The smaller object files will result in faster compile times and faster
>> llvm::Module load times when doing LTO. The type uniquing will also result
>> in significantly smaller debug info for the finished executables,
>> especially for C and Objective-C, which do not support ODR-based type
>> uniquing. This comes at the price of longer initial module build times, as
>> debug info is emitted alongside the module.
>>
>> Design
>> ------
>>
>> Clang modules are designed to be ephemeral build artifacts that live in a
>> shared module cache. Compiling a source file that imports `MyModule`
>> results in `Module.pcm` to be generated to the module cache directory,
>> which contains the serialized AST of the declarations found in the header
>> files that comprise the module.
>>
>> We will change the binary clang module format to became a container (ELF,
>> Mach-O, depending on the platform). Inside the container there will be
>> multiple sections: one containing the serialized AST, and ones containing
>> DWARF5 split debug type information for all types defined in the module
>> that can be encoded in DWARF. By virtue of using type units, each type is
>> emitted into its own type unit which can be identified via a unique type
>> signature. DWARF consumers can use the type signatures to look up type
>> definitions in the module debug info section. For module-aware consumers
>> (LLDB), we will add an index that maps type signatures directly to an
>> offset in the AST section.
>>
>> For an object file that was built using modules, we need to record the
>> fact that a module has been imported. To this end, we add a
>> DW_TAG_compile_unit into a COMDAT .debug_info.dwo section that references
>> the split DWARF inside the module. Similar to split DWARF objects, the
>> module will be identified by its filename and a checksum. The imported unit
>> also contains a couple of extra attributes holding all the information
>> necessary to recreate the module in case the module cache has been flushed.
>
>
> How does the debugging experience work in this case? When do you trigger
> the (possibly-lengthy) rebuild of the source in order to recreate the DWARF
> for the module (is it possible to delay that until the information is
> needed)?
>
>
> The module debugging scenario is primarily aimed at providing a
> better/faster edit-compile-debug cycle. In this scenario, the module would
> most likely still be in the cache. In a case were the binary was build so
> long ago that the module cache has since been flushed it is generally more
> likely the the user also used a DWARF linking step (such as dsymutil on
> Darwin, and maybe dwz on Linux?) because they did a release/archive build
> which would just copy the DWARF out of the module and store it alongside
> the binary. For this reason I’m not very concerned about the time necessary
> for rebuilding the module. But this is all very platform-specific, and
> different platforms may need different defaults.
>
>
> This description is in terms of building a module that has gone missing,
> but just to be clear: a modules-aware debugger probably also needs to
> rebuild modules that have gone out of date, such as when one of their
> headers is modified.
>
>
> In this case were the module is out of date, the debugger should probably
> fall back to the DWARF types, because it cannot guarantee that the
> modifications to the header files did not change the types it wants to look
> up.
>
>
> Sorry, I just realized that this doesn’t make any sense if the DWARF is
> stored in the module. The behavior should be:
> 1. If the module is missing, recreate the module.
> 2. If the module signature does not match the signature in the .o file,
> either print a large warning that types from that module may be bogus, or
> categorically refuse to use them.
>
>
> Maybe this is described elsewhere, but what is the “signature” being used
> here?  Assuming it depends on the detailed contents of the serialized AST:
> currently ASTWriter output is nondeterministic and things like the ID#s for
> identifiers, types, etc. will change every time you build the module; until
> that gets fixed, we would always hit case (2).
>
>
Perhaps it's a reason to be more rigorous about the ID you put in the
module? :)

That said, yes - if you rebuild the module and the number changes then you
can't rely upon the contents. If we can get a number that assures us that
the contents haven't changed then it'll make sense to use that instead.
Algorithm is up to you on how to determine this.

-eric


>
> For long-term debugging users are expected to use a DWARF linker
> (dsymutil, dwz), which archives all types in a future-proof format (DWARF).
>
> -- adrian
>
>
>
> Delaying the module DWARF output until needed (maybe even by the
> debugger!) is an interesting idea. We should definitely measure how
> expensive it is to emit DWARF for an entire module with of types to see if
> this is worthwhile.
>
> How much knowledge does the debugger have/need of Clang's modules to do
> this? Are we just embedding an arbitrary command that can be run to rebuild
> the .dwo if it's missing? And if so, how do we make that safe when (say)
> root attaches a debugger to an arbitrary process?
>
>
> I think it is reasonable to assume that a consumer that can make use of
> clang modules also knows how to rebuild clang modules, which is why the
> example only contained the name of the module, sysroot, include path, and
> defines; not an arbitrary command. On platforms were the debugger does not
> understand clang modules, the whole problem can be dodged by treating the
> modules as explicit build artifacts.
>
>
> You are probably already aware, but you will need a bunch more information
> (language options, target options, header search options) to rebuild a
> module.
>
>
> Thanks, language options and target options were absent from the list
> previously!
>
> -- adrian
>
>
>
>
> Platforms that treat modules as an explicit build artifact do not have
>> this problem. In the .debug_info section all types that are defined in the
>> module are referenced via their unique type signature using
>> DW_FORM_ref_sig8, just as they would be if this were types from a regular
>> DWARF type unit.
>>
>> Example
>> -------
>>
>> Let's say we have a module `MyModule` that defines a type `MyStruct`::
>>  $ cat foo.c
>>  #include <MyModule.h>
>>  MyStruct x;
>>
>> when compiling `foo.c` like this::
>>  clang -fmodules -gmodules foo.c -c
>>
>> clang produces `foo.o` and an ELF or Mach-O container for the module::
>>  /path/to/module-cache/MyModule.pcm
>>
>> In the module container, we have a section for the serialized AST and a
>> split DWARF sections for the debug type info. The exact format is likely
>> still going to evolve a little, but this should give a rough idea::
>>
>>  MyModule.pcm:
>>   .debug_info.dwo:
>>     DW_TAG_compile_unit
>>       DW_AT_dwo_name ("/path/to/MyModule.pcm")
>>       DW_AT_dwo_id   ([unique AST signature])
>>
>>     DW_TAG_type_unit ([hash for MyStruct])
>>        DW_TAG_structure_type
>>           DW_AT_signature ([hash for MyStruct])
>>           DW_AT_name “MyStruct”
>>           ...
>>
>>   .debug_abbrev.dwo:
>>     // abbrevs referenced by .debug_info.dwo
>>   .debug_line.dwo:
>>     // filenames referenced by .debug_info.dwo
>>   .debug_str.dwo:
>>     // strings referenced by .debug_info.dwo
>>
>>   .ast
>>     // Index at the top of the AST section sorted by hash value.
>>     [hash for MyStruct] -> [offset for MyStruct in this section]
>>     ...
>>     // Serialized AST follows
>>     ...
>>
>> The debug info in foo.o will look like this::
>>
>>  .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])
>>
>>       // For LLDB / dsymutil so they can recreate the module
>>       DW_AT_name “MyModule"
>>       DW_AT_LLVM_system_root "/"
>>       DW_AT_LLVM_preprocessor_defines  "-DNDEBUG"
>>       DW_AT_LLVM_include_path "/path/to/MyModule.map"
>>
>>  .debug_info
>>    DW_TAG_compile_unit
>>      DW_TAG_variable
>>        DW_AT_name "x"
>>        DW_AT_type (DW_FORM_ref_sig8) ([hash for MyStruct])
>>
>>
>> Type signatures
>> ---------------
>>
>> We are going to deviate from the DWARF spec by using a more efficient
>> hashing function that uses the type's unique mangled name and the name of
>> the module as input.
>
>
> Why do you need/want the name of the module here? Modules are not a
> namespacing mechanism. How would you compute this name when the same type
> is defined in multiple imported modules?
>
>
> Great point! I’m mostly concerned about non-ODR languages ...
>
>
> For languages that do not have mangled type names or an ODR,
>
>
> The people working on C modules have expressed an intent to apply the ODR
> there too, so it's not clear that Clang modules will support any such
> language in the longer term.
>
>
> ... and this may be the answer to the question!
>
> +Doug: do Objective-C modules have an ODR?
>
>
> we will use the unique identifiers produces by the clang indexer (USRs) as
>> input instead.
>>
>> Extension: Replacing type units with a more efficient storage format
>> --------------------------------------------------------------------
>>
>> As an extension to this proposal, we are thinking of replacing the type
>> units within the module debug info with a more efficient format: Instead of
>> emitting each type into its own type unit (complete with its entire
>> declcontext), it would be much more more efficient to emit one large bag of
>> DWARF together with an index that maps hash values (type signatures) to DIE
>> offsets.
>>
>> Next steps
>> ----------
>>
>> In order to implement this, the next steps would be as follows:
>> 1. Change the clang module format to be an ELF/Mach-O container.
>> 2. Teach clang to emit debug info for module types (e.g., by passing an
>> empty compile unit with retained types to LLVM) into the module container.
>> 3a. Add a -gmodules switch to clang that triggers the emission of type
>> signatures for types coming from a module.
>>
>
> Can you clarify what this flag would do? Does this turn on adding DWARF to
> the .pcm file? Does it turn off generating DWARF for imported modules in
> the current IR module? Both?
>
>
> It would emit references to the type from imported modules instead of the
> types themselves.
> Since the module cache is shared, we could — depending on just expensive
> this is — turn on DWARF generation for .pcm files by default. I’d like to
> measure this first, though.
>
>
>
> I assume this means that the default remains that we build debug
> information for modules as if we didn't have modules (that is, put complete
> DWARF with the object code). Do you think that's the right long-term
> default? I think it's possibly not.
>
>
> I think you’re absolutely right about the long term. In the short term, it
> may be better to have compatibility by default, but I don’t know what the
> official LLVM policy on new features is, if there is one.
>
>
>
> How does this interact with explicit module builds? Can I use a module
> built without -g in a compile that uses -g? And if I do, do I get complete
> debug information, or debug info just for the parts that aren't in the
> module? Does -gmodules let me choose between these?
>
>
> Personally I would expect old-style (full copy of the types) debug
> information if I build agains a module that does not have embedded debug
> information.
>
> thanks,
> adrian
>
>
> 3b. Implement type-signature-based lookup in llvm-dsymutil and lldb.
>> 4a. Emit an index that maps type signatures to AST section offsets into
>> the module container.
>> 4b. Implement direct loading of AST types in lldb.
>> 5a. Improve the efficiency by replace type units in the module debug info
>> with a lookup table that maps type signatures to DIE offsets.
>> 5b. Support this format in lldb and llvm-dsymutil.
>>
>> Let me know what you think!
>>
>> cheers,
>> Adrian
>>
>> [1] For more details about clang modules see
>> http://clang.llvm.org/docs/Modules.html and
>> http://clang.llvm.org/docs/PCHInternals.html
>>
>>
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