> On Dec 1, 2014, at 10:41 AM, Adrian Prantl <apra...@apple.com> wrote:
>
>>
>> On Dec 1, 2014, at 10:32 AM, Adrian Prantl <apra...@apple.com
>> <mailto:apra...@apple.com>> wrote:
>>
>>
>>> On Dec 1, 2014, at 10:27 AM, Ben Langmuir <blangm...@apple.com
>>> <mailto:blangm...@apple.com>> wrote:
>>>
>>>
>>>> On Nov 25, 2014, at 5:25 PM, Adrian Prantl <apra...@apple.com
>>>> <mailto:apra...@apple.com>> wrote:
>>>>
>>>>>
>>>>> On Nov 24, 2014, at 4:55 PM, Richard Smith <rich...@metafoo.co.uk
>>>>> <mailto:rich...@metafoo.co.uk>> wrote:
>>>>>
>>>>> On Fri, Nov 21, 2014 at 5:52 PM, Adrian Prantl <apra...@apple.com
>>>>> <mailto:apra...@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).
>
> 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
>>>>> <http://clang.llvm.org/docs/Modules.html> and
>>>>> http://clang.llvm.org/docs/PCHInternals.html
>>>>> <http://clang.llvm.org/docs/PCHInternals.html>
>>>>>
>>>>>
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