> On Dec 1, 2014, at 10:50 AM, Ben Langmuir <blangm...@apple.com> wrote:
>
>
>> On Dec 1, 2014, at 10:41 AM, Adrian Prantl <apra...@apple.com
>> <mailto: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).
I was actually hoping that we could rely on deterministic output from clang. If
it is infeasible make ASTWriter output deterministic, we can fall back to
something like the DWARF dwo_id signature here.
-- adrian
>
>>
>> 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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