> On Nov 25, 2014, at 5:25 PM, Adrian Prantl <[email protected]> wrote:
>
>>
>> On Nov 24, 2014, at 4:55 PM, Richard Smith <[email protected]
>> <mailto:[email protected]>> wrote:
>>
>> On Fri, Nov 21, 2014 at 5:52 PM, Adrian Prantl <[email protected]
>> <mailto:[email protected]>> 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.
> 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.
>
>>
>> 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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