[cfe-dev] Plans for module debugging

Richard Smith richard at metafoo.co.uk
Tue Nov 25 18:23:15 PST 2014

Thanks for the below; I think these are good answers, at least for now.

On Tue, 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.
> 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.

I think you're essentially saying "you can't reliably and transparently use
implicit modules + module DWARF + gdb" (at least, not if your module might
get rebuilt or the cache might get purged before you debug). That seems a
little unsatisfying, but if we make it easy to copy the DWARF out of the
module cache and into the binary / a .dwo file in your build area at link
time, I think it's not too bad. Can we add such functionality to the link
step somehow?

One case I'm worried about is that the user does a build, then debugs their
program a bit, then reboots their machine (which happens to wipe out their
/tmp and their module cache), and then they can't debug any more.

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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>> cfe-dev at cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
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