[lldb-dev] [llvm-dev] Adding DWARF5 accelerator table support to llvm

[Greg Clayton via lldb-dev]

> On Jun 14, 2018, at 9:36 AM, Adrian Prantl <aprantl at apple.com> wrote:
> 
> 
> 
>> On Jun 14, 2018, at 7:01 AM, Pavel Labath via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>> 
>> Thank you all. I am going to try to reply to all comments in a single email.
>> 
>> Regarding the  .apple_objc idea, I am afraid the situation is not as
>> simple as just flipping a switch.
> 
> Jonas is currently working on adding the support for DWARF5-style Objective-C accelerator tables to LLVM/LLDB/dsymutil. Based on the assumption that DWARF 4 and earlier are unaffected by any of this, I don't think it's necessary to spend any effort of making the transition smooth. I'm fine with having Objective-C on DWARF 5 broken on trunk for two weeks until Jonas is done adding Objective-C support to the DWARF 5 implementation.
> 
> 
>> (If it was, I don't think I would
>> have embarked on this adventure in the first place -- I would just
>> emit .apple_*** everywhere and call it done :)). The issue is that the
>> apple tables have assumptions about the macos debug info distribution
>> model hardcoded in them -- they assume they will either stay in the .o
>> file or be linked by a smart debug-info-aware linker (dsymutil). In
>> particular, this means they are not self-delimiting (no length field
>> as is typical for other dwarf artifacts), so if a linker which is not
>> aware of them would simply concatenate individual .o tables (which elf
>> linkers are really good at), the debugger would have no way to pry
>> them apart. And even if it somehow managed that, it still wouldn't
>> know if the indexes covered all of the compile units in the linked
>> file or only some of them (in case some of the object files were
>> compiled with the tables and some without).
>> 
>> In light of that, I don't think it's worth trying to combine
>> .apple_objc with .debug_names in some way, and it would be much
>> simpler to just extend .debug_names with the necessary information. I
>> think the simplest way of achieving this (one which would require
>> least amount of standard-bending) is to take the index entry for the
>> objc class and add a special attribute to it (DW_IDX_method_list?)
>> with form DW_FORM_blockXXX and just have the references to the method
>> DIEs in the block data. This should make the implementation an almost
>> drop-in for the current .apple_objc functionality (we would still need
>> to figure out what to do with category methods, but it's not clear to
>> me whether lldb actually uses those anywhere).
>> 
>> But, other options may be possible as well. What's not clear to me is
>> whether these tables couldn't be replaced by extra information in the
>> .debug_info section. It seems to me that these tables are trying to
>> work around the issue that there is no straight way to go from a
>> DW_TAG_structure type DIE describing an ObjC class to it's methods. If
>> these methods (their forward declarations) were be present as children
>> of the type DIE (as they are for c++ classes), then these tables may
>> not be necessary. But maybe (probably) that has already been
>> considered and deemed infeasible for some reason. In any case this
>> seemed like a thing best left for people who actually work on ObjC
>> support to figure out.
> 
> That's really a question for Greg or Jim — I don't know why the current representation has the Objective-C methods outside of the structs. One reason might be that an interface's implementation can define more methods than are visible in its public interface in the header file, but we already seem to be aware of this and mark the implementation with DW_AT_APPLE_objc_complete_type. I also am not sure that this is the *only* reason for the objc accelerator table. But I'd like to learn.

New categories to objective C classes can be added and I believe that implementations can be spread across multiple files. Not sure why objective C doesn't contain its DW_TAG_subprogram inside of its DW_TAG_class_type, but that is the way things have been for a while.

> 
> -- adrian
> 
>> As far as the .debug_names size goes, I should also point out that the
>> binary in question was built with -fno-limit-debug-info, which isn't a
>> default setup on linux. I have tried measuring the sizes without that
>> flag and with fission enabled (-gsplit-dwarf) and the results are:
>> without compression:
>> - clang binary: 960 MB
>> - .debug_names: 130 MB (13%)
>> - debug_pubnames: 175 MB (18%)
>> - debug_pubtypes: 204 MB (21%)
>> - median time for setting a breakpoint on non-existent function
>> (variance +/- 2%):
>> real 0m3.526s
>> user 0m3.156s
>> sys 0m0.364s
>> 
>> with -Wl,--compress-debug-sections=zlib:
>> - clang binary: 440 MB
>> - .debug_names: 80MB (18%)
>> - .debug_pubnames: 31 MB (7.2%)
>> - .debug_pubtypes: 42MB (9.5%)
>> - median time for setting a breakpoint on non-existent function:
>> real 0m4.369s
>> user 0m3.948s
>> sys 0m0.416s
>> 
>> So, .debug_names indeed compresses worse than .debug_pubnames/types,
>> but that is not surprising as it has a more condensed encoding to
>> begin with (no inline strings). However, even in it's compressed form
>> its size is only slightly larger that the two other sections combined
>> (while being infinitely more useful). As for the compression, my
>> takeaway from this is that compression definitely has a measurable
>> impact on startup time, but, on the grand scale of things, the impact
>> is not actually that big. And if a user deliberately adds the
>> compression flag to his command line, I would assume he really cares
>> about binary size, and is willing to sacrifice some debug performance
>> in return. So, I would honor his request and compress .debug_names as
>> well.
>> 
>> 
>> I have tried David Anderson's dwarfdump (after Paul pointed it out to
>> me), but as far as I can tell, it has no support from printing out the
>> .debug_names section (the print_debug_names function is stubbed out).
>> **I think** I got the correct source repository
>> (git://git.code.sf.net/p/libdwarf/code) as the last commit there is
>> dated yesterday.
>> 
>> 
>> For testing on the lldb side I have been deliberately trying to avoid
>> adding another dimensions to the ever-growing test matrix. I don't
>> think this functionality is worth it, especially not if you view the
>> test suite as a regression test suite. The entire functionality of
>> this in lldb is encompassed in a single .cpp file which is about 250
>> LOC. The class has about a dozen entry points and most of them are
>> accessible through the lldb-test tool, which I've used to write
>> targeted regression tests for this (it could probably use more of
>> those). I did use the "dotest" suite as an integration test suite, but
>> I did that by simply passing --env CFLAGS_EXTRAS="-mllvm
>> -accel-tables=Dwarf" to dotest (I also tried hacking clang to always
>> emit the new tables to make sure I'm not missing anything).
>> Ironically, if you try that now, you will see one test failing, but
>> that's because I have already added one test passing that flag
>> explicitly (I couldn't find a way to test this functionality through
>> lldb-test) and clang then complains about a duplicate argument. This
>> should go away once we have better -g flag to control this behavior. I
>> haven't yet figured out whether I want to set up a bot to run the
>> tests in this configuration, but I know I don't want to inflict that
>> extra overhead on developers running tests during day-to-day
>> development.
>> 
>> regards,
>> pavel
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