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

[Greg Clayton via lldb-dev]

> On Jun 13, 2018, at 11:18 AM, Jonas Devlieghere via lldb-dev <lldb-dev at lists.llvm.org> wrote:
> 
> Hi Pavel,
> 
>> On Jun 13, 2018, at 6:56 AM, Pavel Labath <labath at google.com <mailto:labath at google.com>> wrote:
>> 
>> Hello again,
>> 
>> It's been nearly six months since my first email, so it's a good time
>> to recap what has been done here so far. I am happy to report that
>> stages 1-3 (i.e. producer/consumer in llvm and integration with lldb)
>> of my original plan are now complete with one caveat.
> 
> Awesome!
> 
>> The caveat is that the .debug_names section is presently not a full
>> drop-in replacement for the .apple_*** sections. The reason for that
>> is that there is no equivalent to the .apple_objc section (which links
>> an objc class/category name  to all of its methods). I did not
>> implement that, because I do not see a way to embed that kind of
>> information to this section without some sort of an extension. Given
>> that this was not required for my use case, I felt it would be best to
>> leave this to the people working on objc support (*looks at Jonas*) to
>> work out the details of how to represent that.
> 
> Definitely :-) I plan to start working on this (+ dsymutil support) very soon. 

It would be great to add "dsymutil --update" support so it can handle both mach-o files and ELF files. What this does is update the accelerator tables of any files that are specified. That way the accelerator tables can be added to any object file that contains DWARF after the fact.

As David Blaike suggested, maybe we can emit the .apple_objc section as a work around for now. 

> 
>> Nonetheless, I believe that the emitted .debug_names section contains
>> all the data that is required by the standard, and it is sufficient to
>> pass all tests in the lldb integration test suite on linux (this
>> doesn't include objc tests).
> 
> How did you (or do you plan to) add this (and DWARF5 in general) in the lldb test suite? It sounds like this might require a new dimension in the test matrix if we want to test all the variants with both Apple and DWARF style accelerator tables. 
> 
>> Simple benchmarks also show a large
>> performance improvement.I have some numbers to illustrate that
>> (measurements taken by using a release build of lldb to debug a debug
>> build of clang, clang was built with -mllvm -accel-tables=Dwarf to
>> enable the accelerator generation, usage of the tables was controlled
>> by a setting in lldb):
>> - setting a breakpoint on a non-existing function without the use of
>> accelerator tables:
>> real    0m5.554s
>> user    0m43.764s
>> sys     0m6.748s
>> (The majority of this time is spend on building a debug info index,
>> which is a one-shot thing. subsequent breakpoints would be fast)
>> 
>> - setting a breakpoint on a non-existing function with accelerator tables:
>> real    0m3.517s
>> user    0m3.136s
>> sys     0m0.376s
>> (With the index already present, we are able to quickly determine that
>> there is no match and finish)
>> 
>> - setting a breakpoint on all "dump" functions without the use of
>> accelerator tables:
>> real    0m21.544s
>> user    0m59.588s
>> sys     0m6.796s
>> (Apart from building the index, now we must also parse a bunch of
>> compile units and line tables to resolve the breakpoint locations)
>> 
>> - setting a breakpoint on all "dump" functions with accelerator tables:
>> real    0m23.644s
>> user    0m22.692s
>> sys     0m0.948s
>> (Here we see that this extra work is actually the bottleneck now.
>> Preliminary analysis shows that majority of this time is spend
>> inserting line table entries into the middle of a vector, which means
>> it should be possible to fix this with a smarter implementation).
>> 
>> As far as object file sizes go, in the resulting clang binary (2.3GB),
>> the new .debug_names section takes up about 160MB (7%), which isn't
>> negligible, but considering that it supersedes the
>> .debug_pubnames/.debug_pubtypes tables whose combined size is 490MB
>> (21% of the binary), switching to this table (and dropping the other
>> two) will have a positive impact on the binary size. Further
>> reductions can be made by merging the individual indexes into one
>> large index as a part of the link step (which will also increase
>> debugger speed), but it's hard to quantify the exact impact of that.
>> 
>> With all of this in mind, I'd like to encourage you to give the new
>> tables a try. All you need to do is pass -mllvm -accel-tables=Dwarf to
>> clang while building your project. lldb should use the generated
>> tables automatically. I'm particularly interested in the interop
>> scenario. I've checked that readelf is able to make sense of the
>> generated tables, but if you have any other producer/consumer of these
>> tables which is independent of llvm, I'd like to know whether we are
>> compatible with it.
> 
> I know of one internal consumer but it ignores the accelerator tables so I don’t expect any issues there.
> 
>> I'd also like to make the new functionality more easily accessible to
>> users. I am not sure what our policy here is, but I was thinking of
>> either including this functionality in -glldb (on non-apple targets);
>> or by adding a separate -g flag for it (-gdebug-names-section?), with
>> the goal of eventual inclusion into -glldb. I exclude apple targets
>> because: a) they already have a thing that works and the lack of
>> .apple_objc would be a pessimization; b) the different debug info
>> distribution model means it requires more testing and code (dsymutil).
> 
> Changing the default on non-Apple targets sounds good. Once we have Objective-C support I’ll do some additional (internal) testing after which we can consider flipping the switch globally. 
> 
>> For other targets this should bring a big performance improvement when
>> debugging with lldb. The lack of .apple_objc will mean lldb will have
>> to either index the objc compile units manually, or implement a more
>> complicated lookup using other information in the section. However,
>> Objective C is not that widespread outside of apple platforms, so the
>> impact of this should be minimal.
>> 
>> What do you think?
>> 
>> regards,
>> pavel
> 
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