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      1 ---
      2 title: 'SAOC LLDB D integration: 5th Weekly Update'
      3 date: '2021-10-21T01:33:00+01:00'
      4 tags: ['saoc', 'saoc2021', 'dlang', 'llvm', 'lldb', 'debug', 'debugging', 'dwarf']
      5 description: "This post describes what I've done on the 5th week of the
      6 Symmetry Autumn of Code 2021, including follow up on LLVM patches, researching
      7 on DWARF standard tags and attributes, investigation of what debug info is
      8 generated among D compilers, written patches to DMD compiler and small updates
      9 for LLDB plugin."
     10 ---
     12 Hi D community!
     14 I'm here again, to describe what I've done during the fifth week of Symmetry
     15 Autumn of Code.
     17 ## LLVM Patches follow up
     19 During the discussion on some patches, two other patches have been raised,
     20 refactoring the usage of string utilities shared among other demanglers.
     22 -
     23 -
     25 ## Second milestone kickoff
     27 This week I started working on the second milestone, mostly covering DWARF
     28 issues that I discovered. The initial idea is to research a bit about what
     29 debug information is provided by each compiler and understand what can be done,
     30 not only on the LLDB side but also improve information generated by the
     31 compilers.
     33 During this week I also took a look at the DWARF5 standard to understand what
     34 to improve. I discovered very interesting tags that can help generalize the
     35 arrays and strings interaction with the debug info.
     37 To give a bit of context, and more deeply into the DWARF standard,
     38 `DW_TAG_array_type` exists to represent array types. Similarly, there is also a
     39 tag to specifically represent string types. These tags can be used with
     40 `DW_TAG_subrange_type` to represent a so-called "slice" and attributes like
     41 `DW_AT_count` or `DW_AT_upper_bound` can be used to represent the slice length.
     42 Interestingly those attributes can refer to runtime variables and therefore
     43 create debug information for a "dynamic array". I checked other compilers and
     44 only C VLAs are using it.
     46 I wrote a document describing some of the issues and comparing the DWARF
     47 capabilities of the 3 most known D compilers. You can read it
     48 [here](../../public/assets/posts/d-saoc-2021-05/
     50 Some of those issues I managed to fix in the compiler backend, others I filed a
     51 bug. Here is a list of patches and bug reports I created:
     53 -
     54 -
     55 -
     56 -
     57 -
     58 -
     59 -
     61 I have an upcoming patch to the DMD backend that I'm currently working on to
     62 refactor some of the DMD DWARF generator, specifically to the abbreviation
     63 code. My motivation for the refactor is the fact that I spent a lot of time
     64 looking into the backend codebase, implementing PR #13202. Later I realized,
     65 when looking into the DWARF standard, that DWARF abbreviations are all LEB128
     66 encoded and the code is not respecting that. I believe some of the errors
     67 produced by the compiler is due to that.
     69 ## LLVM Plugin
     71 To have pretty-printing capabilities I will need to either implement some of
     72 those missing DWARF tags or/and write an LLDB language plugin. The last bug
     73 report on the list above triggers a discussion in that regard. Very soon I'm
     74 going to write a thread on the LLVM mailing list describing the goals of the
     75 plugin and discuss the possibility of upstreaming it, however, I'm not too
     76 confident that will happen, because Rust among other language plugins being
     77 removed from the upstream for "lacking maintainability".
     79 I already started working on some D integration specifically on the LLDB side,
     80 although I haven't pushed any of this code, since some stuff is still janky
     81 experiments and not properly split up.
     83 ## What is next?
     85 Next week, now that I have some scope about DWARF, I'm going to tackle majorly
     86 on the LLDB integration, since that is what the milestone is focused on. I'm
     87 going to clean up the idea I have of the language plugin and hopefully show
     88 some interesting practical results.
     90 You can also read this on the D programming language forum,
     91 [here](,
     92 and discuss there.
     94 Read about the [previous week](../d-saoc-2021-04/) and [next
     95 week](../d-saoc-2021-06/).