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commit 2609773618f3c434879b04ae53735bab47c0d95c
parent ed11ad69e137e489433205a1fc6c6adca9281a78
Author: Luís Ferreira <[email protected]>
Date:   Thu, 21 Oct 2021 02:36:30 +0100

posts: Add 'SAOC LLDB D integration: 5th Weekly Update'

Signed-off-by: Luís Ferreira <[email protected]>

Mcontent/posts/ | 3++-
Acontent/posts/ | 90+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 92 insertions(+), 1 deletion(-)

diff --git a/content/posts/ b/content/posts/ @@ -87,4 +87,5 @@ mentor. On the meanwhile, I made my TODO list open and automatically deployed, at least daily, so you can see what I'm doing during the week [here]( . -Read about the [previous week](../d-saoc-2021-03/). +Read about the [previous week](../d-saoc-2021-03/) and [next +week](../d-saoc-2021-05/). diff --git a/content/posts/ b/content/posts/ @@ -0,0 +1,90 @@ +--- +title: 'SAOC LLDB D integration: 5th Weekly Update' +date: '2021-10-21T01:33:00+01:00' +tags: ['saoc', 'saoc2021', 'dlang', 'llvm', 'lldb', 'debug', 'debugging', 'dwarf'] +description: "This post describes what I've done on the 5th week of the +Symmetry Autumn of Code 2021, including follow up on LLVM patches, researching +on DWARF standard tags and attributes, investigation of what debug info is +generated among D compilers, written patches to DMD compiler and small updates +for LLDB plugin." +--- + +Hi D community! + +I'm here again, to describe what I've done during the fifth week of Symmetry +Autumn of Code. + +## LLVM Patches follow up + +During the discussion on some patches, two other patches have been raised, +refactoring the usage of string utilities shared among other demanglers. + +- +- + +## Second milestone kickoff + +This week I started working on the second milestone, mostly covering DWARF +issues that I discovered. The initial idea is to research a bit about what +debug information is provided by each compiler and understand what can be done, +not only on the LLDB side but also improve information generated by the +compilers. + +During this week I also took a look at the DWARF5 standard to understand what +to improve. I discovered very interesting tags that can help generalize the +arrays and strings interaction with the debug info. + +To give a bit of context, and more deeply into the DWARF standard, +`DW_TAG_array_type` exists to represent array types. Similarly, there is also a +tag to specifically represent string types. These tags can be used with +`DW_TAG_subrange_type` to represent a so-called "slice" and attributes like +`DW_AT_count` or `DW_AT_upper_bound` can be used to represent the slice length. +Interestingly those attributes can refer to runtime variables and therefore +create debug information for a "dynamic array". I checked other compilers and +only C VLAs are using it. + +I wrote a document describing some of the issues and comparing the DWARF +capabilities of the 3 most known D compilers. You can read it +[here]( + +Some of those issues I managed to fix in the compiler backend, others I filed a +bug. Here is a list of patches and bug reports I created: + +- +- +- +- +- +- +- + +I have an upcoming patch to the DMD backend that I'm currently working on to +refactor some of the DMD DWARF generator, specifically to the abbreviation +code. My motivation for the refactor is the fact that I spent a lot of time +looking into the backend codebase, implementing PR #13202. Later I realized, +when looking into the DWARF standard, that DWARF abbreviations are all LEB128 +encoded and the code is not respecting that. I believe some of the errors +produced by the compiler is due to that. + +## LLVM Plugin + +To have pretty-printing capabilities I will need to either implement some of +those missing DWARF tags or/and write an LLDB language plugin. The last bug +report on the list above triggers a discussion in that regard. Very soon I'm +going to write a thread on the LLVM mailing list describing the goals of the +plugin and discuss the possibility of upstreaming it, however, I'm not too +confident that will happen, because Rust among other language plugins being +removed from the upstream for "lacking maintainability". + +I already started working on some D integration specifically on the LLDB side, +although I haven't pushed any of this code, since some stuff is still janky +experiments and not properly split up. + +## What is next? + +Next week, now that I have some scope about DWARF, I'm going to tackle majorly +on the LLDB integration, since that is what the milestone is focused on. I'm +going to clean up the idea I have of the language plugin and hopefully show +some interesting practical results. + +Read about the [previous week](../d-saoc-2021-04/).