[lldb-dev] LLDB Evolution

Tue Aug 9 09:46:47 PDT 2016

09.08.2016, 18:42, "Zachary Turner via lldb-dev" <lldb-dev at lists.llvm.org>:
> There are a lot of reasons for the lack of tests.  Off the top of my head, two of the biggest ones are:
>
> 1) Some areas of LLDB have been historically hard to test.  The unwinder and core dumps come to mind.  You can't really just check in an 800MB core dump into the repo.

Side note: core dumps are often higly compressible, so they may have quite reasonable size e.g. in .xz format

> 2) Tests are very heavyweight.  You have to write a Makefile.  You have to write a Python script that uses the SB API.  you have to write a C program.  Then you have to figure out the right incantation of dotest.py to run your test.  Once you've done this many times it becomes easier, but the barrier to entry is high.
>
> #1 can be improved through the use of unit tests and fuzzing.  Sure, it's hard to generate a full blown executable that contains every edge case the unwinder might ever experience and then have it try to unwind through there.  Especially considering that the unwinder behaves differently on every platform.  But it's much more manageable to write a unit test that constructs a particular sequence of bytes in memory, passes it to the unwinder, and checks the return value of some function that is supposed to handle that.  It's still not entirely simple, since the unwinder is partially heuristic, but at least it's more manageable.
>
> There are also ways we can write IR by hand and have llc generate some byte code for us and then pass that to those same functions.  Again, it's not like we can just start doing this overnight, but there are ways.  The question is just how serious of an effort are we prepared to make and how much time are we prepared to put into making this testable versus implementing new features, fixing bugs, etc.
>
> #2 could potentially be improved by lit style tests.  As I mentioned in my last post, think lldbinline style tests.  Not appropriate for everything, but certainly for a lot of things.  If you only had to have one file which is a C program with some annotations in the file, that is a much lower barrier to entry.
>
> Again, the real question is just how much effort are we actually prepared to put into this?  I'd love it if there were entire days or weeks that were just testing weeks, where all we did was add new tests (or refactor code to make it more testable) and people didn't work on anything else.  I've been inactive for a while because I've had to prioritize work on some things in LLVM, but I could make time for something like that.
>
> On Mon, Aug 8, 2016 at 6:12 PM Vedant Kumar via lldb-dev <lldb-dev at lists.llvm.org> wrote:
>> FWIW, as a happy lldb user, it's exciting to read about these planned
>> developments.
>>
>> I have two follow-up questions about the section on 'Testing Strategy
>> Evaluation': (1) what is lldb's policy on including test updates with bug fix
>> commits and functional changes, and (2) how is this policy enforced?
>>
>> AFAICT, it seems that lldb is not as strict about its test policy as other llvm
>> sub-projects. That could be to its detriment. Here are some very rough numbers
>> on the number of commits which include test updates [1]:
>>
>>   - lldb: 287 of the past 1000 commits
>>   - llvm: 511 of the past 1000 commits
>>   - clang: 622 of the past 1000 commits
>>   - compiler-rt: 543 of the past 1000 commits
>>
>> NFC commits make these numbers a bit noisy. But, unless lldb has a much higher
>> ratio of NFC commits to functional changes as compared to other llvm
>> sub-projects, this is a concerning statistic.
>>
>> best,
>> vedant
>>
>> [1] Based on ToT = r278069.
>>
>> HAS_TEST=0
>> TOTAL=1000
>> for HASH in $(git log --oneline -n$TOTAL | cut -d' ' -f1); do
>>   git show --stat $HASH | grep "|" | grep -q test && HAS_TEST=$((HAS_TEST+1))
>> done
>> echo $HAS_TEST "/" $TOTAL
>>
>>> On Aug 8, 2016, at 2:57 PM, Zachary Turner via lldb-dev <lldb-dev at lists.llvm.org> wrote:
>>>
>>>
>>>
>>> On Mon, Aug 8, 2016 at 2:40 PM Kate Stone via lldb-dev <lldb-dev at lists.llvm.org> wrote:
>>> LLDB has come a long way since the project was first announced.  As a robust debugger for C-family languages and Swift, LLDB is constantly in use by millions of developers.  It has also become a foundation for bringing up debugger support for other languages like Go and RenderScript.  In addition to the original macOS implementation the Linux LLDB port is in active use and Windows support has made significant strides.  IDE and editor integration via both SB APIs and MI have made LLDB available to even more users.  It’s definitely a project every contributor can be proud of and I’d like to take a moment to thank everyone who has been involved in one way or another.
>>>
>>> It’s also a project that shows some signs of strain due to its rapid growth.  We’ve accumulated some technical debt that must be paid off, and in general it seems like a good time to reflect on where we'll be headed next.  We’ve outlined a few goals for discussion below as well as one more short-term action.  Discussion is very much encouraged.
>>>
>>> Forward-Looking Goals
>>>
>>>    1. Testing Strategy Evaluation
>>>
>>> Keeping our code base healthy is next to impossible without a robust testing strategy.  Our existing suite of tests is straightforward to run locally, and serves as a foundation for continuous integration.  That said, it is definitely not exhaustive.  Obvious priorities for improvement include gathering coverage information, investing in more conventional unit tests in addition to the suite of end-to-end tests, and introducing tests in code bases where we depend on debugger-specific behavior (e.g.: for expression evaluation.)
>>> I know this is going to be controversial, but I think we should at least do a serious evaluation of whether using the lit infrastructure would work for LLDB.  Conventional wisdom is that it won't work because LLDB tests are fundamentally different than LLVM tests.  I actually completely agree with the latter part.  They are fundamentally different.
>>>
>>> However, we've seen some effort to move towards lldb inline tests, and in a sense that's conceptually exactly what lit tests are.  My concern is that nobody with experience working on LLDB has a sufficient understanding of what lit is capable of to really figure this out.
>>>
>>> I know when I mentioned this some months ago Jonathan Roelofs chimed in and said that he believes lit is extensible enough to support LLDB's use case.  The argument -- if I remember it correctly -- is that the traditional view of what a lit test (i.e. a sequence of commands that checks the output of a program against expected output) is one particular implementation of a lit-style test.  But that you can make your own which do whatever you want.
>>>
>>> This would not just be busy work either.  I think everyone involved with LLDB has experienced flakiness in the test suite.  Sometimes it's flakiness in LLDB itself, but sometimes it is flakiness in the test infrastructure.  It would be nice to completely eliminate one source of flakiness.
>>>
>>> I think it would be worth having some LLDB experts sit down in person with some lit experts and brainstorm ways to make LLDB use lit.
>>>
>>> Certainly it's worth a serious look, even if nothing comes of it.
>>>
>>>
>>>    4. Good Citizenship in the LLVM Community
>>>
>>> Last, but definitely not least, LLDB should endeavor to be a good citizen of the LLVM community.  We should encourage developers to think of the technology stack as a coherent effort, where common code should be introduced at an appropriate level in the stack.  Opportunities to factor reusable aspects of the LLDB code base up the stack into LLVM will be pursued.
>>>
>>> One arbitrary source of inconsistency at present is LLDB’s coding standard.  That brings us to…
>>>
>>> Near-Term Goal: Standardizing on LLVM-style clang-format Rules
>>>
>>> We’ve heard from several in the community that would prefer to have a single code formatting style to further unify the two code bases.  Using clang-format with the default LLVM conventions would simplify code migration, editor configuration, and coding habits for developers who work in multiple LLVM projects.  There are non-trivial implications to reformatting a code base with this much history.  It can obfuscate history and impact downstream projects by complicating merges.  Ideally, it should be done once with as much advance notice as is practical.  Here’s the timeline we’re proposing:
>>>
>>> Today - mechanical reformatting proposed, comment period begins
>>>
>>> To get a preview of what straightforward reformatting of the code looks like, just follow these steps to get a clean copy of the repository and reformat it:
>>>       • Check out a clean copy of the existing repository
>>>       • Edit .clang-format in the root of the tree, remove all but the line “BasedOnStyle: LLVM”
>>>       • Change your current working directory to the root of the tree to reformat
>>>       • Double-check to make sure you did step 3 ;-)
>>>       • Run the following shell command: "find . -name "*.[c,cpp,h] -exec clang-format -i {} +"
>>> Very excited about this one, personally.  While I have my share of qualms with LLVM's style, the benefit of having consistency is hard to overstate.  It greatly reduces the effort to switch between codebases, a direct consequence of which is that it encourages people with LLVM expertise to jump into the LLDB codebase, which hopefully can help to tear down the invisible wall between the two.
>>>
>>> As a personal aside, this allows me to go back to my normal workflow of having 3 edit source files opened simultaneously and tiled horizontally, which is very nice.
>>>
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-- 
Regards,
Konstantin