[lldb-dev] LLDB Evolution - Final Form

[Enrico Granata via lldb-dev]

A few remarks

> On Sep 19, 2016, at 1:18 PM, Zachary Turner via lldb-dev <lldb-dev at lists.llvm.org> wrote:
> 
> Following up with Kate's post from a few weeks ago, I think the dust has settled on the code reformat and it went over pretty smoothly for the most part.  So I thought it might be worth throwing out some ideas for where we go from here.  I have a large list of ideas (more ideas than time, sadly) that I've been collecting over the past few weeks, so I figured I would throw them out in the open for discussion.
> 
> I’ve grouped the areas for improvement into 3 high level categories.
> 
> De-inventing the wheel - We should use more code from LLVM, and delete code in LLDB where LLVM provides a solution.  In cases where there is an LLVM thing that is *similar* to what we need, we should extend the LLVM thing to support what we need, and then use it.  Following are some areas I've identified.  This list is by no means complete.  For each one, I've given a personal assessment of how likely it is to cause some (temporary) hiccups, how much it would help us in the long run, and how difficult it would be to do.  Without further ado:
> Use llvm::Regex instead of lldb::Regex
> llvm::Regex doesn’t support enhanced mode.  Could we add support for this to llvm::Regex?
> Risk: 6
> Impact: 3
> Difficulty / Effort: 3  (5 if we have to add enhanced mode support)
> Use llvm streams instead of lldb::StreamString
> Supports output re-targeting (stderr, stdout, std::string, etc), printf style formatting, and type-safe streaming operators.
> Interoperates nicely with many existing llvm utility classes
> Risk: 4
> Impact: 5
> Difficulty / Effort: 7
> Use llvm::Error instead of lldb::Error
> llvm::Error is an error class that *requires* you to check whether it succeeded or it will assert.

I assume that assertion would be stripped in Release builds?
We have our own lldbassert() macro currently, which assert()s in Debug mode, but in Release mode produces an error message and continues
It would be great if llvm::Error allowed us to plug that in..

>   In a way, it's similar to a C++ exception, except that it doesn't come with the performance hit associated with exceptions.  It's extensible, and can be easily extended to support the various ways LLDB needs to construct errors and error messages.
> Would need to first rename lldb::Error to LLDBError so that te conversion from LLDBError to llvm::Error could be done incrementally.
> Risk: 7
> Impact: 7
> Difficulty / Effort: 8
> StringRef instead of const char *, len everywhere
> Can do most common string operations in a way that is guaranteed to be safe.
> Reduces string manipulation algorithm complexity by an order of magnitude.
> Can potentially eliminate tens of thousands of string copies across the codebase.
> Simplifies code.
> Risk: 3
> Impact: 8
> Difficulty / Effort: 7
> ArrayRef instead of const void *, len everywhere
> Same analysis as StringRef
> MutableArrayRef instead of void *, len everywhere
> Same analysis as StringRef

I don't think we have a lot of those - IIRC, it's mostly in the SB API where SWIG is supposed to map it back to a Python string

> Delete ConstString, use a modified StringPool that is thread-safe.
> StringPool is a non thread-safe version of ConstString.
> Strings are internally refcounted so they can be cleaned up when they are no longer used.  ConstStrings are a large source of memory in LLDB, so ref-counting and removing stale strings has the potential to be a huge savings.
> Risk: 2
> Impact: 9
> Difficulty / Effort: 6
> thread_local instead of lldb::ThreadLocal
> This fixes a number of bugs on Windows that cannot be fixed otherwise, as they require compiler support.
> Some other compilers may not support this yet?
> Risk: 2
> Impact: 3
> Difficulty: 3
> Use llvm::cl for the command line arguments to the primary lldb executable.
> Risk: 2
> Impact: 3
> Difficulty / Effort: 4
> Testing - Our testing infrastructure is unstable, and our test coverage is lacking.  We should take steps to improve this.
> Port as much as possible to lit
> Simple tests should be trivial to port to lit today.  If nothing else this serves as a proof of concept while increasing the speed and stability of the test suite, since lit is a more stable harness.
> Separate testing tools
> One question that remains open is how to represent the complicated needs of a debugger in lit tests.  Part a) above covers the trivial cases, but what about the difficult cases?  In https://reviews.llvm.org/D24591 <https://reviews.llvm.org/D24591> a number of ideas were discussed.  We started getting to this idea towards the end, about a separate tool which has an interface independent of the command line interface and which can be used to test.  lldb-mi was mentioned.  While I have serious concerns about lldb-mi due to its poorly written and tested codebase, I do agree in principle with the methodology.  In fact, this is the entire philosophy behind lit as used with LLVM, clang, lld, etc.  
> 
> I don’t take full credit for this idea.  I had been toying with a similar idea for some time, but it was further cemented in an offline discussion with a co-worker.  
> 
> There many small, targeted tools in LLVM (e.g. llc, lli, llvm-objdump, etc) whose purpose are to be chained together to do interesting things.  Instead of a command line api as we think of in LLDB where you type commands from an interactive prompt, they have a command line api as you would expect from any tool which is launched from a shell.
> 
> I can imagine many potential candidates for lldb tools of this nature.  Off the top of my head:
> lldb-unwind - A tool for testing the unwinder.  Accepts byte code as input and passes it through to the unwinder, outputting a compressed summary of the steps taken while unwinding, which could be pattern matched in lit.  The output format is entirely controlled by the tool, and not by the unwinder itself, so it would be stable in the face of changes to the underlying unwinder.  Could have various options to enable or disable features of the unwinder in order to force the unwinder into modes that can be tricky to encounter in the wild.
> lldb-symbol - A tool for testing symbol resolution.  Could have options for testing things like:
> Determining if a symbol matches an executable
> looking up a symbol by name in the debug info, and mapping it to an address in the process.  
> Displaying candidate symbols when doing name lookup in a particular scope (e.g. while stopped at a breakpoint).
> lldb-breakpoint - A tool for testing breakpoints and stepping.  Various options could include:
> Set breakpoints and out addresses and/or symbol names where they were resolved to.
> Trigger commands, so that when a breakpoint is hit the tool could automatically continue and try to run to another breakpoint, etc.
> options to inspect certain useful pieces of state about an inferior, to be matched in lit. 
> lldb-interpreter - tests the jitter etc.  I don’t know much about this, but I don’t see why this couldn’t be tested in a manner similar to how lli is tested.
> lldb-platform - tests lldb local and remote platform interfaces.
> lldb-cli -- lldb interactive command line.
> lldb-format - lldb data formatters etc.
> Tests NOW, not later.
> I know we’ve been over this a million times and it’s not worth going over the arguments again.  And I know it’s hard to write tests, often requiring the invention of new SB APIs.  Hopefully those issues will be addressed by above a) and b) above and writing tests will be easier.  Vedant Kumar ran some analytics on the various codebases and found that LLDB has the lowest test / commit ratio of any LLVM project (He didn’t post numbers for lld, so I’m not sure what it is there).
> 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
> This is an alarming statistic, and I would love to see this number closer to 50%.

I am definitely not innocent in this regard. However, it does happen for a reason.

Sometimes, I am writing code in lldb that is the foundation of something I need to do over on the Swift.org <http://swift.org/> side.

I'll lay out foundational work/groundwork/plumbing code/... on the llvm.org <http://llvm.org/> side of the fence, but that code is a no-op there. The real grunt work happens on the Swift support. It's just architecturally sound to have non-Swift-specific bits happen on the llvm.org <http://llvm.org/> side. When that happens, I have no reasonable way (in the current model) to test the groundwork - it's just an intricate no-op that doesn't get activated.

There are tests. They are on a different repo. It's not great, I'll admit. But right now, I would have to design an API around those bits even though I don't need one, or add commands I don't want "just" for testing. That is polluting a valuable user-facing resource with implementation details. I would gladly jump on a testing infrastructure that lets me write tests for this kind of code without extra API/commands.

> Code style / development conventions - Aside from just the column limitations and bracing styles, there are other areas where LLDB differs from LLVM on code style.  We should continue to adopt more of LLVM's style where it makes sense.  I've identified a couple of areas (incomplete list) which I outline below.  
> Clean up the mess of cyclical dependencies and properly layer the libraries.  This is especially important for things like lldb-server that need to link in as little as possible, but regardless it leads to a more robust architecture, faster build and link times, better testability, and is required if we ever want to do a modules build of LLDB
> Use CMake instead of Xcode project (CMake supports Frameworks).  CMake supports Apple Frameworks, so the main roadblock to getting this working is just someone doing it.  Segmenting the build process by platform doesn't make sense for the upstream, especially when there is a perfectly workable solution.  I have no doubt that the resulting Xcode workspace generated automatically by CMake will not be as "nice" as one that is maintained by hand.  We face this problem with Visual Studio on Windows as well.  The solution that most people have adopted is to continue using the IDE for code editing and debugging, but for actually running the build, use CMake with Ninja.  A similar workflow should still be possible with an OSX CMake build, but as I do not work every day on a Mac, all I can say is that it's possible, I have no idea how impactful it would be on peoples' workflows.  

I am very much in the minority on this issue, but +100
I have no particular allegiance to CMake over Xcode, or vice versa, but I definitely don't see the value in our current multiple build system model.

> Variable naming conventions
> I don’t expect anyone is too fond of LLDB’s naming conventions, but if we’re committed to joining the LLVM ecosystem, then let’s go all the way.
> Use more modern C++ and less C
> Old habits die hard, but this isn’t just a matter of style.  It leads to safer, more robust, and less fragile code as well.
> Shorter functions and classes with more narrowly targeted responsibilities
> It’s not uncommon to find functions that are hundreds (and in a few cases even 1,000+) of lines long.  We really need to be better about breaking functions and classes down into smaller responsibilities.  This helps not just for someone coming in to read the function, but also for testing.  Smaller functions are easier to unit test.
> Convert T foo(X, Y, Error &error) functions to Expected<T> foo(X, Y) style (Depends on 1.c)
> llvm::Expected is based on the llvm::Error class described earlier.  It’s used when a function is supposed to return a value, but it could fail.  By packaging the error with the return value, it’s impossible to have a situation where you use the return value even in case of an error, and because llvm::Error has mandatory checking, it’s also impossible to have a sitaution where you don’t check the error.  So it’s very safe.  
> 
> Whew.  That was a lot.  If you made it this far, thanks for reading!
> 
> Obviously if we were to embark on all of the above, it would take many months to complete everything.  So I'm not proposing anyone stop what they're doing to work on this.  This is just my own personal wishlist
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Thanks,
- Enrico
📩 egranata@.com ☎️ 27683

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