[lldb-dev] LLDB Evolution - Final Form
Zachary Turner via lldb-dev
lldb-dev at lists.llvm.org
Mon Sep 19 13:18:19 PDT 2016
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?
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
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. In a way, it's similar to a
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
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
Can potentially eliminate tens of thousands of string copies
across the codebase.
Difficulty / Effort: 7
ArrayRef instead of const void *, len everywhere
Same analysis as StringRef
MutableArrayRef instead of void *, len everywhere
Same analysis as StringRef
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.
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?
Use llvm::cl for the command line arguments to the primary lldb
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
but what about the difficult cases? In
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
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
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
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
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
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
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
just someone doing it. Segmenting the build process by platform doesn't
make sense for the upstream, especially when there is a
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
using the IDE for code editing and debugging, but for actually
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
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
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
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
breaking functions and classes down into smaller
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,
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
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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