forked from OSchip/llvm-project
411 lines
21 KiB
ReStructuredText
411 lines
21 KiB
ReStructuredText
Projects
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========
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The following is a mostly unordered set of the ideas for improvements to the
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LLDB debugger. Some are fairly deep, some would require less effort.
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.. contents::
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:local:
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Speed up type realization in lldb
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---------------------------------
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The type of problem I'm addressing here is the situation where you are
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debugging a large program (lldb built with debug clang/swift will do) and you
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go to print a simple expression, and lldb goes away for 30 seconds. When you
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sample it, it is always busily churning through all the CU's in the world
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looking for something. The problem isn't that looking for something in
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particular is slow, but rather that we somehow turned an bounded search (maybe
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a subtype of "std::string" into an unbounded search (all things with the name
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of that subtype.) Or didn't stop when we got a reasonable answer proximate to
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the context of the search, but let the search leak out globally. And quite
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likely there are other issues that I haven't guessed yet. But if you end up
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churning though 3 or 4 Gig of debug info, that's going to be slow no matter how
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well written your debug reader is...
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My guess is the work will be more in the general symbol lookup than in the
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DWARF parser in particular, but it may be a combination of both.
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As a user debugging a largish program, this is the most obvious lameness of
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lldb.
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Symbol name completion in the expression parser
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-----------------------------------------------
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This is the other obvious lameness of lldb. You can do:
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::
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(lldb) frame var foo.b
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and we will tell you it is "foo.bar". But you can't do that in the expression
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parser. This will require collaboration with the clang/swift folks to get the
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right extension points in the compiler. And whatever they are, lldb will need
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use them to tell the compiler about what names are available. It will be
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important to avoid the pitfalls of #1 where we wander into the entire DWARF
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world.
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Make a high speed asynchronous communication channel
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----------------------------------------------------
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All lldb debugging nowadays is done by talking to a debug agent. We used the
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gdb-remote protocol because that is universal, and good enough, and you have to
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support it anyway since so many little devices & JTAG's and VM's etc support
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it. But it is really old, not terribly high performance, and can't really
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handle sending or receiving messages while the process is supposedly running.
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It should have compression built in, remove the hand-built checksums and rely
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on the robust communication protocols we always have nowadays, allow for
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out-of-order requests/replies, allow for reconnecting to a temporarily
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disconnected debug session, regularize all of the packet formatting into JSON
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or BSON or whatever while including a way to do large binary transfers. It must
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be possible to come up with something faster, and better tunable for the many
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communications pathways we end up supporting.
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Fix local variable lookup in the lldb expression parser
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-------------------------------------------------------
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The injection of local variables into the clang expression parser is
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currently done incorrectly - it happens too late in the lookup. This results
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in namespace variables & functions, same named types and ivars shadowing
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locals when it should be the other way around. An attempt was made to fix
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this by manually inserting all the visible local variables into wrapper
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function in the expression text. This mostly gets the job done but that
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method means you have to realize all the types and locations of all local
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variables for even the simplest of expressions, and when run on large
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programs (e.g. lldb) it would cause unacceptable delays. And it was very
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fragile since an error in realizing any of the locals would cause all
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expressions run in that context to fail. We need to fix this by adjusting
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the points where name lookup calls out to lldb in clang.
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Support calling SB & commands everywhere and support non-stop debugging
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-----------------------------------------------------------------------
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There is a fairly ad-hoc system to handle when it is safe to run SB API's and
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command line commands. This is actually a bit of a tricky problem, since we
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allow access to the command line and SB API from some funky places in lldb. The
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Operating System plugins are the most obvious instance, since they get run
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right after lldb is told by debugserver that the process has stopped, but
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before it has finished collating the information from the stop for presentation
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to the higher levels. But breakpoint callbacks have some of the same problems,
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and other things like the scripted stepping operations and any fancier
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extension points we want to add to the debugger are going to be hard to
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implement robustly till we work on a finer-grained and more explicit control
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over who gets to control the process state.
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We also won't have any chance of supporting non-stop debugging - which is a
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useful mode for programs that have a lot of high-priority or real-time worker
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threads - until we get this sorted out.
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Finish the language abstraction and remove all the unnecessary API's
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--------------------------------------------------------------------
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An important part of making lldb a more useful "debugger toolkit" as opposed to
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a C/C++/ObjC/Swift debugger is to have a clean abstraction for language
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support. We did most, but not all, of the physical separation. We need to
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finish that. And then by force of necessity the API's really look like the
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interface to a C++ type system with a few swift bits added on. How you would
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go about adding a new language is unclear and much more trouble than it is
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worth at present. But if we made this nice, we could add a lot of value to
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other language projects.
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Add some syntax to generate data formatters from type definitions
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-----------------------------------------------------------------
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Uses of the data formatters fall into two types. There are data formatters for
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types where the structure elements pretty much tell you how to present the
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data, you just need a little expression language to express how to turn them
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into what the user expects to see. Then there are the ones (like pretty much
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all our Foundation/AppKit/UIKit formatters) that use deep magic to figure out
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how the type is actually laid out. The latter are pretty much always going to
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have to be done by hand.
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But for the ones where the information is expressed in the fields, it would be
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great to have a way to express the instructions to produce summaries and
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children in some form you could embed next to the types and have the compiler
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produce a byte code form of the instructions and then make that available to
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lldb along with the library. This isn't as simple as having clang run over the
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headers and produce something from the types directly. After all, clang has no
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way of knowing that the interesting thing about a std::vector is the elements
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that you get by calling size (for the summary) and [] for the elements. But it
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shouldn't be hard to come up with a generic markup to express this.
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Allow the expression parser to access dynamic type/data formatter information
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-----------------------------------------------------------------------------
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This seems like a smaller one. The symptom is your object is Foo child of
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Bar, and in the Locals view you see all the fields of Foo, but because the
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static type of the object is Bar, you can't see any of the fields of Foo.
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But if you could get this working, you could hijack the mechanism to make
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the results of the value object summaries/synthetic children available to
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expressions. And if you can do that, you could add other properties to an
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object externally (through Python or some other extension point) and then
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have these also available in the expression parser. You could use this to
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express invariants for data structures, or other more advanced uses of types
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in the debugger.
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Another version of this is to allow access to synthetic children in the
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expression parser. Otherwise you end up in situations like:
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::
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(lldb) print return_a_foo()
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(SomeVectorLikeType) $1 = {
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[0] = 0
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[1] = 1
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[2] = 2
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[3] = 3
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[4] = 4
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}
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That's good but:
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::
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(lldb) print return_a_foo()[2]
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fails because the expression parser doesn't know anything about the
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array-like nature of SomeVectorLikeType that it gets from the synthetic
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children.
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Recover thread information lazily
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---------------------------------
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LLDB stores all the user intentions for a thread in the ThreadPlans stored in
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the Thread class. That allows us to reliably implement a very natural model for
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users moving through a debug session. For example, if step-over stops at a
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breakpoint in an function in a younger region of the stack, continue will
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complete the step-over rather than having to manually step out. But that means
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that it is important that the Thread objects live as long as the Threads they
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represent. For programs with many threads, but only one that you are debugging,
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that makes stepping less efficient, since now you have to fetch the thread list
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on every step or stepping doesn't work correctly. This is especially an issue
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when the threads are provided by an Operating System plugin, where it may take
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non-trivial work to reconstruct the thread list. It would be better to fetch
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threads lazily but keep "unseen" threads in a holding area, and only retire
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them when we know we've fetched the whole thread list and ensured they are no
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longer alive.
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Make Python-backed commands first class citizens
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------------------------------------------------
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As it stands, Python commands have no way to advertise their options. They are
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required to parse their arguments by hand. That leads to inconsistency, and
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more importantly means they can't take advantage of auto-generated help and
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command completion. This leaves python-backed commands feeling worse than
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built-in ones.
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As part of this job, it would also be great to hook automatically hook the
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"type" of an option value or argument (e.g. eArgTypeShlibName) to sensible
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default completers. You need to be able to over-ride this in more complicated
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scenarios (like in "break set" where the presence of a "-s" option limits the
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search for completion of a "-n" option.) But in common cases it is unnecessary
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busy-work to have to supply the completer AND the type. If this worked, then it
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would be easier for Python commands to also get correct completers.
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Reimplement the command interpreter commands using the SB API
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-------------------------------------------------------------
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Currently, all the CommandObject::DoExecute methods are implemented using the
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lldb_private API's. That generally means that there's code that gets duplicated
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between the CommandObject and the SB API that does roughly the same thing. We
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would reduce this code duplication, present a single coherent face to the users
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of lldb, and keep ourselves more honest about what we need in the SB API's if
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we implemented the CommandObjects::DoExecute methods using the SB API's.
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BTW, it is only the way it was much easier to develop lldb if it had a
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functioning command-line early on. So we did that first, and developed the SB
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API's when lldb was more mature. There's no good technical reason to have the
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commands use the lldb_private API's.
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Documentation and better examples
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---------------------------------
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We need to put the lldb syntax docs in the tutorial somewhere that is more
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easily accessible. On suggestion is to add non-command based help to the help
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system, and then have a "help lldb" or "help syntax" type command with this
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info. Be nice if the non-command based help could be hierarchical so you could
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make topics.
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There's a fair bit of docs about the SB API's, but it is spotty. Some classes
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are well documented in the Python "help (lldb.SBWhatever)" and some are not.
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We need more conceptual docs. And we need more examples. And we could provide a
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clean pluggable example for using LLDB standalone from Python. The
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process_events.py is a start of this, but it just handles process events, and
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it is really a quick sketch not a polished expandable proto-tool.
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Make a more accessible plugin architecture for lldb
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---------------------------------------------------
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Right now, you can only use the Python or SB API's to extend an extant lldb.
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You can't implement any of the actual lldb Plugins as plugins. That means
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anybody that wants to add new Object file/Process/Language etc support has to
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build and distribute their own lldb. This is tricky because the API's the
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plugins use are currently not stable (and recently have been changing quite a
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lot.) We would have to define a subset of lldb_private that you could use, and
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some way of telling whether the plugins were compatible with the lldb. But
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long-term, making this sort of extension possible will make lldb more appealing
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for research and 3rd party uses.
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Use instruction emulation to reduce the overhead for breakpoints
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----------------------------------------------------------------
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At present, breakpoints are implemented by inserting a trap instruction, then
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when the trap is hit, replace the trap with the actual instruction and single
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step. Then swap back and continue. This causes problems for read only text, and
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also means that no-stop debugging ust either stop all threads briefly to handle
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this two-step or risk missing some breakpoint hits. If you emulated the
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instruction and wrote back the results, you wouldn't have these problems, and
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it would also save a stop per breakpoint hit. Since we use breakpoints to
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implement stepping, this savings could be significant on slow connections.
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Use the JIT to speed up conditional breakpoint evaluation
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---------------------------------------------------------
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We already JIT and cache the conditional expressions for breakpoints for the C
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family of languages, so we aren't re-compiling every time you hit the
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breakpoint. And if we couldn't IR interpret the expression, we leave the JIT'ed
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code in place for reuse. But it would be even better if we could also insert
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the "stop or not" decision into the code at the breakpoint, so you would only
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actually stop the process when the condition was true. Greg's idea was that if
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you had a conditional breakpoint set when you started the debug session, Xcode
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could rebuild and insert enough no-ops that we could instrument the breakpoint
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site and call the conditional expression, and only trap if the conditional was
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true.
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Broaden the idea in "target stop-hook" to cover more events in the debugger
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---------------------------------------------------------------------------
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Shared library loads, command execution, User directed memory/register reads
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and writes are all places where you would reasonably want to hook into the
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debugger.
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Mock classes for testing
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------------------------
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We need "ProcessMock" and "ObjectFileMock" and the like. These would be real
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plugin implementations for their underlying lldb classes, with the addition
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that you can prime them from some sort of text based input files. For classes
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that manage changes over time (like process) you would need to program the
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state at StopPoint 0, StopPoint 1, etc. These could then be used for testing
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reactions to complex threading problems & the like, and also for simulating
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hard-to-test environments (like bare board debugging).
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Expression parser needs syntax for "{symbol,type} A in CU B.cpp"
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----------------------------------------------------------------
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Sometimes you need to specify non-visible or ambiguous types to the expression
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parser. We were planning to do $b_dot_cpp$A or something like. You might want
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to specify a static in a function, in a source file, or in a shared library. So
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the syntax should support all these.
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Add a "testButDontAbort" style test to the UnitTest framework
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-------------------------------------------------------------
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The way we use unittest now (maybe this is the only way it can work, I don't
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know) you can't report a real failure and continue with the test. That is
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appropriate in some cases: if I'm supposed to hit breakpoint A before I
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evaluate an expression, and don't hit breakpoint A, the test should fail. But
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it means that if I want to test five different expressions, I can either do it
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in one test, which is good because it means I only have to fire up one process,
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attach to it, and get it to a certain point. But it also means if the first
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test fails, the other four don't even get run. So though at first we wrote a
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bunch of test like this, as time went on we switched more to writing "one at a
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time" tests because they were more robust against a single failure. That makes
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the test suite run much more slowly. It would be great to add a
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"test_but_dont_abort" variant of the tests, then we could gang tests that all
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drive to the same place and do similar things. As an added benefit, it would
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allow us to be more thorough in writing tests, since each test would have lower
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costs.
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Convert the dotest style tests to use lldbutil.run_to_source_breakpoint
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-----------------------------------------------------------------------
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run_to_source_breakpoint & run_to_name_breakpoint provide a compact API that
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does in one line what the first 10 or 20 lines of most of the old tests now do
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by hand. Using these functions makes tests much more readable, and by
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centralizing common functionality will make maintaining the testsuites easier
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in the future. This is more of a finger exercise, and perhaps best implemented
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by a rule like: "If you touch a test case, and it isn't using
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run_to_source_breakpoint, please make it do so".
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Unify Watchpoint's & Breakpoints
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--------------------------------
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Option handling isn't shared, and more importantly the PerformAction's have a
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lot of duplicated common code, most of which works less well on the Watchpoint
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side.
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Reverse debugging
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-----------------
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This is kind of a holy grail, it's hard to support for complex apps (many
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threads, shared memory, etc.) But it would be SO nice to have...
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Non-stop debugging
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------------------
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By this I mean allowing some threads in the target program to run while
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stopping other threads. This is supported in name in lldb at present, but lldb
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makes the assumption "If I get a stop, I won't get another stop unless I
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actually run the program." in a bunch of places so getting it to work reliably
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will be some a good bit of work. And figuring out how to present this in the UI
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will also be tricky.
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Fix and continue
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----------------
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We did this in gdb without a real JIT. The implementation shouldn't be that
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hard, especially if you can build the executable for fix and continue. The
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tricky part is how to verify that the user can only do the kinds of fixes that
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are safe to do. No changing object sizes is easy to detect, but there were many
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more subtle changes (function you are fixing is on the stack...) that take more
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work to prevent. And then you have to explain these conditions the user in some
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helpful way.
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Unified IR interpreter
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----------------------
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Currently IRInterpreter implements a portion of the LLVM IR, but it doesn't
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handle vector data types and there are plenty of instructions it also doesn't
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support. Conversely, lli supports most of LLVM's IR but it doesn't handle
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remote memory and its function calling support is very rudimentary. It would be
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useful to unify these and make the IR interpreter -- both for LLVM and LLDB --
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better. An alternate strategy would be simply to JIT into the current process
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but have callbacks for non-stack memory access.
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Teach lldb to predict exception propagation at the throw site
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-------------------------------------------------------------
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There are a bunch of places in lldb where we need to know at the point where an
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exception is thrown, what frame will catch the exception.
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For instance, if an expression throws an exception, we need to know whether the
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exception will be caught in the course of the expression evaluation. If so it
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would be safe to let the expression continue. But since we would destroy the
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state of the thread if we let the exception escape the expression, we currently
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stop the expression evaluation if we see a throw. If we knew where it would be
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caught we could distinguish these two cases.
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Similarly, when you step over a call that throws, you want to stop at the throw
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point if you know the exception will unwind past the frame you were stepping in,
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but it would annoying to have the step abort every time an exception was thrown.
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If we could predict the catching frame, we could do this right.
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And of course, this would be a useful piece of information to display when stopped
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at a throw point.
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Add predicates to the nodes of settings
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---------------------------------------
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It would be very useful to be able to give values to settings that are dependent
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on the triple, or executable name, for targets, or on whether a process is local
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or remote, or on the name of a thread, etc. The original intent (and there is
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a sketch of this in the settings parsing code) was to be able to say:
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::
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(lldb) settings set target{arch=x86_64}.process.thread{name=foo}...
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The exact details are still to be worked out, however.
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