[lldb-dev] RFC: Break/Watchpoint refactor

[Zachary Turner via lldb-dev]

On Tue, Sep 27, 2016 at 1:09 PM Daniel Austin Noland via lldb-dev <
lldb-dev at lists.llvm.org> wrote:

> 4. All of these classes are "old school" (not necessarily bad, but
> potentially a problem).  For example:
>    a. lots of const char* running around.
>
We should use llvm::StringRef here.


>    b. DISALLOW_COPY_AND_ASSIGN(BreakpointEventData) to make ctors and
> such private rather than using ctor() = delete (which provides better
> compiler errors)
>    c. use of Error& args in function signatures as opposed to
> Expected<ReturnType>.
>
LLDB already has its own class called Error, and it makes it confusing if
we're going to be using llvm::Error as well.  In an earlier thread I
proposed changing lldb::Error to LLDBError for exactly this reason.  I
would suggest doing this first.


>    d. callback implementation uses function pointers (an ever confusing
> topic, especially for new programmers) where I think we could use
> templated methods (or just a parent Callback class) to significant effect.
>
We should consider using llvm::function_ref<> here.

More details on each of these below.


>
> Phase 2: Restructure / Modernize the Private API / Implementation
> -----------------------------------------------------------------
>
> * Change Error& out parameters to Expected<ReturnType>.
>
As mentioned, we should rename lldb::Error first so as to avoid naming
conflicts with llvm.  Granted, they are each in their own namespace, but
still, it will lead to confusion, and prevents the use of "using namespace
llvm;" as it currently stands.


> * Get rid of all the const char* vars / args in favor of a better string
> type (which?)
>
llvm::StringRef.  Anyone returning a const char* is saying they don't own
the backing memory.  That's exactly what a StringRef is for, but it has
many other benefits such as very efficient searching, substring, and
parsing methods.


> * Prefer explicitly deleted copy ctor / assignments over multiline macro
> DISALLOW_COPY_AND_ASSIGN
> * Try to reduce the (perhaps excessive) use of friendship between the
> support classes.  For instance, is it necessary to give
> BreakpointLocation access to private data members from
> BreakpointLocationList, Process, BreakpointSite, and
> StopInfoBreakpoint?  Wouldn't it be better to expand the functionality
> of those other classes?
>
> A more significant change could be a rewrite of the callback functionality.
>
> There are some problems with the way callbacks are implemented in terms
> of maintainability and extensibility.  I think that using templates and
> simple inheritance we could shift to using callback objects instead of
> function pointers.  I have a crude code sketch of this plan in the works
> now, and I will submit that if there is interest in this idea.
> Essentially, the idea is to let the user define their own Breakpoint or
> Watchpoint (or whatever) child class which overrides a pure virtual run
> method from a parent StoppointCallback templated class.  The template
> parameter of StoppointCallback would allow us to define a
> std::unique_ptr<UserData> member where the user can hang any data they
> want for the callback.  That saves us from void pointers (which I find
> very awkward).
>
I'm not a huge fan of this.  Inheriting from Breakpoint or Watchpoint
sounds old school to me.  Implementation inheritance in general should be
avoided wherever possible IMO.  Also, you've mentioned that
StoppointCallback would only have a template argument, but it's possible
that different types of callback take completely incompatible types and
numbers of arguments.  This is something that can't be described by a
single function with a template parameter.

I think the canonical pattern for what you want here is "double dispatch".
You've got something like this:

struct CallbackArgs {
  virtual void dispatch(StoppointBase &point) = 0;
};
struct WatchpointArgs : public CallbackArgs {
private:
  int Old;
  int New;
public:
  void dispatch(StoppointBase &point) override {
    static_cast<Watchpoint&>(point).Callback(Old, New);
  };
};

struct BreakpointArgs : public CallbackArgs {
private:
  int id;
public:
  void dispatch(StoppointBase &point) override {
    static_cast<Breakpoint&>(point).Callback(id);
  }
};

It's a little bit awkward though, so you can perhaps simplify this by
making use of llvm's casting infrastructure.  Check out
llvm/include/Casting.h.  But basically you could move all this dispatch
logic into a single function that looks like this:

if (auto wp = llvm::dyn_cast<Watchpoint*>(stop_point)) {
  wp->Callback( ... );
} else if (auto bp = llvm::dyn_cast<Breakpoint*>(stop_point)) {
  bp->Callback( ... );
}

And these callback functions could have completely different signatures.



>
> template <class UserData>
> class StoppointCallback {
> private:
>     std::unique_ptr<UserData> m_data;
> // ...
> };
>
Yes, this kind of pattern is good.  Then just pass StoppointUserData& into
the callback and let the caller cast it to the appropriate derived type.


>
> * GDB style tracepoints.  This may be very difficult but it seems very
> desirable.
>

 I don't see why this would be hard.  but I also don't think it's
sufficiently different from a breakpoint that it needs to be *that*
special.  A tracepoint is just a breakpoint that does some stuff and then
automatically continues.  We can already stop at breakpoints, run commands,
and resume commands.  Why can't we just do all of those in a single
scripted operation?  I can envision a command like:

break set -n foo --exec "print p" --exec "break set -n bar" --continue

which sets a breakpoint at foo, and when it gets hit prints the value of p
and sets another breakpoint in bar, then resumes.
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