[lldb-dev] RFC: Break/Watchpoint refactor

[Daniel Austin Noland via lldb-dev]

On 09/27/2016 03:23 PM, Zachary Turner wrote:

>
>
>
> On Tue, Sep 27, 2016 at 1:09 PM Daniel Austin Noland via lldb-dev 
> <lldb-dev at lists.llvm.org <mailto: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.
Sounds good to me.  Just wanted to make sure I'm on the same page as 
everyone else.
>
>        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.
I will look into the llvm::function_ref<> as that may be the real answer 
here.
I also tend to agree that inheritance is mostly undesirable.  My 
argument for it in this case is that it would be nice to enforce a 
consistent interface, but that may be better done manually.
I always try to avoid casting, but that may be unrealistic here.
>
>
>     * 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.
That is correct for an infinite speed computer.  The issue is that GDB 
Tracepoints run their instructions without talking to the debugger 
(until the next time the program stops).  This allows you to watch 
variables in applications that are timing sensitive (e.g., you introduce 
a bug in a VOIP application just from the lag associated with watching 
the data).
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