[Lldb-commits] [lldb] Initial step in targets DAP support (PR #86623)

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jimingham wrote:

> @jimingham, thanks for detailed comment.
> 
> "Step in targets" or "Step Into Specific" is an IDE feature that works in two steps:
> 
> 1. Show a list of call sites before stepping to allow users specify which call to step into
> 2. Perform step into with user choosing call site function to stop.
>    https://www.tabsoverspaces.com/233742-step-into-a-specific-call-when-debugging-in-visual-studio contains a good screenshot.
> 
> Note: in step 1, we will have to show users the callsite target function **before** performing stepping. For indirect/virtual call, you can't reliable resolve the target function name without running a smaller interpreter to simulate the execution.
> 
> > I don't think there's any way you are going to know what address "method_call"
> > will resolve to in this context. You'd have to know what "this" actually is, and deal with overload resolution, etc. That does > not sound very doable to me.
> 
> I am not proposing using the function target PC, but the call instruction address in the call site side. For example:
> 
> ```
>     0x55555555a426 <+934>:  callq  0x555555558990 ; bar at main.cpp:64
>     0x55555555a42b <+939>:  movl   %eax, -0xb0(%rbp)
>     0x55555555a431 <+945>:  callq  0x5555555589a0 ; bar2 at main.cpp:68
>     0x55555555a436 <+950>:  movl   -0xb0(%rbp), %edi
>     0x55555555a43c <+956>:  movl   %eax, %esi
>     0x55555555a43e <+958>:  callq  0x555555558970 ; foo at main.cpp:60
> ```
> 
> For the above step range above, if user wants to step into "bar2()" function, I propose us pass `0x55555555a431` as an extra callsite PC to underlying `ThreadPlanStepInRange`, so `ThreadPlanStepInRange::DoWillResume()` can detect the current IP would match the user specified call-site PC `0x55555555a431`. Then `ThreadPlanStepInRange::DefaultShouldStopHereCallback()` can detect the state that there is a previous matching callsite PC, so return true to stop when eFrameCompareYounger.
> 
> Hope this makes sense.

That sounds fine.  I try to stay out of the game of telling people what is going to run in any given range of code because it is pretty hard to get that right from assembly, whereas if you watch execution you are always right.  But if that's what the DAP requires, that seems to be your fate.

It would be fine to add an `m_step_into_target_addr` to ThreadPlanStepInRange and check that in its ShouldStopHereCallback.  Not sure why you mention DoWillResume, that's not where ShouldStopHere gets called, and deciding your step was done then would be way too late in the whole "I've stopped, what should I do next" negotiation.  But if you put it in the ShouldStopHereCallback it will get called at the right time.

https://github.com/llvm/llvm-project/pull/86623