[lldb-dev] [Bug 28910] New: Add frame-finish breakpoints

via lldb-dev lldb-dev at lists.llvm.org
Mon Aug 8 15:17:29 PDT 2016


            Bug ID: 28910
           Summary: Add frame-finish breakpoints
           Product: lldb
           Version: 3.8
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: All Bugs
          Assignee: lldb-dev at lists.llvm.org
          Reporter: christian.convey at gmail.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

It would be helpful for LLDB's Python API to provide a notification when a
particular frame of interest is no longer on the call stack.  

The motivating use-case is a script which performs per-function-activation
analysis, and needs to perform book-keeping when the function of interest is
both activated and completed.

These features in particular would be great:

(a) Users have a reliable mechanism to correlate the activation and completion
of each frame.

(b) Sometimes a frame is completed not because it executed a "return"
statement, but because of a C++ exception or a call to "longjmp".  In these
cases, it would be very helpful if the notification still occurred.  Ideally
the notification would occur before another call was made, on that same thread,
to the function of interest.  But it would still be useful if the notifications
were deferred until the overall thread completed execution.

(c) Performance is better than what one would get by having a script
single-step through the inferior's execution, examining the state of the call
stack after each step.


(1) In a discussion on llvm-dev:
http://lists.llvm.org/pipermail/lldb-dev/2016-August/010865.html , Jim Ingham
provided a Python class which approximates the desired behavior.  Casual
testing on be a novice LLDB user (me) seems to show that his approach works for
normal return paths, but does not detect frame-completions arising from
"longjump" calls, as described above.  The effect of C++ exceptions were not

(2) The lldb command "thread step-out" seems to behave similarly to (1) in the
presence / absence of "longjmp" calls.

(3) GDB provides a "finish" command, which attempts to resume execution
(approximately) until the currently selected stack frame is complete.  That gdb
command seems equivalent to creating a 'gdb.FinishBreakpoint' object in gdb's
Python API.  Testing seems to show that 'gdb.FinishBreakpoint' objects do
eventually get triggered even when a "longjmp" occurred, but perhaps not until
the next time that thread's execution is paused.

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