Axing --breakpoint switch, C-DEBUG-BREAK-AT

Development Debugger
1

I'm killing off features for whom their maintenance is not paying off.

One is the "tick" based usermode --breakpoint feature, implemented on the command line as:

        "--breakpoint" end (
            c-debug-break-at to-integer param-or-die "BREAKPOINT"
        )

The complex problem of taking a "tick" count to break on via a command line switch is that since command line processing is in usermode, you have a kind of observer-affects-the-experiment situation. The code for adding in the breakpoint messes with the tick count you're trying to reproduce.

I had ideas to work around this. So there's the /COMPENSATE feature of C-DEBUG-BREAK, which I don't even know if it works or if my idea is fundamentally flawed:

; Taking a command-line `--breakpoint NNN` parameter is helpful if a
; problem is reproducible, and you have a tick count in hand from a
; panic(), REBSER.tick, Frame.tick, REBVAL.extra.tick, etc.  But there's
; an entanglement issue, as any otherwise-deterministic tick from a prior
; run would be thrown off by the **ticks added by the userspace parameter
; processing of the command-line for `--breakpoint`**!  :-/
;
; The /COMPENSATE option addresses this problem.  Pass it a reasonable
; upper bound for how many ticks you think could have been added to the
; parse, if `--breakpoint` was processed (even though it might not have
; been processed).  Regardless of whether the switch was present or not,
; the tick count rounds up to a reproducible value, using this method:
;
; https://math.stackexchange.com/q/2521219/
;
; At time of writing, 1000 ticks should be *way* more than enough for both
; the PARSE steps and the evaluation steps `--breakpoint` adds.  Yet some
; things could affect this, e.g. a complex userspace TRACE which was
; run during boot.
;
attempt [c-debug-break-at/compensate 1000]  ; fails in release build

Here was more of it:

  #if !defined(NDEBUG) && DEBUG_COUNT_TICKS
    if (REF(compensate)) {
        //
        // Imagine two runs of Rebol console initialization.  In the first,
        // the tick count is 304 when C-DEBUG-BREAK/COMPENSATE is called,
        // right after command line parsing.  Later on a panic() is hit and
        // reports tick count 1020 in the crash log.
        //
        // Wishing to pick apart the bug before it happens, the Rebol Core
        // Developer then re-runs the program with `--breakpoint=1020`, hoping
        // to break at that tick, to catch the downstream appearance of the
        // tick in the panic().  But since command-line processing is in
        // usermode, the addition of the parameter throws off the ticks!
        //
        // https://en.wikipedia.org/wiki/Observer_effect_(physics)
        //
        // Let's say that after the command line processing, it still runs
        // C-DEBUG-BREAK/COMPENSATE, this time at tick 403.  Imagine our goal
        // is to make the parameter to /COMPENSATE something that can be used
        // to conservatively guess the same value to set the tick to, and
        // that /COMPENSATE ARG(bound) that gives a maximum of how far off we
        // could possibly be from the "real" tick. (e.g. "argument processing
        // took no more than 200 additional ticks", which this is consistent
        // with...since 403-304 = 99).
        //
        // The reasoning for why the formula below works for this rounding is
        // given in this StackExchange question and answer:
        //
        // https://math.stackexchange.com/q/2521219/
        //
        Tick one = 1; // MSVC gives misguided warning for cast(Tick, 1)
        TG_tick =
            (one << (ceil_log2(TG_tick) + 1))
            + VAL_INT64(ARG(tick))
            - 1;
        return nullptr;
    }

Also, I've never used the /RELATIVE tick feature, killing that too:

  if (REF(relative))
      TG_break_at_tick = frame_->tick + 1 + VAL_INT64(ARG(tick));

I'm going to be deleting a lot of things, I think, in the pursuit of simplification--now that isotopes are showing the way.

3 Likes