[ENG-1794] Support for start/stop time in read_vcd Command

[stanminlee]

Add -start_time and -stop_time arguments to read_vcd to only count transitions/signals from the specified timestamps.

To do this, we add support for start/stop time to be passed in to relevant functions (most of the changes are for this).

• IncrCounts: We check if we have reached the filter window, only count as we were previously if we are in the window.
• highTime: Function used at the very END for duty cycle calculation
• We cover the case where the signal is 1 before the filter window start, as to not count the time it was 1 before

[linear]

ENG-1794

[greptile-apps]

Greptile Summary

This PR adds -start_time / -end_time filter arguments to read_vcd, threading the values from Tcl through SWIG into a new VcdCount::setFilter() static method that gates both high-time accumulation and transition counting to the requested window.

• P1 — incorrect duty cycle for signals whose transition to '0' falls after end_time: incrCounts always writes back prev_time_ / prev_value_ even when in_window=false. If a signal was '1' inside the window and transitions to '0' at t > end_time, the tail of that high period is never accumulated and highTime(end_time) returns an underestimated value. The test VCD avoids this because all value changes occur at #50; real VCDs will expose incorrect duty cycles.
• P1 — static filter_start_ / filter_end_: sharing filter state as class-level statics works today because setFilter is always called before each parse, but makes every VcdCount instance sensitive to global side-effects and is fragile under future refactoring.

Confidence Score: 3/5

Not safe to merge: the end_time clipping bug will silently produce wrong duty cycles for any real VCD where transitions occur after the specified end_time.

Two P1 findings remain: (1) out-of-window incrCounts calls overwrite prev_value_, discarding the tail of the last high interval and causing incorrect duty cycle calculations; (2) static filter fields are fragile. The test suite passes only because the test VCD has no value changes after any tested end_time boundary.

power/VcdReader.cc — specifically the incrCounts and highTime methods and the static VcdCount::filter_start_/filter_end_ members

Important Files Changed

Filename	Overview
power/VcdReader.cc	Core filtering logic; contains P1 bug where out-of-window transitions overwrite prev_value_, silently dropping the tail of the last high interval, plus static filter fields design smell
power/VcdParse.cc	Correctly pre-sets time_min to start_time and gates the initial setTimeMin call; setTimeMax at end of parseVarValues honours end_time_ when specified
power/Power.tcl	Adds -start_time/-end_time keys with -1 defaults; public flag name inconsistency vs PR description (stop_time vs end_time)
power/Power.i	Adds stdint.i include and int64_t params to read_vcd_file; straightforward SWIG binding update
test/vcd_timestamp.vcd	Test VCD only has value changes at t=0 and t=50 with parallaxsw#100 as an empty timestamp; no transitions after any tested end_time, limiting bug coverage

_{Reviews (2): Last reviewed commit: "ok this is a real fix" | Re-trigger Greptile}

[akashlevy]

@stanminlee can you address greptile and lmk when ready for review?

[stanminlee]

@greptile review my pull request again

[greptile-apps]

Static filter fields make VcdCount non-reentrant across read_vcd calls

filter_start_ and filter_end_ are static class members, so they are shared by every VcdCount instance in the entire process. setFilter() is called once per readActivities(), which is correct for a single sequential call, but introduces a latent issue: if read_vcd is ever invoked without a filter after a previous call that did set a filter, the statics retain the old window values until the next setFilter(). Since setFilter is always called at the top of readActivities() before parsing, this is currently safe — but only by convention. A future refactor that skips the no-filter case could silently re-apply the previous call's filter.

Consider making filter_start_ / filter_end_ instance members (non-static) and passing them via the constructor or a per-instance setFilter call.