[PATCH 11/11] gdb: optimize all_matching_threads_iterator
Pedro Alves
pedro@palves.net
Mon Jul 5 15:52:44 GMT 2021
On 2021-06-22 5:57 p.m., Simon Marchi via Gdb-patches wrote:
> all_matching_threads_iterator is used extensively in some pretty fast
> paths, often under the all_non_exited_threads function.
>
> If a filter target and thread-specific ptid are given, it iterates on
> all threads of all inferiors of that target, to ultimately yield exactly
> on thread. And this happens quite often, which means we unnecessarily
> spend time iterating on threads to find the one we are looking for. The
> same thing happens if an inferior-specific ptid is given, although there
> the iterator yields all the threads of that inferior.
>
> In those cases, the callers of all_non_exited_threads could have
> different behaviors depending on the kind of ptid, to avoid this
> inefficiency, but that would be very tedious. Using
> all_non_exited_threads has the advantage that one simple implementation
> can work seamlessly on multiple threads or on one specific thread, just
> by playing with the ptid.
>
> Instead, optimize all_matching_threads_iterator directly to detect these
> different cases and limiting what we iterate on to just what we need.
>
> - if filter_ptid is minus_one_ptid, do as we do now: filter inferiors
> based on filter_target, iterate on all of the matching inferiors'
> threads
> - if filter_ptid is a pid-only ptid (then a filter_target must
> necessarily be given), look up that inferior and iterate on all its
> threads
> - otherwise, filter_ptid is a thread-specific ptid, so look up that
> specific thread and "iterate" only on it
>
> For the last case, what was an iteration on all threads of the filter
> target now becomes a call to find_thread_ptid, which is quite efficient
> now thanks to inferior::ptid_thread_map.
>
> gdb/ChangeLog:
>
> * thread-iter.h (class all_matching_threads_iterator)
> <all_matching_threads_iterator>: Use default.
> <enum class mode>: New.
> <m_inf, m_thr>: Initialize.
> <m_filter_ptid>: Remove.
> * thread-iter.c (all_matching_threads_iterator::m_inf_matches):
> Don't filter on m_filter_ptid.
> (all_matching_threads_iterator::all_matching_threads_iterator):
> Choose path based on filter_ptid (all threads, all threads of
> inferior, single thread).
> (all_matching_threads_iterator::advance): Likewise.
OK.
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