Re: [PATCH 11/11] gdb: optimize all_matching_threads_iterator

[Pedro Alves <pedro@palves.net>]

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.