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

[Tom de Vries <tdevries@suse.de>]

On 7/5/21 5:52 PM, Pedro Alves wrote:
> 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.
> 

FTR, this caused an internal-error, filed at
https://sourceware.org/bugzilla/show_bug.cgi?id=28086 .

Thanks,
- Tom