Exploratory Queries with `--tree-callees` in `perf focus`

Matches : 12 Not Matches: 1311 Percentage : 0% ``` Here we used the `..` operator to ask "how often do we have `do_mir_borrowck` on the stack and then, later, some function whose name begins with `rustc::traits`?" (basically, code in that module). It turns out the answer is "almost never" — only 12 samples fit that description (if you ever see *no* samples, that often indicates your query is messed up). If you're curious, you can find out exactly which samples by using the `--print-match` option. This will print out the full backtrace for each sample. The `|` at the front of the line indicates the part that the regular expression matched. ### Example: Where does MIR borrowck spend its time? Often we want to do more "explorational" queries. Like, we know that MIR borrowck is 29% of the time, but where does that time get spent? For that, the `--tree-callees` option is often the best tool. You usually also want to give `--tree-min-percent` or `--tree-max-depth`. The result looks like this: ```bash $ perf focus '{do_mir_borrowck}' --tree-callees --tree-min-percent 3 Matcher : {do_mir_borrowck} Matches : 577 Not Matches: 746 Percentage : 43% Tree | matched `{do_mir_borrowck}` (43% total, 0% self) : | rustc_borrowck::nll::compute_regions (20% total, 0% self) : : | rustc_borrowck::nll::type_check::type_check_internal (13% total, 0% self) : : : | core::ops::function::FnOnce::call_once (5% total, 0% self) : : : : | rustc_borrowck::nll::type_check::liveness::generate (5% total, 3% self) : : : | <rustc_borrowck::nll::type_check::TypeVerifier<'a, 'b, 'tcx> as rustc::mir::visit::Visitor<'tcx>>::visit_mir (3% total, 0% self) : | rustc::mir::visit::Visitor::visit_mir (8% total, 6% self) : | <rustc_borrowck::MirBorrowckCtxt<'cx, 'tcx> as rustc_mir_dataflow::DataflowResultsConsumer<'cx, 'tcx>>::visit_statement_entry (5% total, 0% self) : | rustc_mir_dataflow::do_dataflow (3% total, 0% self) ``` What happens with `--tree-callees` is that - we find each sample matching the regular expression - we look at the code that occurs *after* the regex match and try to build up a call tree

This section explains how to use the `--tree-callees` option in `perf focus` for exploratory queries. It demonstrates how to identify where time is spent after a regex match, effectively building a call tree. The example shows how to find the code executed after `do_mir_borrowck` by using `perf focus '{do_mir_borrowck}' --tree-callees --tree-min-percent 3`. The explanation also includes that the tool finds each sample matching the regular expression, then inspects the code that occurs after the regex match to build up a call tree.