Unrolled build for rust-lang#130263

Rollup merge of rust-lang#130263 - Zalathar:sums, r=compiler-errors

coverage: Simplify creation of sum counters

A small and self-contained improvement, extracted from some larger changes that I'm still working on.

Ultimately I want to avoid creating these sum counter-expressions in some cases (in favour of just adding physical counters directly to the nodes we care about), so a good incremental move towards that is splitting the “gather edge counters” step out from the ”build a sum of those counters” step.

Creating an extra intermediate vector should have negligible cost (and coverage isn't exercised by the benchmark suite anyway). The removed logging is redundant with the `#[instrument(..)]` logging we already have on the underlying method calls.

Author: rust-timer

compiler/rustc_mir_transform/src/coverage/counters.rs

@@ -155,12 +155,14 @@ impl CoverageCounters {
         BcbCounter::Expression { id }
     }
 
-    /// Variant of `make_expression` that makes `lhs` optional and assumes [`Op::Add`].
+    /// Creates a counter that is the sum of the given counters.
     ///
-    /// This is useful when using [`Iterator::fold`] to build an arbitrary-length sum.
-    fn make_sum_expression(&mut self, lhs: Option<BcbCounter>, rhs: BcbCounter) -> BcbCounter {
-        let Some(lhs) = lhs else { return rhs };
-        self.make_expression(lhs, Op::Add, rhs)
+    /// Returns `None` if the given list of counters was empty.
+    fn make_sum(&mut self, counters: &[BcbCounter]) -> Option<BcbCounter> {
+        counters
+            .iter()
+            .copied()
+            .reduce(|accum, counter| self.make_expression(accum, Op::Add, counter))
     }
 
     pub(super) fn num_counters(&self) -> usize {
@@ -315,20 +317,17 @@ impl<'a> MakeBcbCounters<'a> {
         // For each out-edge other than the one that was chosen to get an expression,
         // ensure that it has a counter (existing counter/expression or a new counter),
         // and accumulate the corresponding counters into a single sum expression.
-        let sum_of_all_other_out_edges: BcbCounter = {
-            let _span = debug_span!("sum_of_all_other_out_edges", ?expression_to_bcb).entered();
-            successors
-                .iter()
-                .copied()
-                // Skip the chosen edge, since we'll calculate its count from this sum.
-                .filter(|&to_bcb| to_bcb != expression_to_bcb)
-                .fold(None, |accum, to_bcb| {
-                    let _span = debug_span!("to_bcb", ?accum, ?to_bcb).entered();
-                    let edge_counter = self.get_or_make_edge_counter(from_bcb, to_bcb);
-                    Some(self.coverage_counters.make_sum_expression(accum, edge_counter))
-                })
-                .expect("there must be at least one other out-edge")
-        };
+        let other_out_edge_counters = successors
+            .iter()
+            .copied()
+            // Skip the chosen edge, since we'll calculate its count from this sum.
+            .filter(|&to_bcb| to_bcb != expression_to_bcb)
+            .map(|to_bcb| self.get_or_make_edge_counter(from_bcb, to_bcb))
+            .collect::<Vec<_>>();
+        let sum_of_all_other_out_edges: BcbCounter = self
+            .coverage_counters
+            .make_sum(&other_out_edge_counters)
+            .expect("there must be at least one other out-edge");
 
         // Now create an expression for the chosen edge, by taking the counter
         // for its source node and subtracting the sum of its sibling out-edges.
@@ -375,20 +374,15 @@ impl<'a> MakeBcbCounters<'a> {
 
         // A BCB with multiple incoming edges can compute its count by ensuring that counters
         // exist for each of those edges, and then adding them up to get a total count.
-        let sum_of_in_edges: BcbCounter = {
-            let _span = debug_span!("sum_of_in_edges", ?bcb).entered();
-            // We avoid calling `self.bcb_predecessors` here so that we can
-            // call methods on `&mut self` inside the fold.
-            self.basic_coverage_blocks.predecessors[bcb]
-                .iter()
-                .copied()
-                .fold(None, |accum, from_bcb| {
-                    let _span = debug_span!("from_bcb", ?accum, ?from_bcb).entered();
-                    let edge_counter = self.get_or_make_edge_counter(from_bcb, bcb);
-                    Some(self.coverage_counters.make_sum_expression(accum, edge_counter))
-                })
-                .expect("there must be at least one in-edge")
-        };
+        let in_edge_counters = self.basic_coverage_blocks.predecessors[bcb]
+            .iter()
+            .copied()
+            .map(|from_bcb| self.get_or_make_edge_counter(from_bcb, bcb))
+            .collect::<Vec<_>>();
+        let sum_of_in_edges: BcbCounter = self
+            .coverage_counters
+            .make_sum(&in_edge_counters)
+            .expect("there must be at least one in-edge");
 
         debug!("{bcb:?} gets a new counter (sum of predecessor counters): {sum_of_in_edges:?}");
         self.coverage_counters.set_bcb_counter(bcb, sum_of_in_edges)