Auto merge of rust-lang#115586 - Zalathar:query, r=cjgillot

coverage: Simplify the `coverageinfo` query

The `coverageinfo` query walks through a `mir::Body`'s statements to find the total number of coverage counter IDs and coverage expression IDs that have been used, as this information is needed by coverage codegen.

This PR makes 3 nice simplifications to that query:
- Extract a common iterator over coverage statements, shared by both coverage-related queries
- Simplify the query's visitor from two passes to just one pass
- Explicitly track the highest seen IDs in the visitor, and only convert to a count right at the end

I also updated some related comments. Some had been invalidated by these changes, while others had already been invalidated by previous coverage changes.

compiler/rustc_mir_transform/src/coverage/query.rs

@@ -1,5 +1,6 @@
 use super::*;
 
+use rustc_data_structures::captures::Captures;
 use rustc_middle::mir::coverage::*;
 use rustc_middle::mir::{self, Body, Coverage, CoverageInfo};
 use rustc_middle::query::Providers;
@@ -12,87 +13,62 @@ pub(crate) fn provide(providers: &mut Providers) {
     providers.covered_code_regions = |tcx, def_id| covered_code_regions(tcx, def_id);
 }
 
-/// The `num_counters` argument to `llvm.instrprof.increment` is the max counter_id + 1, or in
-/// other words, the number of counter value references injected into the MIR (plus 1 for the
-/// reserved `ZERO` counter, which uses counter ID `0` when included in an expression). Injected
-/// counters have a counter ID from `1..num_counters-1`.
-///
-/// `num_expressions` is the number of counter expressions added to the MIR body.
-///
-/// Both `num_counters` and `num_expressions` are used to initialize new vectors, during backend
-/// code generate, to lookup counters and expressions by simple u32 indexes.
+/// Coverage codegen needs to know the total number of counter IDs and expression IDs that have
+/// been used by a function's coverage mappings. These totals are used to create vectors to hold
+/// the relevant counter and expression data, and the maximum counter ID (+ 1) is also needed by
+/// the `llvm.instrprof.increment` intrinsic.
 ///
 /// MIR optimization may split and duplicate some BasicBlock sequences, or optimize out some code
 /// including injected counters. (It is OK if some counters are optimized out, but those counters
 /// are still included in the total `num_counters` or `num_expressions`.) Simply counting the
 /// calls may not work; but computing the number of counters or expressions by adding `1` to the
 /// highest ID (for a given instrumented function) is valid.
 ///
-/// This visitor runs twice, first with `add_missing_operands` set to `false`, to find the maximum
-/// counter ID and maximum expression ID based on their enum variant `id` fields; then, as a
-/// safeguard, with `add_missing_operands` set to `true`, to find any other counter or expression
-/// IDs referenced by expression operands, if not already seen.
-///
-/// Ideally, each operand ID in a MIR `CoverageKind::Expression` will have a separate MIR `Coverage`
-/// statement for the `Counter` or `Expression` with the referenced ID. but since current or future
-/// MIR optimizations can theoretically optimize out segments of a MIR, it may not be possible to
-/// guarantee this, so the second pass ensures the `CoverageInfo` counts include all referenced IDs.
+/// It's possible for a coverage expression to remain in MIR while one or both of its operands
+/// have been optimized away. To avoid problems in codegen, we include those operands' IDs when
+/// determining the maximum counter/expression ID, even if the underlying counter/expression is
+/// no longer present.
 struct CoverageVisitor {
-    info: CoverageInfo,
-    add_missing_operands: bool,
+    max_counter_id: CounterId,
+    max_expression_id: ExpressionId,
 }
 
 impl CoverageVisitor {
-    /// Updates `num_counters` to the maximum encountered counter ID plus 1.
+    /// Updates `max_counter_id` to the maximum encountered counter ID.
     #[inline(always)]
-    fn update_num_counters(&mut self, counter_id: CounterId) {
-        let counter_id = counter_id.as_u32();
-        self.info.num_counters = std::cmp::max(self.info.num_counters, counter_id + 1);
+    fn update_max_counter_id(&mut self, counter_id: CounterId) {
+        self.max_counter_id = self.max_counter_id.max(counter_id);
     }
 
-    /// Updates `num_expressions` to the maximum encountered expression ID plus 1.
+    /// Updates `max_expression_id` to the maximum encountered expression ID.
     #[inline(always)]
-    fn update_num_expressions(&mut self, expression_id: ExpressionId) {
-        let expression_id = expression_id.as_u32();
-        self.info.num_expressions = std::cmp::max(self.info.num_expressions, expression_id + 1);
+    fn update_max_expression_id(&mut self, expression_id: ExpressionId) {
+        self.max_expression_id = self.max_expression_id.max(expression_id);
     }
 
     fn update_from_expression_operand(&mut self, operand: Operand) {
         match operand {
-            Operand::Counter(id) => self.update_num_counters(id),
-            Operand::Expression(id) => self.update_num_expressions(id),
+            Operand::Counter(id) => self.update_max_counter_id(id),
+            Operand::Expression(id) => self.update_max_expression_id(id),
             Operand::Zero => {}
         }
     }
 
     fn visit_body(&mut self, body: &Body<'_>) {
-        for bb_data in body.basic_blocks.iter() {
-            for statement in bb_data.statements.iter() {
-                if let StatementKind::Coverage(box ref coverage) = statement.kind {
-                    if is_inlined(body, statement) {
-                        continue;
-                    }
-                    self.visit_coverage(coverage);
-                }
-            }
+        for coverage in all_coverage_in_mir_body(body) {
+            self.visit_coverage(coverage);
         }
     }
 
     fn visit_coverage(&mut self, coverage: &Coverage) {
-        if self.add_missing_operands {
-            match coverage.kind {
-                CoverageKind::Expression { lhs, rhs, .. } => {
-                    self.update_from_expression_operand(lhs);
-                    self.update_from_expression_operand(rhs);
-                }
-                _ => {}
-            }
-        } else {
-            match coverage.kind {
-                CoverageKind::Counter { id, .. } => self.update_num_counters(id),
-                CoverageKind::Expression { id, .. } => self.update_num_expressions(id),
-                _ => {}
+        match coverage.kind {
+            CoverageKind::Counter { id, .. } => self.update_max_counter_id(id),
+            CoverageKind::Expression { id, lhs, rhs, .. } => {
+                self.update_max_expression_id(id);
+                self.update_from_expression_operand(lhs);
+                self.update_from_expression_operand(rhs);
             }
+            CoverageKind::Unreachable => {}
         }
     }
 }
@@ -101,37 +77,40 @@ fn coverageinfo<'tcx>(tcx: TyCtxt<'tcx>, instance_def: ty::InstanceDef<'tcx>) ->
     let mir_body = tcx.instance_mir(instance_def);
 
     let mut coverage_visitor = CoverageVisitor {
-        info: CoverageInfo { num_counters: 0, num_expressions: 0 },
-        add_missing_operands: false,
+        max_counter_id: CounterId::START,
+        max_expression_id: ExpressionId::START,
     };
 
     coverage_visitor.visit_body(mir_body);
 
-    coverage_visitor.add_missing_operands = true;
-    coverage_visitor.visit_body(mir_body);
-
-    coverage_visitor.info
+    // Add 1 to the highest IDs to get the total number of IDs.
+    CoverageInfo {
+        num_counters: (coverage_visitor.max_counter_id + 1).as_u32(),
+        num_expressions: (coverage_visitor.max_expression_id + 1).as_u32(),
+    }
 }
 
 fn covered_code_regions(tcx: TyCtxt<'_>, def_id: DefId) -> Vec<&CodeRegion> {
     let body = mir_body(tcx, def_id);
-    body.basic_blocks
-        .iter()
-        .flat_map(|data| {
-            data.statements.iter().filter_map(|statement| match statement.kind {
-                StatementKind::Coverage(box ref coverage) => {
-                    if is_inlined(body, statement) {
-                        None
-                    } else {
-                        coverage.code_region.as_ref() // may be None
-                    }
-                }
-                _ => None,
-            })
-        })
+    all_coverage_in_mir_body(body)
+        // Not all coverage statements have an attached code region.
+        .filter_map(|coverage| coverage.code_region.as_ref())
         .collect()
 }
 
+fn all_coverage_in_mir_body<'a, 'tcx>(
+    body: &'a Body<'tcx>,
+) -> impl Iterator<Item = &'a Coverage> + Captures<'tcx> {
+    body.basic_blocks.iter().flat_map(|bb_data| &bb_data.statements).filter_map(|statement| {
+        match statement.kind {
+            StatementKind::Coverage(box ref coverage) if !is_inlined(body, statement) => {
+                Some(coverage)
+            }
+            _ => None,
+        }
+    })
+}
+
 fn is_inlined(body: &Body<'_>, statement: &Statement<'_>) -> bool {
     let scope_data = &body.source_scopes[statement.source_info.scope];
     scope_data.inlined.is_some() || scope_data.inlined_parent_scope.is_some()