coverage: Use a query to find counters/expressions that must be zero

This query (`coverage_ids_info`) already determines which counter/expression
IDs are unused, so it only takes a little extra effort to also determine which
counters/expressions must have a value of zero.

compiler/rustc_codegen_llvm/src/coverageinfo/map_data.rs

@@ -1,9 +1,7 @@
 use rustc_data_structures::captures::Captures;
-use rustc_data_structures::fx::FxIndexSet;
-use rustc_index::bit_set::BitSet;
 use rustc_middle::mir::coverage::{
-    CounterId, CovTerm, CoverageIdsInfo, Expression, ExpressionId, FunctionCoverageInfo, Mapping,
-    MappingKind, Op, SourceRegion,
+    CovTerm, CoverageIdsInfo, Expression, FunctionCoverageInfo, Mapping, MappingKind, Op,
+    SourceRegion,
 };
 use rustc_middle::ty::Instance;
 use tracing::debug;
@@ -55,92 +53,17 @@ impl<'tcx> FunctionCoverageCollector<'tcx> {
         Self { function_coverage_info, ids_info, is_used }
     }
 
-    /// Identify expressions that will always have a value of zero, and note
-    /// their IDs in [`ZeroExpressions`]. Mappings that refer to a zero expression
-    /// can instead become mappings to a constant zero value.
-    ///
-    /// This method mainly exists to preserve the simplifications that were
-    /// already being performed by the Rust-side expression renumbering, so that
-    /// the resulting coverage mappings don't get worse.
-    fn identify_zero_expressions(&self) -> ZeroExpressions {
-        // The set of expressions that either were optimized out entirely, or
-        // have zero as both of their operands, and will therefore always have
-        // a value of zero. Other expressions that refer to these as operands
-        // can have those operands replaced with `CovTerm::Zero`.
-        let mut zero_expressions = ZeroExpressions::default();
-
-        // Simplify a copy of each expression based on lower-numbered expressions,
-        // and then update the set of always-zero expressions if necessary.
-        // (By construction, expressions can only refer to other expressions
-        // that have lower IDs, so one pass is sufficient.)
-        for (id, expression) in self.function_coverage_info.expressions.iter_enumerated() {
-            if !self.is_used || !self.ids_info.expressions_seen.contains(id) {
-                // If an expression was not seen, it must have been optimized away,
-                // so any operand that refers to it can be replaced with zero.
-                zero_expressions.insert(id);
-                continue;
-            }
-
-            // We don't need to simplify the actual expression data in the
-            // expressions list; we can just simplify a temporary copy and then
-            // use that to update the set of always-zero expressions.
-            let Expression { mut lhs, op, mut rhs } = *expression;
-
-            // If an expression has an operand that is also an expression, the
-            // operand's ID must be strictly lower. This is what lets us find
-            // all zero expressions in one pass.
-            let assert_operand_expression_is_lower = |operand_id: ExpressionId| {
-                assert!(
-                    operand_id < id,
-                    "Operand {operand_id:?} should be less than {id:?} in {expression:?}",
-                )
-            };
-
-            // If an operand refers to a counter or expression that is always
-            // zero, then that operand can be replaced with `CovTerm::Zero`.
-            let maybe_set_operand_to_zero = |operand: &mut CovTerm| {
-                if let CovTerm::Expression(id) = *operand {
-                    assert_operand_expression_is_lower(id);
-                }
-
-                if is_zero_term(&self.ids_info.counters_seen, &zero_expressions, *operand) {
-                    *operand = CovTerm::Zero;
-                }
-            };
-            maybe_set_operand_to_zero(&mut lhs);
-            maybe_set_operand_to_zero(&mut rhs);
-
-            // Coverage counter values cannot be negative, so if an expression
-            // involves subtraction from zero, assume that its RHS must also be zero.
-            // (Do this after simplifications that could set the LHS to zero.)
-            if lhs == CovTerm::Zero && op == Op::Subtract {
-                rhs = CovTerm::Zero;
-            }
-
-            // After the above simplifications, if both operands are zero, then
-            // we know that this expression is always zero too.
-            if lhs == CovTerm::Zero && rhs == CovTerm::Zero {
-                zero_expressions.insert(id);
-            }
-        }
-
-        zero_expressions
-    }
-
     pub(crate) fn into_finished(self) -> FunctionCoverage<'tcx> {
-        let zero_expressions = self.identify_zero_expressions();
         let FunctionCoverageCollector { function_coverage_info, ids_info, is_used, .. } = self;
 
-        FunctionCoverage { function_coverage_info, ids_info, is_used, zero_expressions }
+        FunctionCoverage { function_coverage_info, ids_info, is_used }
     }
 }
 
 pub(crate) struct FunctionCoverage<'tcx> {
     pub(crate) function_coverage_info: &'tcx FunctionCoverageInfo,
     ids_info: &'tcx CoverageIdsInfo,
     is_used: bool,
-
-    zero_expressions: ZeroExpressions,
 }
 
 impl<'tcx> FunctionCoverage<'tcx> {
@@ -195,37 +118,6 @@ impl<'tcx> FunctionCoverage<'tcx> {
     }
 
     fn is_zero_term(&self, term: CovTerm) -> bool {
-        !self.is_used || is_zero_term(&self.ids_info.counters_seen, &self.zero_expressions, term)
-    }
-}
-
-/// Set of expression IDs that are known to always evaluate to zero.
-/// Any mapping or expression operand that refers to these expressions can have
-/// that reference replaced with a constant zero value.
-#[derive(Default)]
-struct ZeroExpressions(FxIndexSet<ExpressionId>);
-
-impl ZeroExpressions {
-    fn insert(&mut self, id: ExpressionId) {
-        self.0.insert(id);
-    }
-
-    fn contains(&self, id: ExpressionId) -> bool {
-        self.0.contains(&id)
-    }
-}
-
-/// Returns `true` if the given term is known to have a value of zero, taking
-/// into account knowledge of which counters are unused and which expressions
-/// are always zero.
-fn is_zero_term(
-    counters_seen: &BitSet<CounterId>,
-    zero_expressions: &ZeroExpressions,
-    term: CovTerm,
-) -> bool {
-    match term {
-        CovTerm::Zero => true,
-        CovTerm::Counter(id) => !counters_seen.contains(id),
-        CovTerm::Expression(id) => zero_expressions.contains(id),
+        !self.is_used || self.ids_info.is_zero_term(term)
     }
 }

compiler/rustc_middle/src/mir/coverage.rs

@@ -320,7 +320,7 @@ pub struct MCDCDecisionSpan {
 #[derive(Clone, TyEncodable, TyDecodable, Debug, HashStable)]
 pub struct CoverageIdsInfo {
     pub counters_seen: BitSet<CounterId>,
-    pub expressions_seen: BitSet<ExpressionId>,
+    pub zero_expressions: BitSet<ExpressionId>,
 }
 
 impl CoverageIdsInfo {
@@ -337,4 +337,15 @@ impl CoverageIdsInfo {
         // used. Fixing this would require adding a renumbering step somewhere.
         self.counters_seen.last_set_in(..).map_or(0, |max| max.as_u32() + 1)
     }
+
+    /// Returns `true` if the given term is known to have a value of zero, taking
+    /// into account knowledge of which counters are unused and which expressions
+    /// are always zero.
+    pub fn is_zero_term(&self, term: CovTerm) -> bool {
+        match term {
+            CovTerm::Zero => true,
+            CovTerm::Counter(id) => !self.counters_seen.contains(id),
+            CovTerm::Expression(id) => self.zero_expressions.contains(id),
+        }
+    }
 }

compiler/rustc_mir_transform/src/coverage/query.rs

@@ -1,7 +1,10 @@
 use rustc_data_structures::captures::Captures;
 use rustc_index::bit_set::BitSet;
 use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags;
-use rustc_middle::mir::coverage::{CovTerm, CoverageIdsInfo, CoverageKind, MappingKind};
+use rustc_middle::mir::coverage::{
+    CounterId, CovTerm, CoverageIdsInfo, CoverageKind, Expression, ExpressionId,
+    FunctionCoverageInfo, MappingKind, Op,
+};
 use rustc_middle::mir::{Body, Statement, StatementKind};
 use rustc_middle::query::TyCtxtAt;
 use rustc_middle::ty::{self, TyCtxt};
@@ -87,10 +90,10 @@ fn coverage_ids_info<'tcx>(
 ) -> CoverageIdsInfo {
     let mir_body = tcx.instance_mir(instance_def);
 
-    let Some(fn_cov_info) = mir_body.function_coverage_info.as_ref() else {
+    let Some(fn_cov_info) = mir_body.function_coverage_info.as_deref() else {
         return CoverageIdsInfo {
             counters_seen: BitSet::new_empty(0),
-            expressions_seen: BitSet::new_empty(0),
+            zero_expressions: BitSet::new_empty(0),
         };
     };
 
@@ -123,7 +126,10 @@ fn coverage_ids_info<'tcx>(
         }
     }
 
-    CoverageIdsInfo { counters_seen, expressions_seen }
+    let zero_expressions =
+        identify_zero_expressions(fn_cov_info, &counters_seen, &expressions_seen);
+
+    CoverageIdsInfo { counters_seen, zero_expressions }
 }
 
 fn all_coverage_in_mir_body<'a, 'tcx>(
@@ -141,3 +147,94 @@ 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()
 }
+
+/// Identify expressions that will always have a value of zero, and note
+/// their IDs in a `BitSet`. Mappings that refer to a zero expression
+/// can instead become mappings to a constant zero value.
+///
+/// This function mainly exists to preserve the simplifications that were
+/// already being performed by the Rust-side expression renumbering, so that
+/// the resulting coverage mappings don't get worse.
+fn identify_zero_expressions(
+    fn_cov_info: &FunctionCoverageInfo,
+    counters_seen: &BitSet<CounterId>,
+    expressions_seen: &BitSet<ExpressionId>,
+) -> BitSet<ExpressionId> {
+    // The set of expressions that either were optimized out entirely, or
+    // have zero as both of their operands, and will therefore always have
+    // a value of zero. Other expressions that refer to these as operands
+    // can have those operands replaced with `CovTerm::Zero`.
+    let mut zero_expressions = BitSet::new_empty(fn_cov_info.expressions.len());
+
+    // Simplify a copy of each expression based on lower-numbered expressions,
+    // and then update the set of always-zero expressions if necessary.
+    // (By construction, expressions can only refer to other expressions
+    // that have lower IDs, so one pass is sufficient.)
+    for (id, expression) in fn_cov_info.expressions.iter_enumerated() {
+        if !expressions_seen.contains(id) {
+            // If an expression was not seen, it must have been optimized away,
+            // so any operand that refers to it can be replaced with zero.
+            zero_expressions.insert(id);
+            continue;
+        }
+
+        // We don't need to simplify the actual expression data in the
+        // expressions list; we can just simplify a temporary copy and then
+        // use that to update the set of always-zero expressions.
+        let Expression { mut lhs, op, mut rhs } = *expression;
+
+        // If an expression has an operand that is also an expression, the
+        // operand's ID must be strictly lower. This is what lets us find
+        // all zero expressions in one pass.
+        let assert_operand_expression_is_lower = |operand_id: ExpressionId| {
+            assert!(
+                operand_id < id,
+                "Operand {operand_id:?} should be less than {id:?} in {expression:?}",
+            )
+        };
+
+        // If an operand refers to a counter or expression that is always
+        // zero, then that operand can be replaced with `CovTerm::Zero`.
+        let maybe_set_operand_to_zero = |operand: &mut CovTerm| {
+            if let CovTerm::Expression(id) = *operand {
+                assert_operand_expression_is_lower(id);
+            }
+
+            if is_zero_term(&counters_seen, &zero_expressions, *operand) {
+                *operand = CovTerm::Zero;
+            }
+        };
+        maybe_set_operand_to_zero(&mut lhs);
+        maybe_set_operand_to_zero(&mut rhs);
+
+        // Coverage counter values cannot be negative, so if an expression
+        // involves subtraction from zero, assume that its RHS must also be zero.
+        // (Do this after simplifications that could set the LHS to zero.)
+        if lhs == CovTerm::Zero && op == Op::Subtract {
+            rhs = CovTerm::Zero;
+        }
+
+        // After the above simplifications, if both operands are zero, then
+        // we know that this expression is always zero too.
+        if lhs == CovTerm::Zero && rhs == CovTerm::Zero {
+            zero_expressions.insert(id);
+        }
+    }
+
+    zero_expressions
+}
+
+/// Returns `true` if the given term is known to have a value of zero, taking
+/// into account knowledge of which counters are unused and which expressions
+/// are always zero.
+fn is_zero_term(
+    counters_seen: &BitSet<CounterId>,
+    zero_expressions: &BitSet<ExpressionId>,
+    term: CovTerm,
+) -> bool {
+    match term {
+        CovTerm::Zero => true,
+        CovTerm::Counter(id) => !counters_seen.contains(id),
+        CovTerm::Expression(id) => zero_expressions.contains(id),
+    }
+}