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),
+ }
+}