Rollup merge of #134029 - Zalathar:zero, r=oli-obk

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

As of #133446, this query (`coverage_ids_info`) determines which counter/expression IDs are unused. So with only a little extra work, we can take the code that was using that information to determine which coverage counters/expressions must be zero, and move that inside the query as well.

There should be no change in compiler output.

compiler/rustc_middle/src/mir/coverage.rs

@@ -3,6 +3,7 @@
 use std::fmt::{self, Debug, Formatter};
 
 use rustc_index::IndexVec;
+use rustc_index::bit_set::BitSet;
 use rustc_macros::{HashStable, TyDecodable, TyEncodable, TypeFoldable, TypeVisitable};
 use rustc_span::Span;
 
@@ -310,3 +311,41 @@ pub struct MCDCDecisionSpan {
     pub decision_depth: u16,
     pub num_conditions: usize,
 }
+
+/// Summarizes coverage IDs inserted by the `InstrumentCoverage` MIR pass
+/// (for compiler option `-Cinstrument-coverage`), after MIR optimizations
+/// have had a chance to potentially remove some of them.
+///
+/// Used by the `coverage_ids_info` query.
+#[derive(Clone, TyEncodable, TyDecodable, Debug, HashStable)]
+pub struct CoverageIdsInfo {
+    pub counters_seen: BitSet<CounterId>,
+    pub zero_expressions: BitSet<ExpressionId>,
+}
+
+impl CoverageIdsInfo {
+    /// Coverage codegen needs to know how many coverage counters are ever
+    /// incremented within a function, so that it can set the `num-counters`
+    /// argument of the `llvm.instrprof.increment` intrinsic.
+    ///
+    /// This may be less than the highest counter ID emitted by the
+    /// InstrumentCoverage MIR pass, if the highest-numbered counter increments
+    /// were removed by MIR optimizations.
+    pub fn num_counters_after_mir_opts(&self) -> u32 {
+        // FIXME(Zalathar): Currently this treats an unused counter as "used"
+        // if its ID is less than that of the highest counter that really is
+        // 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_middle/src/mir/query.rs

@@ -8,7 +8,7 @@ use rustc_abi::{FieldIdx, VariantIdx};
 use rustc_data_structures::fx::FxIndexMap;
 use rustc_errors::ErrorGuaranteed;
 use rustc_hir::def_id::LocalDefId;
-use rustc_index::bit_set::{BitMatrix, BitSet};
+use rustc_index::bit_set::BitMatrix;
 use rustc_index::{Idx, IndexVec};
 use rustc_macros::{HashStable, TyDecodable, TyEncodable, TypeFoldable, TypeVisitable};
 use rustc_span::Span;
@@ -16,7 +16,6 @@ use rustc_span::symbol::Symbol;
 use smallvec::SmallVec;
 
 use super::{ConstValue, SourceInfo};
-use crate::mir;
 use crate::ty::fold::fold_regions;
 use crate::ty::{self, CoroutineArgsExt, OpaqueHiddenType, Ty, TyCtxt};
 
@@ -351,30 +350,3 @@ pub struct DestructuredConstant<'tcx> {
     pub variant: Option<VariantIdx>,
     pub fields: &'tcx [(ConstValue<'tcx>, Ty<'tcx>)],
 }
-
-/// Summarizes coverage IDs inserted by the `InstrumentCoverage` MIR pass
-/// (for compiler option `-Cinstrument-coverage`), after MIR optimizations
-/// have had a chance to potentially remove some of them.
-///
-/// Used by the `coverage_ids_info` query.
-#[derive(Clone, TyEncodable, TyDecodable, Debug, HashStable)]
-pub struct CoverageIdsInfo {
-    pub counters_seen: BitSet<mir::coverage::CounterId>,
-    pub expressions_seen: BitSet<mir::coverage::ExpressionId>,
-}
-
-impl CoverageIdsInfo {
-    /// Coverage codegen needs to know how many coverage counters are ever
-    /// incremented within a function, so that it can set the `num-counters`
-    /// argument of the `llvm.instrprof.increment` intrinsic.
-    ///
-    /// This may be less than the highest counter ID emitted by the
-    /// InstrumentCoverage MIR pass, if the highest-numbered counter increments
-    /// were removed by MIR optimizations.
-    pub fn num_counters_after_mir_opts(&self) -> u32 {
-        // FIXME(Zalathar): Currently this treats an unused counter as "used"
-        // if its ID is less than that of the highest counter that really is
-        // used. Fixing this would require adding a renumbering step somewhere.
-        self.counters_seen.last_set_in(..).map_or(0, |max| max.as_u32() + 1)
-    }
-}

compiler/rustc_middle/src/query/mod.rs

@@ -581,7 +581,7 @@ rustc_queries! {
     /// Summarizes coverage IDs inserted by the `InstrumentCoverage` MIR pass
     /// (for compiler option `-Cinstrument-coverage`), after MIR optimizations
     /// have had a chance to potentially remove some of them.
-    query coverage_ids_info(key: ty::InstanceKind<'tcx>) -> &'tcx mir::CoverageIdsInfo {
+    query coverage_ids_info(key: ty::InstanceKind<'tcx>) -> &'tcx mir::coverage::CoverageIdsInfo {
         desc { |tcx| "retrieving coverage IDs info from MIR for `{}`", tcx.def_path_str(key.def_id()) }
         arena_cache
     }