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

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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.
This commit is contained in:
Zalathar 2024-12-06 22:33:24 +11:00
parent f3f7c20f7b
commit 2022ef7f12
3 changed files with 117 additions and 117 deletions
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116
compiler/rustc_codegen_llvm/src/coverageinfo/map_data.rs
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@ -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;
@ -55,83 +53,10 @@ 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 }
}
}
@ -139,8 +64,6 @@ 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)
}
}