bjoernager/rust
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coverage: Explicitly simplify coverage expressions in codegen

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After coverage instrumentation and MIR transformations, we can sometimes end up
with coverage expressions that always have a value of zero. Any expression
operand that refers to an always-zero expression can be replaced with a literal
`Operand::Zero`, making the emitted coverage mapping data smaller and simpler.

This simplification step is mostly redundant with the simplifications performed
inline in `expressions_with_regions`, except that it does a slightly more
thorough job in some cases (because it checks for always-zero expressions
*after* other simplifications).

However, adding this simplification step will then let us greatly simplify that
code, without affecting the quality of the emitted coverage maps.
This commit is contained in:
Zalathar 2023-08-14 18:01:28 +10:00
parent 659575aade
commit 527c629cd9
3 changed files with 185 additions and 137 deletions
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53
compiler/rustc_codegen_llvm/src/coverageinfo/map_data.rs
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@ -1,5 +1,6 @@
use crate::coverageinfo::ffi::{Counter, CounterExpression, ExprKind};
use rustc_data_structures::fx::FxIndexSet;
use rustc_index::{IndexSlice, IndexVec};
use rustc_middle::bug;
use rustc_middle::mir::coverage::{
@ -128,6 +129,58 @@ impl<'tcx> FunctionCoverage<'tcx> {
self.unreachable_regions.push(region)
}
/// Perform some simplifications to make the final coverage mappings
/// slightly smaller.
///
/// 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.
pub(crate) fn simplify_expressions(&mut self) {
// 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 `Operand::Zero`.
let mut zero_expressions = FxIndexSet::default();
// For each expression, perform simplifications 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 simplification pass is sufficient.)
for (id, maybe_expression) in self.expressions.iter_enumerated_mut() {
let Some(expression) = maybe_expression else {
// If an expression is missing, it must have been optimized away,
// so any operand that refers to it can be replaced with zero.
zero_expressions.insert(id);
continue;
};
// If an operand refers to an expression that is always zero, then
// that operand can be replaced with `Operand::Zero`.
let maybe_set_operand_to_zero = |operand: &mut Operand| match &*operand {
Operand::Expression(id) if zero_expressions.contains(id) => {
*operand = Operand::Zero;
}
_ => (),
};
maybe_set_operand_to_zero(&mut expression.lhs);
maybe_set_operand_to_zero(&mut expression.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 let Expression { lhs: Operand::Zero, op: Op::Subtract, .. } = expression {
expression.rhs = Operand::Zero;
}
// After the above simplifications, if both operands are zero, then
// we know that this expression is always zero too.
if let Expression { lhs: Operand::Zero, rhs: Operand::Zero, .. } = expression {
zero_expressions.insert(id);
}
}
}
/// Return the source hash, generated from the HIR node structure, and used to indicate whether
/// or not the source code structure changed between different compilations.
pub fn source_hash(&self) -> u64 {