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coverage: Overhaul the search for unused functions
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@Zalathar
Zalathar committed Mar 26, 2024
1 parent 5ddc4f2 commit e3f66b2
Showing 1 changed file with 68 additions and 62 deletions.
130 changes: 68 additions & 62 deletions compiler/rustc_codegen_llvm/src/coverageinfo/mapgen.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -6,9 +6,8 @@ use crate::llvm;

use itertools::Itertools as _;
use rustc_codegen_ssa::traits::{BaseTypeMethods, ConstMethods};
use rustc_data_structures::fx::{FxIndexMap, FxIndexSet};
use rustc_hir::def::DefKind;
use rustc_hir::def_id::DefId;
use rustc_data_structures::fx::{FxHashSet, FxIndexMap, FxIndexSet};
use rustc_hir::def_id::{DefId, LocalDefId};
use rustc_index::IndexVec;
use rustc_middle::bug;
use rustc_middle::mir;
Expand Down Expand Up @@ -335,16 +334,9 @@ fn save_function_record(
);
}

/// When finalizing the coverage map, `FunctionCoverage` only has the `CodeRegion`s and counters for
/// the functions that went through codegen; such as public functions and "used" functions
/// (functions referenced by other "used" or public items). Any other functions considered unused,
/// or "Unreachable", were still parsed and processed through the MIR stage, but were not
/// codegenned. (Note that `-Clink-dead-code` can force some unused code to be codegenned, but
/// that flag is known to cause other errors, when combined with `-C instrument-coverage`; and
/// `-Clink-dead-code` will not generate code for unused generic functions.)
///
/// We can find the unused functions (including generic functions) by the set difference of all MIR
/// `DefId`s (`tcx` query `mir_keys`) minus the codegenned `DefId`s (`codegenned_and_inlined_items`).
/// Each CGU will normally only emit coverage metadata for the functions that it actually generates.
/// But since we don't want unused functions to disappear from coverage reports, we also scan for
/// functions that were instrumented but are not participating in codegen.
///
/// These unused functions don't need to be codegenned, but we do need to add them to the function
/// coverage map (in a single designated CGU) so that we still emit coverage mappings for them.
Expand All @@ -354,78 +346,92 @@ fn add_unused_functions(cx: &CodegenCx<'_, '_>) {
assert!(cx.codegen_unit.is_code_coverage_dead_code_cgu());

let tcx = cx.tcx;
let usage = prepare_usage_sets(tcx);

let is_unused_fn = |def_id: LocalDefId| -> bool {
let def_id = def_id.to_def_id();

// To be eligible for "unused function" mappings, a definition must:
// - Be function-like
// - Not participate directly in codegen
// - Not have any coverage statements inlined into codegenned functions
tcx.def_kind(def_id).is_fn_like()
&& !usage.all_mono_items.contains(&def_id)
&& !usage.used_via_inlining.contains(&def_id)
};

let eligible_def_ids = tcx.mir_keys(()).iter().filter_map(|local_def_id| {
let def_id = local_def_id.to_def_id();
let kind = tcx.def_kind(def_id);
// `mir_keys` will give us `DefId`s for all kinds of things, not
// just "functions", like consts, statics, etc. Filter those out.
if !matches!(kind, DefKind::Fn | DefKind::AssocFn | DefKind::Closure) {
return None;
}
// Scan for unused functions that were instrumented for coverage.
for def_id in tcx.mir_keys(()).iter().copied().filter(|&def_id| is_unused_fn(def_id)) {
// Get the coverage info from MIR, skipping functions that were never instrumented.
let body = tcx.optimized_mir(def_id);
let Some(function_coverage_info) = body.function_coverage_info.as_deref() else { continue };

// FIXME(79651): Consider trying to filter out dummy instantiations of
// unused generic functions from library crates, because they can produce
// "unused instantiation" in coverage reports even when they are actually
// used by some downstream crate in the same binary.

Some(local_def_id.to_def_id())
});

let codegenned_def_ids = codegenned_and_inlined_items(tcx);

// For each `DefId` that should have coverage instrumentation but wasn't
// codegenned, add it to the function coverage map as an unused function.
for def_id in eligible_def_ids.filter(|id| !codegenned_def_ids.contains(id)) {
// Skip any function that didn't have coverage data added to it by the
// coverage instrumentor.
let body = tcx.instance_mir(ty::InstanceDef::Item(def_id));
let Some(function_coverage_info) = body.function_coverage_info.as_deref() else {
continue;
};

debug!("generating unused fn: {def_id:?}");
add_unused_function_coverage(cx, def_id, function_coverage_info);
}
}

/// All items participating in code generation together with (instrumented)
/// items inlined into them.
fn codegenned_and_inlined_items(tcx: TyCtxt<'_>) -> DefIdSet {
let (items, cgus) = tcx.collect_and_partition_mono_items(());
let mut visited = DefIdSet::default();
let mut result = items.clone();

for cgu in cgus {
for item in cgu.items().keys() {
if let mir::mono::MonoItem::Fn(ref instance) = item {
let did = instance.def_id();
if !visited.insert(did) {
continue;
}
let body = tcx.instance_mir(instance.def);
for block in body.basic_blocks.iter() {
for statement in &block.statements {
let mir::StatementKind::Coverage(_) = statement.kind else { continue };
let scope = statement.source_info.scope;
if let Some(inlined) = scope.inlined_instance(&body.source_scopes) {
result.insert(inlined.def_id());
}
}
}
struct UsageSets<'tcx> {
all_mono_items: &'tcx DefIdSet,
used_via_inlining: FxHashSet<DefId>,
}

/// Prepare sets of definitions that are relevant to deciding whether something
/// is an "unused function" for coverage purposes.
fn prepare_usage_sets<'tcx>(tcx: TyCtxt<'tcx>) -> UsageSets<'tcx> {
let (all_mono_items, cgus) = tcx.collect_and_partition_mono_items(());

// Obtain a MIR body for each function participating in codegen, via an
// arbitrary instance.
let mut def_ids_seen = FxHashSet::default();
let def_and_mir_for_all_mono_fns = cgus
.iter()
.flat_map(|cgu| cgu.items().keys())
.filter_map(|item| match item {
mir::mono::MonoItem::Fn(instance) => Some(instance),
mir::mono::MonoItem::Static(_) | mir::mono::MonoItem::GlobalAsm(_) => None,
})
// We only need one arbitrary instance per definition.
.filter(move |instance| def_ids_seen.insert(instance.def_id()))
.map(|instance| {
// We don't care about the instance, just its underlying MIR.
let body = tcx.instance_mir(instance.def);
(instance.def_id(), body)
});

// Functions whose coverage statments were found inlined into other functions.
let mut used_via_inlining = FxHashSet::default();

for (_def_id, body) in def_and_mir_for_all_mono_fns {
// Inspect every coverage statement in the function's MIR.
for stmt in body
.basic_blocks
.iter()
.flat_map(|block| &block.statements)
.filter(|stmt| matches!(stmt.kind, mir::StatementKind::Coverage(_)))
{
if let Some(inlined) = stmt.source_info.scope.inlined_instance(&body.source_scopes) {
// This coverage statement was inlined from another function.
used_via_inlining.insert(inlined.def_id());
}
}
}

result
UsageSets { all_mono_items, used_via_inlining }
}

fn add_unused_function_coverage<'tcx>(
cx: &CodegenCx<'_, 'tcx>,
def_id: DefId,
def_id: LocalDefId,
function_coverage_info: &'tcx mir::coverage::FunctionCoverageInfo,
) {
let tcx = cx.tcx;
let def_id = def_id.to_def_id();

// Make a dummy instance that fills in all generics with placeholders.
let instance = ty::Instance::new(
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