rustc_codegen_llvm/coverageinfo/mapgen/
covfun.rs

1//! For each function that was instrumented for coverage, we need to embed its
2//! corresponding coverage mapping metadata inside the `__llvm_covfun`[^win]
3//! linker section of the final binary.
4//!
5//! [^win]: On Windows the section name is `.lcovfun`.
6
7use std::ffi::CString;
8use std::sync::Arc;
9
10use rustc_abi::Align;
11use rustc_codegen_ssa::traits::{BaseTypeCodegenMethods as _, ConstCodegenMethods};
12use rustc_middle::mir::coverage::{
13    BasicCoverageBlock, CounterId, CovTerm, CoverageIdsInfo, Expression, ExpressionId,
14    FunctionCoverageInfo, Mapping, MappingKind, Op,
15};
16use rustc_middle::ty::{Instance, TyCtxt};
17use rustc_span::{SourceFile, Span};
18use rustc_target::spec::HasTargetSpec;
19use tracing::debug;
20
21use crate::common::CodegenCx;
22use crate::coverageinfo::mapgen::{GlobalFileTable, VirtualFileMapping, spans};
23use crate::coverageinfo::{ffi, llvm_cov};
24use crate::llvm;
25
26/// Intermediate coverage metadata for a single function, used to help build
27/// the final record that will be embedded in the `__llvm_covfun` section.
28#[derive(Debug)]
29pub(crate) struct CovfunRecord<'tcx> {
30    /// Not used directly, but helpful in debug messages.
31    _instance: Instance<'tcx>,
32
33    mangled_function_name: &'tcx str,
34    source_hash: u64,
35    is_used: bool,
36
37    virtual_file_mapping: VirtualFileMapping,
38    expressions: Vec<ffi::CounterExpression>,
39    regions: llvm_cov::Regions,
40}
41
42impl<'tcx> CovfunRecord<'tcx> {
43    /// Iterator that yields all source files referred to by this function's
44    /// coverage mappings. Used to build the global file table for the CGU.
45    pub(crate) fn all_source_files(&self) -> impl Iterator<Item = &SourceFile> {
46        self.virtual_file_mapping.local_file_table.iter().map(Arc::as_ref)
47    }
48}
49
50pub(crate) fn prepare_covfun_record<'tcx>(
51    tcx: TyCtxt<'tcx>,
52    instance: Instance<'tcx>,
53    is_used: bool,
54) -> Option<CovfunRecord<'tcx>> {
55    let fn_cov_info = tcx.instance_mir(instance.def).function_coverage_info.as_deref()?;
56    let ids_info = tcx.coverage_ids_info(instance.def)?;
57
58    let expressions = prepare_expressions(ids_info);
59
60    let mut covfun = CovfunRecord {
61        _instance: instance,
62        mangled_function_name: tcx.symbol_name(instance).name,
63        source_hash: if is_used { fn_cov_info.function_source_hash } else { 0 },
64        is_used,
65        virtual_file_mapping: VirtualFileMapping::default(),
66        expressions,
67        regions: llvm_cov::Regions::default(),
68    };
69
70    fill_region_tables(tcx, fn_cov_info, ids_info, &mut covfun);
71
72    if covfun.regions.has_no_regions() {
73        debug!(?covfun, "function has no mappings to embed; skipping");
74        return None;
75    }
76
77    Some(covfun)
78}
79
80pub(crate) fn counter_for_term(term: CovTerm) -> ffi::Counter {
81    use ffi::Counter;
82    match term {
83        CovTerm::Zero => Counter::ZERO,
84        CovTerm::Counter(id) => {
85            Counter { kind: ffi::CounterKind::CounterValueReference, id: CounterId::as_u32(id) }
86        }
87        CovTerm::Expression(id) => {
88            Counter { kind: ffi::CounterKind::Expression, id: ExpressionId::as_u32(id) }
89        }
90    }
91}
92
93/// Convert the function's coverage-counter expressions into a form suitable for FFI.
94fn prepare_expressions(ids_info: &CoverageIdsInfo) -> Vec<ffi::CounterExpression> {
95    // We know that LLVM will optimize out any unused expressions before
96    // producing the final coverage map, so there's no need to do the same
97    // thing on the Rust side unless we're confident we can do much better.
98    // (See `CounterExpressionsMinimizer` in `CoverageMappingWriter.cpp`.)
99    ids_info
100        .expressions
101        .iter()
102        .map(move |&Expression { lhs, op, rhs }| ffi::CounterExpression {
103            lhs: counter_for_term(lhs),
104            kind: match op {
105                Op::Add => ffi::ExprKind::Add,
106                Op::Subtract => ffi::ExprKind::Subtract,
107            },
108            rhs: counter_for_term(rhs),
109        })
110        .collect::<Vec<_>>()
111}
112
113/// Populates the mapping region tables in the current function's covfun record.
114fn fill_region_tables<'tcx>(
115    tcx: TyCtxt<'tcx>,
116    fn_cov_info: &'tcx FunctionCoverageInfo,
117    ids_info: &'tcx CoverageIdsInfo,
118    covfun: &mut CovfunRecord<'tcx>,
119) {
120    // If this function is unused, replace all counters with zero.
121    let counter_for_bcb = |bcb: BasicCoverageBlock| -> ffi::Counter {
122        let term = if covfun.is_used {
123            ids_info.term_for_bcb[bcb].expect("every BCB in a mapping was given a term")
124        } else {
125            CovTerm::Zero
126        };
127        counter_for_term(term)
128    };
129
130    // Currently a function's mappings must all be in the same file, so use the
131    // first mapping's span to determine the file.
132    let source_map = tcx.sess.source_map();
133    let Some(first_span) = (try { fn_cov_info.mappings.first()?.span }) else {
134        debug_assert!(false, "function has no mappings: {covfun:?}");
135        return;
136    };
137    let source_file = source_map.lookup_source_file(first_span.lo());
138
139    let local_file_id = covfun.virtual_file_mapping.push_file(&source_file);
140
141    // In rare cases, _all_ of a function's spans are discarded, and coverage
142    // codegen needs to handle that gracefully to avoid #133606.
143    // It's hard for tests to trigger this organically, so instead we set
144    // `-Zcoverage-options=discard-all-spans-in-codegen` to force it to occur.
145    let discard_all = tcx.sess.coverage_options().discard_all_spans_in_codegen;
146    let make_coords = |span: Span| {
147        if discard_all { None } else { spans::make_coords(source_map, &source_file, span) }
148    };
149
150    let llvm_cov::Regions {
151        code_regions,
152        expansion_regions: _, // FIXME(Zalathar): Fill out support for expansion regions
153        branch_regions,
154    } = &mut covfun.regions;
155
156    // For each counter/region pair in this function+file, convert it to a
157    // form suitable for FFI.
158    for &Mapping { ref kind, span } in &fn_cov_info.mappings {
159        let Some(coords) = make_coords(span) else { continue };
160        let cov_span = coords.make_coverage_span(local_file_id);
161
162        match *kind {
163            MappingKind::Code { bcb } => {
164                code_regions.push(ffi::CodeRegion { cov_span, counter: counter_for_bcb(bcb) });
165            }
166            MappingKind::Branch { true_bcb, false_bcb } => {
167                branch_regions.push(ffi::BranchRegion {
168                    cov_span,
169                    true_counter: counter_for_bcb(true_bcb),
170                    false_counter: counter_for_bcb(false_bcb),
171                });
172            }
173        }
174    }
175}
176
177/// Generates the contents of the covfun record for this function, which
178/// contains the function's coverage mapping data. The record is then stored
179/// as a global variable in the `__llvm_covfun` section.
180pub(crate) fn generate_covfun_record<'tcx>(
181    cx: &mut CodegenCx<'_, 'tcx>,
182    global_file_table: &GlobalFileTable,
183    covfun: &CovfunRecord<'tcx>,
184) {
185    let &CovfunRecord {
186        _instance,
187        mangled_function_name,
188        source_hash,
189        is_used,
190        ref virtual_file_mapping,
191        ref expressions,
192        ref regions,
193    } = covfun;
194
195    let Some(local_file_table) = virtual_file_mapping.resolve_all(global_file_table) else {
196        debug_assert!(
197            false,
198            "all local files should be present in the global file table: \
199                global_file_table = {global_file_table:?}, \
200                virtual_file_mapping = {virtual_file_mapping:?}"
201        );
202        return;
203    };
204
205    // Encode the function's coverage mappings into a buffer.
206    let coverage_mapping_buffer =
207        llvm_cov::write_function_mappings_to_buffer(&local_file_table, expressions, regions);
208
209    // A covfun record consists of four target-endian integers, followed by the
210    // encoded mapping data in bytes. Note that the length field is 32 bits.
211    // <https://llvm.org/docs/CoverageMappingFormat.html#llvm-ir-representation>
212    // See also `src/llvm-project/clang/lib/CodeGen/CoverageMappingGen.cpp` and
213    // `COVMAP_V3` in `src/llvm-project/llvm/include/llvm/ProfileData/InstrProfData.inc`.
214    let func_name_hash = llvm_cov::hash_bytes(mangled_function_name.as_bytes());
215    let covfun_record = cx.const_struct(
216        &[
217            cx.const_u64(func_name_hash),
218            cx.const_u32(coverage_mapping_buffer.len() as u32),
219            cx.const_u64(source_hash),
220            cx.const_u64(global_file_table.filenames_hash),
221            cx.const_bytes(&coverage_mapping_buffer),
222        ],
223        // This struct needs to be packed, so that the 32-bit length field
224        // doesn't have unexpected padding.
225        true,
226    );
227
228    // Choose a variable name to hold this function's covfun data.
229    // Functions that are used have a suffix ("u") to distinguish them from
230    // unused copies of the same function (from different CGUs), so that if a
231    // linker sees both it won't discard the used copy's data.
232    let u = if is_used { "u" } else { "" };
233    let covfun_var_name = CString::new(format!("__covrec_{func_name_hash:X}{u}")).unwrap();
234    debug!("function record var name: {covfun_var_name:?}");
235
236    let covfun_global = llvm::add_global(cx.llmod, cx.val_ty(covfun_record), &covfun_var_name);
237    llvm::set_initializer(covfun_global, covfun_record);
238    llvm::set_global_constant(covfun_global, true);
239    llvm::set_linkage(covfun_global, llvm::Linkage::LinkOnceODRLinkage);
240    llvm::set_visibility(covfun_global, llvm::Visibility::Hidden);
241    llvm::set_section(covfun_global, cx.covfun_section_name());
242    // LLVM's coverage mapping format specifies 8-byte alignment for items in this section.
243    // <https://llvm.org/docs/CoverageMappingFormat.html>
244    llvm::set_alignment(covfun_global, Align::EIGHT);
245    if cx.target_spec().supports_comdat() {
246        llvm::set_comdat(cx.llmod, covfun_global, &covfun_var_name);
247    }
248
249    cx.add_used_global(covfun_global);
250}
rustc_codegen_llvm/coverageinfo/mapgen/covfun.rs

rustc_codegen_llvm/coverageinfo/mapgen/
covfun.rs
