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, CovTerm, CoverageIdsInfo, Expression, FunctionCoverageInfo, Mapping,
14    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    mangled_function_name: &'tcx str,
31    source_hash: u64,
32    is_used: bool,
33
34    virtual_file_mapping: VirtualFileMapping,
35    expressions: Vec<ffi::CounterExpression>,
36    regions: ffi::Regions,
37}
38
39impl<'tcx> CovfunRecord<'tcx> {
40    /// Iterator that yields all source files referred to by this function's
41    /// coverage mappings. Used to build the global file table for the CGU.
42    pub(crate) fn all_source_files(&self) -> impl Iterator<Item = &SourceFile> {
43        self.virtual_file_mapping.local_file_table.iter().map(Arc::as_ref)
44    }
45}
46
47pub(crate) fn prepare_covfun_record<'tcx>(
48    tcx: TyCtxt<'tcx>,
49    instance: Instance<'tcx>,
50    is_used: bool,
51) -> Option<CovfunRecord<'tcx>> {
52    let fn_cov_info = tcx.instance_mir(instance.def).function_coverage_info.as_deref()?;
53    let ids_info = tcx.coverage_ids_info(instance.def)?;
54
55    let expressions = prepare_expressions(ids_info);
56
57    let mut covfun = CovfunRecord {
58        mangled_function_name: tcx.symbol_name(instance).name,
59        source_hash: if is_used { fn_cov_info.function_source_hash } else { 0 },
60        is_used,
61        virtual_file_mapping: VirtualFileMapping::default(),
62        expressions,
63        regions: ffi::Regions::default(),
64    };
65
66    fill_region_tables(tcx, fn_cov_info, ids_info, &mut covfun);
67
68    if covfun.regions.has_no_regions() {
69        debug!(?covfun, "function has no mappings to embed; skipping");
70        return None;
71    }
72
73    Some(covfun)
74}
75
76/// Convert the function's coverage-counter expressions into a form suitable for FFI.
77fn prepare_expressions(ids_info: &CoverageIdsInfo) -> Vec<ffi::CounterExpression> {
78    let counter_for_term = ffi::Counter::from_term;
79
80    // We know that LLVM will optimize out any unused expressions before
81    // producing the final coverage map, so there's no need to do the same
82    // thing on the Rust side unless we're confident we can do much better.
83    // (See `CounterExpressionsMinimizer` in `CoverageMappingWriter.cpp`.)
84    ids_info
85        .expressions
86        .iter()
87        .map(move |&Expression { lhs, op, rhs }| ffi::CounterExpression {
88            lhs: counter_for_term(lhs),
89            kind: match op {
90                Op::Add => ffi::ExprKind::Add,
91                Op::Subtract => ffi::ExprKind::Subtract,
92            },
93            rhs: counter_for_term(rhs),
94        })
95        .collect::<Vec<_>>()
96}
97
98/// Populates the mapping region tables in the current function's covfun record.
99fn fill_region_tables<'tcx>(
100    tcx: TyCtxt<'tcx>,
101    fn_cov_info: &'tcx FunctionCoverageInfo,
102    ids_info: &'tcx CoverageIdsInfo,
103    covfun: &mut CovfunRecord<'tcx>,
104) {
105    // Currently a function's mappings must all be in the same file, so use the
106    // first mapping's span to determine the file.
107    let source_map = tcx.sess.source_map();
108    let Some(first_span) = (try { fn_cov_info.mappings.first()?.span }) else {
109        debug_assert!(false, "function has no mappings: {:?}", covfun.mangled_function_name);
110        return;
111    };
112    let source_file = source_map.lookup_source_file(first_span.lo());
113
114    let local_file_id = covfun.virtual_file_mapping.push_file(&source_file);
115
116    // In rare cases, _all_ of a function's spans are discarded, and coverage
117    // codegen needs to handle that gracefully to avoid #133606.
118    // It's hard for tests to trigger this organically, so instead we set
119    // `-Zcoverage-options=discard-all-spans-in-codegen` to force it to occur.
120    let discard_all = tcx.sess.coverage_discard_all_spans_in_codegen();
121    let make_coords = |span: Span| {
122        if discard_all { None } else { spans::make_coords(source_map, &source_file, span) }
123    };
124
125    let ffi::Regions {
126        code_regions,
127        expansion_regions: _, // FIXME(Zalathar): Fill out support for expansion regions
128        branch_regions,
129        mcdc_branch_regions,
130        mcdc_decision_regions,
131    } = &mut covfun.regions;
132
133    // For each counter/region pair in this function+file, convert it to a
134    // form suitable for FFI.
135    for &Mapping { ref kind, span } in &fn_cov_info.mappings {
136        // If this function is unused, replace all counters with zero.
137        let counter_for_bcb = |bcb: BasicCoverageBlock| -> ffi::Counter {
138            let term = if covfun.is_used {
139                ids_info.term_for_bcb[bcb].expect("every BCB in a mapping was given a term")
140            } else {
141                CovTerm::Zero
142            };
143            ffi::Counter::from_term(term)
144        };
145
146        let Some(coords) = make_coords(span) else { continue };
147        let cov_span = coords.make_coverage_span(local_file_id);
148
149        match *kind {
150            MappingKind::Code { bcb } => {
151                code_regions.push(ffi::CodeRegion { cov_span, counter: counter_for_bcb(bcb) });
152            }
153            MappingKind::Branch { true_bcb, false_bcb } => {
154                branch_regions.push(ffi::BranchRegion {
155                    cov_span,
156                    true_counter: counter_for_bcb(true_bcb),
157                    false_counter: counter_for_bcb(false_bcb),
158                });
159            }
160            MappingKind::MCDCBranch { true_bcb, false_bcb, mcdc_params } => {
161                mcdc_branch_regions.push(ffi::MCDCBranchRegion {
162                    cov_span,
163                    true_counter: counter_for_bcb(true_bcb),
164                    false_counter: counter_for_bcb(false_bcb),
165                    mcdc_branch_params: ffi::mcdc::BranchParameters::from(mcdc_params),
166                });
167            }
168            MappingKind::MCDCDecision(mcdc_decision_params) => {
169                mcdc_decision_regions.push(ffi::MCDCDecisionRegion {
170                    cov_span,
171                    mcdc_decision_params: ffi::mcdc::DecisionParameters::from(mcdc_decision_params),
172                });
173            }
174        }
175    }
176}
177
178/// Generates the contents of the covfun record for this function, which
179/// contains the function's coverage mapping data. The record is then stored
180/// as a global variable in the `__llvm_covfun` section.
181pub(crate) fn generate_covfun_record<'tcx>(
182    cx: &mut CodegenCx<'_, 'tcx>,
183    global_file_table: &GlobalFileTable,
184    covfun: &CovfunRecord<'tcx>,
185) {
186    let &CovfunRecord {
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
