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::{
12    BaseTypeCodegenMethods as _, ConstCodegenMethods, StaticCodegenMethods,
13};
14use rustc_middle::mir::coverage::{
15    BasicCoverageBlock, CovTerm, CoverageIdsInfo, Expression, FunctionCoverageInfo, Mapping,
16    MappingKind, Op,
17};
18use rustc_middle::ty::{Instance, TyCtxt};
19use rustc_span::{SourceFile, Span};
20use rustc_target::spec::HasTargetSpec;
21use tracing::debug;
22
23use crate::common::CodegenCx;
24use crate::coverageinfo::mapgen::{GlobalFileTable, VirtualFileMapping, spans};
25use crate::coverageinfo::{ffi, llvm_cov};
26use crate::llvm;
27
28/// Intermediate coverage metadata for a single function, used to help build
29/// the final record that will be embedded in the `__llvm_covfun` section.
30#[derive(Debug)]
31pub(crate) struct CovfunRecord<'tcx> {
32    mangled_function_name: &'tcx str,
33    source_hash: u64,
34    is_used: bool,
35
36    virtual_file_mapping: VirtualFileMapping,
37    expressions: Vec<ffi::CounterExpression>,
38    regions: ffi::Regions,
39}
40
41impl<'tcx> CovfunRecord<'tcx> {
42    /// Iterator that yields all source files referred to by this function's
43    /// coverage mappings. Used to build the global file table for the CGU.
44    pub(crate) fn all_source_files(&self) -> impl Iterator<Item = &SourceFile> {
45        self.virtual_file_mapping.local_file_table.iter().map(Arc::as_ref)
46    }
47}
48
49pub(crate) fn prepare_covfun_record<'tcx>(
50    tcx: TyCtxt<'tcx>,
51    instance: Instance<'tcx>,
52    is_used: bool,
53) -> Option<CovfunRecord<'tcx>> {
54    let fn_cov_info = tcx.instance_mir(instance.def).function_coverage_info.as_deref()?;
55    let ids_info = tcx.coverage_ids_info(instance.def)?;
56
57    let expressions = prepare_expressions(ids_info);
58
59    let mut covfun = CovfunRecord {
60        mangled_function_name: tcx.symbol_name(instance).name,
61        source_hash: if is_used { fn_cov_info.function_source_hash } else { 0 },
62        is_used,
63        virtual_file_mapping: VirtualFileMapping::default(),
64        expressions,
65        regions: ffi::Regions::default(),
66    };
67
68    fill_region_tables(tcx, fn_cov_info, ids_info, &mut covfun);
69
70    if covfun.regions.has_no_regions() {
71        debug!(?covfun, "function has no mappings to embed; skipping");
72        return None;
73    }
74
75    Some(covfun)
76}
77
78/// Convert the function's coverage-counter expressions into a form suitable for FFI.
79fn prepare_expressions(ids_info: &CoverageIdsInfo) -> Vec<ffi::CounterExpression> {
80    let counter_for_term = ffi::Counter::from_term;
81
82    // We know that LLVM will optimize out any unused expressions before
83    // producing the final coverage map, so there's no need to do the same
84    // thing on the Rust side unless we're confident we can do much better.
85    // (See `CounterExpressionsMinimizer` in `CoverageMappingWriter.cpp`.)
86    ids_info
87        .expressions
88        .iter()
89        .map(move |&Expression { lhs, op, rhs }| ffi::CounterExpression {
90            lhs: counter_for_term(lhs),
91            kind: match op {
92                Op::Add => ffi::ExprKind::Add,
93                Op::Subtract => ffi::ExprKind::Subtract,
94            },
95            rhs: counter_for_term(rhs),
96        })
97        .collect::<Vec<_>>()
98}
99
100/// Populates the mapping region tables in the current function's covfun record.
101fn fill_region_tables<'tcx>(
102    tcx: TyCtxt<'tcx>,
103    fn_cov_info: &'tcx FunctionCoverageInfo,
104    ids_info: &'tcx CoverageIdsInfo,
105    covfun: &mut CovfunRecord<'tcx>,
106) {
107    // Currently a function's mappings must all be in the same file, so use the
108    // first mapping's span to determine the file.
109    let source_map = tcx.sess.source_map();
110    let Some(first_span) = (try { fn_cov_info.mappings.first()?.span }) else {
111        debug_assert!(false, "function has no mappings: {:?}", covfun.mangled_function_name);
112        return;
113    };
114    let source_file = source_map.lookup_source_file(first_span.lo());
115
116    let local_file_id = covfun.virtual_file_mapping.push_file(&source_file);
117
118    // In rare cases, _all_ of a function's spans are discarded, and coverage
119    // codegen needs to handle that gracefully to avoid #133606.
120    // It's hard for tests to trigger this organically, so instead we set
121    // `-Zcoverage-options=discard-all-spans-in-codegen` to force it to occur.
122    let discard_all = tcx.sess.coverage_discard_all_spans_in_codegen();
123    let make_coords = |span: Span| {
124        if discard_all { None } else { spans::make_coords(source_map, &source_file, span) }
125    };
126
127    let ffi::Regions {
128        code_regions,
129        expansion_regions: _, // FIXME(Zalathar): Fill out support for expansion regions
130        branch_regions,
131        mcdc_branch_regions,
132        mcdc_decision_regions,
133    } = &mut covfun.regions;
134
135    // For each counter/region pair in this function+file, convert it to a
136    // form suitable for FFI.
137    for &Mapping { ref kind, span } in &fn_cov_info.mappings {
138        // If this function is unused, replace all counters with zero.
139        let counter_for_bcb = |bcb: BasicCoverageBlock| -> ffi::Counter {
140            let term = if covfun.is_used {
141                ids_info.term_for_bcb[bcb].expect("every BCB in a mapping was given a term")
142            } else {
143                CovTerm::Zero
144            };
145            ffi::Counter::from_term(term)
146        };
147
148        let Some(coords) = make_coords(span) else { continue };
149        let cov_span = coords.make_coverage_span(local_file_id);
150
151        match *kind {
152            MappingKind::Code { bcb } => {
153                code_regions.push(ffi::CodeRegion { cov_span, counter: counter_for_bcb(bcb) });
154            }
155            MappingKind::Branch { true_bcb, false_bcb } => {
156                branch_regions.push(ffi::BranchRegion {
157                    cov_span,
158                    true_counter: counter_for_bcb(true_bcb),
159                    false_counter: counter_for_bcb(false_bcb),
160                });
161            }
162            MappingKind::MCDCBranch { true_bcb, false_bcb, mcdc_params } => {
163                mcdc_branch_regions.push(ffi::MCDCBranchRegion {
164                    cov_span,
165                    true_counter: counter_for_bcb(true_bcb),
166                    false_counter: counter_for_bcb(false_bcb),
167                    mcdc_branch_params: ffi::mcdc::BranchParameters::from(mcdc_params),
168                });
169            }
170            MappingKind::MCDCDecision(mcdc_decision_params) => {
171                mcdc_decision_regions.push(ffi::MCDCDecisionRegion {
172                    cov_span,
173                    mcdc_decision_params: ffi::mcdc::DecisionParameters::from(mcdc_decision_params),
174                });
175            }
176        }
177    }
178}
179
180/// Generates the contents of the covfun record for this function, which
181/// contains the function's coverage mapping data. The record is then stored
182/// as a global variable in the `__llvm_covfun` section.
183pub(crate) fn generate_covfun_record<'tcx>(
184    cx: &CodegenCx<'_, 'tcx>,
185    global_file_table: &GlobalFileTable,
186    covfun: &CovfunRecord<'tcx>,
187) {
188    let &CovfunRecord {
189        mangled_function_name,
190        source_hash,
191        is_used,
192        ref virtual_file_mapping,
193        ref expressions,
194        ref regions,
195    } = covfun;
196
197    let Some(local_file_table) = virtual_file_mapping.resolve_all(global_file_table) else {
198        debug_assert!(
199            false,
200            "all local files should be present in the global file table: \
201                global_file_table = {global_file_table:?}, \
202                virtual_file_mapping = {virtual_file_mapping:?}"
203        );
204        return;
205    };
206
207    // Encode the function's coverage mappings into a buffer.
208    let coverage_mapping_buffer =
209        llvm_cov::write_function_mappings_to_buffer(&local_file_table, expressions, regions);
210
211    // A covfun record consists of four target-endian integers, followed by the
212    // encoded mapping data in bytes. Note that the length field is 32 bits.
213    // <https://llvm.org/docs/CoverageMappingFormat.html#llvm-ir-representation>
214    // See also `src/llvm-project/clang/lib/CodeGen/CoverageMappingGen.cpp` and
215    // `COVMAP_V3` in `src/llvm-project/llvm/include/llvm/ProfileData/InstrProfData.inc`.
216    let func_name_hash = llvm_cov::hash_bytes(mangled_function_name.as_bytes());
217    let covfun_record = cx.const_struct(
218        &[
219            cx.const_u64(func_name_hash),
220            cx.const_u32(coverage_mapping_buffer.len() as u32),
221            cx.const_u64(source_hash),
222            cx.const_u64(global_file_table.filenames_hash),
223            cx.const_bytes(&coverage_mapping_buffer),
224        ],
225        // This struct needs to be packed, so that the 32-bit length field
226        // doesn't have unexpected padding.
227        true,
228    );
229
230    // Choose a variable name to hold this function's covfun data.
231    // Functions that are used have a suffix ("u") to distinguish them from
232    // unused copies of the same function (from different CGUs), so that if a
233    // linker sees both it won't discard the used copy's data.
234    let u = if is_used { "u" } else { "" };
235    let covfun_var_name = CString::new(format!("__covrec_{func_name_hash:X}{u}")).unwrap();
236    debug!("function record var name: {covfun_var_name:?}");
237
238    let covfun_global = llvm::add_global(cx.llmod, cx.val_ty(covfun_record), &covfun_var_name);
239    llvm::set_initializer(covfun_global, covfun_record);
240    llvm::set_global_constant(covfun_global, true);
241    llvm::set_linkage(covfun_global, llvm::Linkage::LinkOnceODRLinkage);
242    llvm::set_visibility(covfun_global, llvm::Visibility::Hidden);
243    llvm::set_section(covfun_global, cx.covfun_section_name());
244    // LLVM's coverage mapping format specifies 8-byte alignment for items in this section.
245    // <https://llvm.org/docs/CoverageMappingFormat.html>
246    llvm::set_alignment(covfun_global, Align::EIGHT);
247    if cx.target_spec().supports_comdat() {
248        llvm::set_comdat(cx.llmod, covfun_global, &covfun_var_name);
249    }
250
251    cx.add_used_global(covfun_global);
252}
rustc_codegen_llvm/coverageinfo/mapgen/covfun.rs

rustc_codegen_llvm/coverageinfo/mapgen/
covfun.rs
