rustc_codegen_llvm/coverageinfo/mapgen/
covfun.rs

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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;
8
9use rustc_abi::Align;
10use rustc_codegen_ssa::traits::{
11    BaseTypeCodegenMethods as _, ConstCodegenMethods, StaticCodegenMethods,
12};
13use rustc_middle::mir::coverage::{
14    BasicCoverageBlock, CovTerm, CoverageIdsInfo, Expression, FunctionCoverageInfo, Mapping,
15    MappingKind, Op,
16};
17use rustc_middle::ty::{Instance, TyCtxt};
18use rustc_span::Span;
19use rustc_target::spec::HasTargetSpec;
20use tracing::debug;
21
22use crate::common::CodegenCx;
23use crate::coverageinfo::mapgen::{GlobalFileTable, VirtualFileMapping, spans};
24use crate::coverageinfo::{ffi, llvm_cov};
25use crate::llvm;
26
27/// Intermediate coverage metadata for a single function, used to help build
28/// the final record that will be embedded in the `__llvm_covfun` section.
29#[derive(Debug)]
30pub(crate) struct CovfunRecord<'tcx> {
31    mangled_function_name: &'tcx str,
32    source_hash: u64,
33    is_used: bool,
34
35    virtual_file_mapping: VirtualFileMapping,
36    expressions: Vec<ffi::CounterExpression>,
37    regions: ffi::Regions,
38}
39
40impl<'tcx> CovfunRecord<'tcx> {
41    /// FIXME(Zalathar): Make this the responsibility of the code that determines
42    /// which functions are unused.
43    pub(crate) fn mangled_function_name_if_unused(&self) -> Option<&'tcx str> {
44        (!self.is_used).then_some(self.mangled_function_name)
45    }
46}
47
48pub(crate) fn prepare_covfun_record<'tcx>(
49    tcx: TyCtxt<'tcx>,
50    global_file_table: &mut GlobalFileTable,
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, global_file_table, 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    global_file_table: &mut GlobalFileTable,
104    fn_cov_info: &'tcx FunctionCoverageInfo,
105    ids_info: &'tcx CoverageIdsInfo,
106    covfun: &mut CovfunRecord<'tcx>,
107) {
108    // Currently a function's mappings must all be in the same file, so use the
109    // first mapping's span to determine the file.
110    let source_map = tcx.sess.source_map();
111    let Some(first_span) = (try { fn_cov_info.mappings.first()?.span }) else {
112        debug_assert!(false, "function has no mappings: {:?}", covfun.mangled_function_name);
113        return;
114    };
115    let source_file = source_map.lookup_source_file(first_span.lo());
116
117    // Look up the global file ID for that file.
118    let global_file_id = global_file_table.global_file_id_for_file(&source_file);
119
120    // Associate that global file ID with a local file ID for this function.
121    let local_file_id = covfun.virtual_file_mapping.local_id_for_global(global_file_id);
122
123    // In rare cases, _all_ of a function's spans are discarded, and coverage
124    // codegen needs to handle that gracefully to avoid #133606.
125    // It's hard for tests to trigger this organically, so instead we set
126    // `-Zcoverage-options=discard-all-spans-in-codegen` to force it to occur.
127    let discard_all = tcx.sess.coverage_discard_all_spans_in_codegen();
128    let make_coords = |span: Span| {
129        if discard_all { None } else { spans::make_coords(source_map, &source_file, span) }
130    };
131
132    let ffi::Regions {
133        code_regions,
134        expansion_regions: _, // FIXME(Zalathar): Fill out support for expansion regions
135        branch_regions,
136        mcdc_branch_regions,
137        mcdc_decision_regions,
138    } = &mut covfun.regions;
139
140    // For each counter/region pair in this function+file, convert it to a
141    // form suitable for FFI.
142    for &Mapping { ref kind, span } in &fn_cov_info.mappings {
143        // If this function is unused, replace all counters with zero.
144        let counter_for_bcb = |bcb: BasicCoverageBlock| -> ffi::Counter {
145            let term = if covfun.is_used {
146                ids_info.term_for_bcb[bcb].expect("every BCB in a mapping was given a term")
147            } else {
148                CovTerm::Zero
149            };
150            ffi::Counter::from_term(term)
151        };
152
153        let Some(coords) = make_coords(span) else { continue };
154        let cov_span = coords.make_coverage_span(local_file_id);
155
156        match *kind {
157            MappingKind::Code { bcb } => {
158                code_regions.push(ffi::CodeRegion { cov_span, counter: counter_for_bcb(bcb) });
159            }
160            MappingKind::Branch { true_bcb, false_bcb } => {
161                branch_regions.push(ffi::BranchRegion {
162                    cov_span,
163                    true_counter: counter_for_bcb(true_bcb),
164                    false_counter: counter_for_bcb(false_bcb),
165                });
166            }
167            MappingKind::MCDCBranch { true_bcb, false_bcb, mcdc_params } => {
168                mcdc_branch_regions.push(ffi::MCDCBranchRegion {
169                    cov_span,
170                    true_counter: counter_for_bcb(true_bcb),
171                    false_counter: counter_for_bcb(false_bcb),
172                    mcdc_branch_params: ffi::mcdc::BranchParameters::from(mcdc_params),
173                });
174            }
175            MappingKind::MCDCDecision(mcdc_decision_params) => {
176                mcdc_decision_regions.push(ffi::MCDCDecisionRegion {
177                    cov_span,
178                    mcdc_decision_params: ffi::mcdc::DecisionParameters::from(mcdc_decision_params),
179                });
180            }
181        }
182    }
183}
184
185/// Generates the contents of the covfun record for this function, which
186/// contains the function's coverage mapping data. The record is then stored
187/// as a global variable in the `__llvm_covfun` section.
188pub(crate) fn generate_covfun_record<'tcx>(
189    cx: &CodegenCx<'_, 'tcx>,
190    filenames_hash: u64,
191    covfun: &CovfunRecord<'tcx>,
192) {
193    let &CovfunRecord {
194        mangled_function_name,
195        source_hash,
196        is_used,
197        ref virtual_file_mapping,
198        ref expressions,
199        ref regions,
200    } = covfun;
201
202    // Encode the function's coverage mappings into a buffer.
203    let coverage_mapping_buffer = llvm_cov::write_function_mappings_to_buffer(
204        &virtual_file_mapping.to_vec(),
205        expressions,
206        regions,
207    );
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(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
