coverage: Extract function metadata handling to a covfun submodule

compiler/rustc_codegen_llvm/src/coverageinfo/mapgen/covfun.rs

@@ -0,0 +1,198 @@
+//! For each function that was instrumented for coverage, we need to embed its
+//! corresponding coverage mapping metadata inside the `__llvm_covfun`[^win]
+//! linker section of the final binary.
+//!
+//! [^win]: On Windows the section name is `.lcovfun`.
+
+use std::ffi::CString;
+
+use rustc_abi::Align;
+use rustc_codegen_ssa::traits::{
+    BaseTypeCodegenMethods, ConstCodegenMethods, StaticCodegenMethods,
+};
+use rustc_middle::bug;
+use rustc_middle::mir::coverage::MappingKind;
+use rustc_middle::ty::{Instance, TyCtxt};
+use rustc_target::spec::HasTargetSpec;
+use tracing::debug;
+
+use crate::common::CodegenCx;
+use crate::coverageinfo::map_data::FunctionCoverage;
+use crate::coverageinfo::mapgen::{GlobalFileTable, VirtualFileMapping, span_file_name};
+use crate::coverageinfo::{ffi, llvm_cov};
+use crate::llvm;
+
+pub(crate) fn prepare_and_generate_covfun_record<'ll, 'tcx>(
+    cx: &CodegenCx<'ll, 'tcx>,
+    global_file_table: &GlobalFileTable,
+    filenames_ref: u64,
+    unused_function_names: &mut Vec<&'tcx str>,
+    instance: Instance<'tcx>,
+    function_coverage: &FunctionCoverage<'tcx>,
+) {
+    let tcx = cx.tcx;
+
+    let mangled_function_name = tcx.symbol_name(instance).name;
+    let source_hash = function_coverage.source_hash();
+    let is_used = function_coverage.is_used();
+
+    let coverage_mapping_buffer =
+        encode_mappings_for_function(tcx, global_file_table, function_coverage);
+
+    if coverage_mapping_buffer.is_empty() {
+        if function_coverage.is_used() {
+            bug!(
+                "A used function should have had coverage mapping data but did not: {}",
+                mangled_function_name
+            );
+        } else {
+            debug!("unused function had no coverage mapping data: {}", mangled_function_name);
+            return;
+        }
+    }
+
+    if !is_used {
+        unused_function_names.push(mangled_function_name);
+    }
+
+    generate_covfun_record(
+        cx,
+        mangled_function_name,
+        source_hash,
+        filenames_ref,
+        coverage_mapping_buffer,
+        is_used,
+    );
+}
+
+/// Using the expressions and counter regions collected for a single function,
+/// generate the variable-sized payload of its corresponding `__llvm_covfun`
+/// entry. The payload is returned as a vector of bytes.
+///
+/// Newly-encountered filenames will be added to the global file table.
+fn encode_mappings_for_function(
+    tcx: TyCtxt<'_>,
+    global_file_table: &GlobalFileTable,
+    function_coverage: &FunctionCoverage<'_>,
+) -> Vec<u8> {
+    let counter_regions = function_coverage.counter_regions();
+    if counter_regions.is_empty() {
+        return Vec::new();
+    }
+
+    let expressions = function_coverage.counter_expressions().collect::<Vec<_>>();
+
+    let mut virtual_file_mapping = VirtualFileMapping::default();
+    let mut code_regions = vec![];
+    let mut branch_regions = vec![];
+    let mut mcdc_branch_regions = vec![];
+    let mut mcdc_decision_regions = vec![];
+
+    // Currently a function's mappings must all be in the same file as its body span.
+    let file_name = span_file_name(tcx, function_coverage.function_coverage_info.body_span);
+
+    // Look up the global file ID for that filename.
+    let global_file_id = global_file_table.global_file_id_for_file_name(file_name);
+
+    // Associate that global file ID with a local file ID for this function.
+    let local_file_id = virtual_file_mapping.local_id_for_global(global_file_id);
+    debug!("  file id: {local_file_id:?} => {global_file_id:?} = '{file_name:?}'");
+
+    // For each counter/region pair in this function+file, convert it to a
+    // form suitable for FFI.
+    for (mapping_kind, region) in counter_regions {
+        debug!("Adding counter {mapping_kind:?} to map for {region:?}");
+        let span = ffi::CoverageSpan::from_source_region(local_file_id, region);
+        match mapping_kind {
+            MappingKind::Code(term) => {
+                code_regions.push(ffi::CodeRegion { span, counter: ffi::Counter::from_term(term) });
+            }
+            MappingKind::Branch { true_term, false_term } => {
+                branch_regions.push(ffi::BranchRegion {
+                    span,
+                    true_counter: ffi::Counter::from_term(true_term),
+                    false_counter: ffi::Counter::from_term(false_term),
+                });
+            }
+            MappingKind::MCDCBranch { true_term, false_term, mcdc_params } => {
+                mcdc_branch_regions.push(ffi::MCDCBranchRegion {
+                    span,
+                    true_counter: ffi::Counter::from_term(true_term),
+                    false_counter: ffi::Counter::from_term(false_term),
+                    mcdc_branch_params: ffi::mcdc::BranchParameters::from(mcdc_params),
+                });
+            }
+            MappingKind::MCDCDecision(mcdc_decision_params) => {
+                mcdc_decision_regions.push(ffi::MCDCDecisionRegion {
+                    span,
+                    mcdc_decision_params: ffi::mcdc::DecisionParameters::from(mcdc_decision_params),
+                });
+            }
+        }
+    }
+
+    // Encode the function's coverage mappings into a buffer.
+    llvm_cov::write_function_mappings_to_buffer(
+        &virtual_file_mapping.into_vec(),
+        &expressions,
+        &code_regions,
+        &branch_regions,
+        &mcdc_branch_regions,
+        &mcdc_decision_regions,
+    )
+}
+
+/// Generates the contents of the covfun record for this function, which
+/// contains the function's coverage mapping data. The record is then stored
+/// as a global variable in the `__llvm_covfun` section.
+fn generate_covfun_record(
+    cx: &CodegenCx<'_, '_>,
+    mangled_function_name: &str,
+    source_hash: u64,
+    filenames_ref: u64,
+    coverage_mapping_buffer: Vec<u8>,
+    is_used: bool,
+) {
+    // Concatenate the encoded coverage mappings
+    let coverage_mapping_size = coverage_mapping_buffer.len();
+    let coverage_mapping_val = cx.const_bytes(&coverage_mapping_buffer);
+
+    let func_name_hash = llvm_cov::hash_bytes(mangled_function_name.as_bytes());
+    let func_name_hash_val = cx.const_u64(func_name_hash);
+    let coverage_mapping_size_val = cx.const_u32(coverage_mapping_size as u32);
+    let source_hash_val = cx.const_u64(source_hash);
+    let filenames_ref_val = cx.const_u64(filenames_ref);
+    let func_record_val = cx.const_struct(
+        &[
+            func_name_hash_val,
+            coverage_mapping_size_val,
+            source_hash_val,
+            filenames_ref_val,
+            coverage_mapping_val,
+        ],
+        /*packed=*/ true,
+    );
+
+    // Choose a variable name to hold this function's covfun data.
+    // Functions that are used have a suffix ("u") to distinguish them from
+    // unused copies of the same function (from different CGUs), so that if a
+    // linker sees both it won't discard the used copy's data.
+    let func_record_var_name =
+        CString::new(format!("__covrec_{:X}{}", func_name_hash, if is_used { "u" } else { "" }))
+            .unwrap();
+    debug!("function record var name: {:?}", func_record_var_name);
+
+    let llglobal = llvm::add_global(cx.llmod, cx.val_ty(func_record_val), &func_record_var_name);
+    llvm::set_initializer(llglobal, func_record_val);
+    llvm::set_global_constant(llglobal, true);
+    llvm::set_linkage(llglobal, llvm::Linkage::LinkOnceODRLinkage);
+    llvm::set_visibility(llglobal, llvm::Visibility::Hidden);
+    llvm::set_section(llglobal, cx.covfun_section_name());
+    // LLVM's coverage mapping format specifies 8-byte alignment for items in this section.
+    // <https://llvm.org/docs/CoverageMappingFormat.html>
+    llvm::set_alignment(llglobal, Align::EIGHT);
+    if cx.target_spec().supports_comdat() {
+        llvm::set_comdat(cx.llmod, llglobal, &func_record_var_name);
+    }
+    cx.add_used_global(llglobal);
+}