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

use crate::common::CodegenCx;
use crate::coverageinfo;
use crate::llvm;

use llvm::coverageinfo::CounterMappingRegion;
use rustc_codegen_ssa::coverageinfo::map::{Counter, CounterExpression};
use rustc_codegen_ssa::traits::{ConstMethods, CoverageInfoMethods};
use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexSet};
use rustc_hir::def_id::{DefId, DefIdSet, LOCAL_CRATE};
use rustc_llvm::RustString;
use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags;
use rustc_middle::mir::coverage::CodeRegion;
use rustc_span::Symbol;

use std::ffi::CString;

use tracing::debug;

/// Generates and exports the Coverage Map.
///
/// This Coverage Map complies with Coverage Mapping Format version 4 (zero-based encoded as 3),
/// as defined at [LLVM Code Coverage Mapping Format](https://github.com/rust-lang/llvm-project/blob/rustc/11.0-2020-10-12/llvm/docs/CoverageMappingFormat.rst#llvm-code-coverage-mapping-format)
/// and published in Rust's November 2020 fork of LLVM. This version is supported by the LLVM
/// coverage tools (`llvm-profdata` and `llvm-cov`) bundled with Rust's fork of LLVM.
///
/// Consequently, Rust's bundled version of Clang also generates Coverage Maps compliant with
/// the same version. Clang's implementation of Coverage Map generation was referenced when
/// implementing this Rust version, and though the format documentation is very explicit and
/// detailed, some undocumented details in Clang's implementation (that may or may not be important)
/// were also replicated for Rust's Coverage Map.
pub fn finalize<'ll, 'tcx>(cx: &CodegenCx<'ll, 'tcx>) {
    let tcx = cx.tcx;

    // Ensure LLVM supports Coverage Map Version 4 (encoded as a zero-based value: 3).
    // If not, the LLVM Version must be less than 11.
    let version = coverageinfo::mapping_version();
    if version != 3 {
        tcx.sess.fatal("rustc option `-Z instrument-coverage` requires LLVM 11 or higher.");
    }

    debug!("Generating coverage map for CodegenUnit: `{}`", cx.codegen_unit.name());

    // In order to show that unused functions have coverage counts of zero (0), LLVM requires the
    // functions exist. Generate synthetic functions with a (required) single counter, and add the
    // MIR `Coverage` code regions to the `function_coverage_map`, before calling
    // `ctx.take_function_coverage_map()`.
    if !tcx.sess.instrument_coverage_except_unused_functions() {
        add_unused_functions(cx);
    }

    let function_coverage_map = match cx.coverage_context() {
        Some(ctx) => ctx.take_function_coverage_map(),
        None => return,
    };

    if function_coverage_map.is_empty() {
        // This module has no functions with coverage instrumentation
        return;
    }

    let mut mapgen = CoverageMapGenerator::new();

    // Encode coverage mappings and generate function records
    let mut function_data = Vec::new();
    for (instance, function_coverage) in function_coverage_map {
        debug!("Generate function coverage for {}, {:?}", cx.codegen_unit.name(), instance);
        let mangled_function_name = tcx.symbol_name(instance).to_string();
        let source_hash = function_coverage.source_hash();
        let is_used = function_coverage.is_used();
        let (expressions, counter_regions) =
            function_coverage.get_expressions_and_counter_regions();

        let coverage_mapping_buffer = llvm::build_byte_buffer(|coverage_mapping_buffer| {
            mapgen.write_coverage_mapping(expressions, counter_regions, coverage_mapping_buffer);
        });
        debug_assert!(
            coverage_mapping_buffer.len() > 0,
            "Every `FunctionCoverage` should have at least one counter"
        );

        function_data.push((mangled_function_name, source_hash, is_used, coverage_mapping_buffer));
    }

    // Encode all filenames referenced by counters/expressions in this module
    let filenames_buffer = llvm::build_byte_buffer(|filenames_buffer| {
        coverageinfo::write_filenames_section_to_buffer(&mapgen.filenames, filenames_buffer);
    });

    let filenames_size = filenames_buffer.len();
    let filenames_val = cx.const_bytes(&filenames_buffer[..]);
    let filenames_ref = coverageinfo::hash_bytes(filenames_buffer);

    // Generate the LLVM IR representation of the coverage map and store it in a well-known global
    let cov_data_val = mapgen.generate_coverage_map(cx, version, filenames_size, filenames_val);

    for (mangled_function_name, source_hash, is_used, coverage_mapping_buffer) in function_data {
        save_function_record(
            cx,
            mangled_function_name,
            source_hash,
            filenames_ref,
            coverage_mapping_buffer,
            is_used,
        );
    }

    // Save the coverage data value to LLVM IR
    coverageinfo::save_cov_data_to_mod(cx, cov_data_val);
}

struct CoverageMapGenerator {
    filenames: FxIndexSet<CString>,
}

impl CoverageMapGenerator {
    fn new() -> Self {
        Self { filenames: FxIndexSet::default() }
    }

    /// Using the `expressions` and `counter_regions` collected for the current function, generate
    /// the `mapping_regions` and `virtual_file_mapping`, and capture any new filenames. Then use
    /// LLVM APIs to encode the `virtual_file_mapping`, `expressions`, and `mapping_regions` into
    /// the given `coverage_mapping` byte buffer, compliant with the LLVM Coverage Mapping format.
    fn write_coverage_mapping(
        &mut self,
        expressions: Vec<CounterExpression>,
        counter_regions: impl Iterator<Item = (Counter, &'a CodeRegion)>,
        coverage_mapping_buffer: &RustString,
    ) {
        let mut counter_regions = counter_regions.collect::<Vec<_>>();
        if counter_regions.is_empty() {
            return;
        }

        let mut virtual_file_mapping = Vec::new();
        let mut mapping_regions = Vec::new();
        let mut current_file_name = None;
        let mut current_file_id = 0;

        // Convert the list of (Counter, CodeRegion) pairs to an array of `CounterMappingRegion`, sorted
        // by filename and position. Capture any new files to compute the `CounterMappingRegion`s
        // `file_id` (indexing files referenced by the current function), and construct the
        // function-specific `virtual_file_mapping` from `file_id` to its index in the module's
        // `filenames` array.
        counter_regions.sort_unstable_by_key(|(_counter, region)| *region);
        for (counter, region) in counter_regions {
            let CodeRegion { file_name, start_line, start_col, end_line, end_col } = *region;
            let same_file = current_file_name.as_ref().map_or(false, |p| *p == file_name);
            if !same_file {
                if current_file_name.is_some() {
                    current_file_id += 1;
                }
                current_file_name = Some(file_name);
                let c_filename = CString::new(file_name.to_string())
                    .expect("null error converting filename to C string");
                debug!("  file_id: {} = '{:?}'", current_file_id, c_filename);
                let (filenames_index, _) = self.filenames.insert_full(c_filename);
                virtual_file_mapping.push(filenames_index as u32);
            }
            debug!("Adding counter {:?} to map for {:?}", counter, region);
            mapping_regions.push(CounterMappingRegion::code_region(
                counter,
                current_file_id,
                start_line,
                start_col,
                end_line,
                end_col,
            ));
        }

        // Encode and append the current function's coverage mapping data
        coverageinfo::write_mapping_to_buffer(
            virtual_file_mapping,
            expressions,
            mapping_regions,
            coverage_mapping_buffer,
        );
    }

    /// Construct coverage map header and the array of function records, and combine them into the
    /// coverage map. Save the coverage map data into the LLVM IR as a static global using a
    /// specific, well-known section and name.
    fn generate_coverage_map(
        self,
        cx: &CodegenCx<'ll, 'tcx>,
        version: u32,
        filenames_size: usize,
        filenames_val: &'ll llvm::Value,
    ) -> &'ll llvm::Value {
        debug!("cov map: filenames_size = {}, 0-based version = {}", filenames_size, version);

        // Create the coverage data header (Note, fields 0 and 2 are now always zero,
        // as of `llvm::coverage::CovMapVersion::Version4`.)
        let zero_was_n_records_val = cx.const_u32(0);
        let filenames_size_val = cx.const_u32(filenames_size as u32);
        let zero_was_coverage_size_val = cx.const_u32(0);
        let version_val = cx.const_u32(version);
        let cov_data_header_val = cx.const_struct(
            &[zero_was_n_records_val, filenames_size_val, zero_was_coverage_size_val, version_val],
            /*packed=*/ false,
        );

        // Create the complete LLVM coverage data value to add to the LLVM IR
        cx.const_struct(&[cov_data_header_val, filenames_val], /*packed=*/ false)
    }
}

/// Construct a function record and combine it with the function's coverage mapping data.
/// Save the function record into the LLVM IR as a static global using a
/// specific, well-known section and name.
fn save_function_record(
    cx: &CodegenCx<'ll, 'tcx>,
    mangled_function_name: String,
    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 = coverageinfo::hash_str(&mangled_function_name);
    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,
    );

    coverageinfo::save_func_record_to_mod(cx, func_name_hash, func_record_val, is_used);
}

/// When finalizing the coverage map, `FunctionCoverage` only has the `CodeRegion`s and counters for
/// the functions that went through codegen; such as public functions and "used" functions
/// (functions referenced by other "used" or public items). Any other functions considered unused,
/// or "Unreachable", were still parsed and processed through the MIR stage, but were not
/// codegenned. (Note that `-Clink-dead-code` can force some unused code to be codegenned, but
/// that flag is known to cause other errors, when combined with `-Z instrument-coverage`; and
/// `-Clink-dead-code` will not generate code for unused generic functions.)
///
/// We can find the unused functions (including generic functions) by the set difference of all MIR
/// `DefId`s (`tcx` query `mir_keys`) minus the codegenned `DefId`s (`tcx` query
/// `collect_and_partition_mono_items`).
///
/// *HOWEVER* the codegenned `DefId`s are partitioned across multiple `CodegenUnit`s (CGUs), and
/// this function is processing a `function_coverage_map` for the functions (`Instance`/`DefId`)
/// allocated to only one of those CGUs. We must NOT inject any unused functions's `CodeRegion`s
/// more than once, so we have to pick a CGUs `function_coverage_map` into which the unused
/// function will be inserted.
fn add_unused_functions<'ll, 'tcx>(cx: &CodegenCx<'ll, 'tcx>) {
    let tcx = cx.tcx;

    // FIXME(#79622): Can this solution be simplified and/or improved? Are there other sources
    // of compiler state data that might help (or better sources that could be exposed, but
    // aren't yet)?

    let ignore_unused_generics = tcx.sess.instrument_coverage_except_unused_generics();

    let all_def_ids: DefIdSet = tcx
        .mir_keys(LOCAL_CRATE)
        .iter()
        .filter_map(|local_def_id| {
            let def_id = local_def_id.to_def_id();
            if ignore_unused_generics && tcx.generics_of(def_id).requires_monomorphization(tcx) {
                return None;
            }
            Some(local_def_id.to_def_id())
        })
        .collect();

    let codegenned_def_ids = tcx.codegened_and_inlined_items(LOCAL_CRATE);

    let mut unused_def_ids_by_file: FxHashMap<Symbol, Vec<DefId>> = FxHashMap::default();
    for &non_codegenned_def_id in all_def_ids.difference(codegenned_def_ids) {
        let codegen_fn_attrs = tcx.codegen_fn_attrs(non_codegenned_def_id);
        if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::NO_COVERAGE) {
            continue;
        }
        // Make sure the non-codegenned (unused) function has a file_name
        if let Some(non_codegenned_file_name) = tcx.covered_file_name(non_codegenned_def_id) {
            let def_ids =
                unused_def_ids_by_file.entry(*non_codegenned_file_name).or_insert_with(Vec::new);
            def_ids.push(non_codegenned_def_id);
        }
    }

    if unused_def_ids_by_file.is_empty() {
        // There are no unused functions with file names to add (in any CGU)
        return;
    }

    // Each `CodegenUnit` (CGU) has its own function_coverage_map, and generates a specific binary
    // with its own coverage map.
    //
    // Each covered function `Instance` can be included in only one coverage map, produced from a
    // specific function_coverage_map, from a specific CGU.
    //
    // Since unused functions did not generate code, they are not associated with any CGU yet.
    //
    // To avoid injecting the unused functions in multiple coverage maps (for multiple CGUs)
    // determine which function_coverage_map has the responsibility for publishing unreachable
    // coverage, based on file name: For each unused function, find the CGU that generates the
    // first function (based on sorted `DefId`) from the same file.
    //
    // Add a new `FunctionCoverage` to the `function_coverage_map`, with unreachable code regions
    // for each region in it's MIR.

    // Convert the `HashSet` of `codegenned_def_ids` to a sortable vector, and sort them.
    let mut sorted_codegenned_def_ids: Vec<DefId> =
        codegenned_def_ids.iter().map(|def_id| *def_id).collect();
    sorted_codegenned_def_ids.sort_unstable();

    let mut first_covered_def_id_by_file: FxHashMap<Symbol, DefId> = FxHashMap::default();
    for &def_id in sorted_codegenned_def_ids.iter() {
        if let Some(covered_file_name) = tcx.covered_file_name(def_id) {
            // Only add files known to have unused functions
            if unused_def_ids_by_file.contains_key(covered_file_name) {
                first_covered_def_id_by_file.entry(*covered_file_name).or_insert(def_id);
            }
        }
    }

    // Get the set of def_ids with coverage regions, known by *this* CoverageContext.
    let cgu_covered_def_ids: DefIdSet = match cx.coverage_context() {
        Some(ctx) => ctx
            .function_coverage_map
            .borrow()
            .keys()
            .map(|&instance| instance.def.def_id())
            .collect(),
        None => return,
    };

    let cgu_covered_files: FxHashSet<Symbol> = first_covered_def_id_by_file
        .iter()
        .filter_map(
            |(&file_name, def_id)| {
                if cgu_covered_def_ids.contains(def_id) { Some(file_name) } else { None }
            },
        )
        .collect();

    // For each file for which this CGU is responsible for adding unused function coverage,
    // get the `def_id`s for each unused function (if any), define a synthetic function with a
    // single LLVM coverage counter, and add the function's coverage `CodeRegion`s. to the
    // function_coverage_map.
    for covered_file_name in cgu_covered_files {
        for def_id in unused_def_ids_by_file.remove(&covered_file_name).into_iter().flatten() {
            cx.define_unused_fn(def_id);
        }
    }
}