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

use std::sync::Arc;

use itertools::Itertools;
use rustc_abi::Align;
use rustc_codegen_ssa::traits::{
    BaseTypeCodegenMethods, ConstCodegenMethods, StaticCodegenMethods,
};
use rustc_data_structures::fx::FxIndexMap;
use rustc_index::IndexVec;
use rustc_middle::ty::TyCtxt;
use rustc_session::RemapFileNameExt;
use rustc_session::config::RemapPathScopeComponents;
use rustc_span::{SourceFile, StableSourceFileId};
use tracing::debug;

use crate::common::CodegenCx;
use crate::coverageinfo::llvm_cov;
use crate::coverageinfo::mapgen::covfun::prepare_covfun_record;
use crate::llvm;

mod covfun;
mod spans;
mod unused;

/// Generates and exports the coverage map, which is embedded in special
/// linker sections in the final binary.
///
/// Those sections are then read and understood by LLVM's `llvm-cov` tool,
/// which is distributed in the `llvm-tools` rustup component.
pub(crate) fn finalize(cx: &CodegenCx<'_, '_>) {
    let tcx = cx.tcx;

    // Ensure that LLVM is using a version of the coverage mapping format that
    // agrees with our Rust-side code. Expected versions (encoded as n-1) are:
    // - `CovMapVersion::Version7` (6) used by LLVM 18-19
    let covmap_version = {
        let llvm_covmap_version = llvm_cov::mapping_version();
        let expected_versions = 6..=6;
        assert!(
            expected_versions.contains(&llvm_covmap_version),
            "Coverage mapping version exposed by `llvm-wrapper` is out of sync; \
            expected {expected_versions:?} but was {llvm_covmap_version}"
        );
        // This is the version number that we will embed in the covmap section:
        llvm_covmap_version
    };

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

    // FIXME(#132395): Can this be none even when coverage is enabled?
    let instances_used = match cx.coverage_cx {
        Some(ref cx) => cx.instances_used.borrow(),
        None => return,
    };

    let mut covfun_records = instances_used
        .iter()
        .copied()
        // Sort by symbol name, so that the global file table is built in an
        // order that doesn't depend on the stable-hash-based order in which
        // instances were visited during codegen.
        .sorted_by_cached_key(|&instance| tcx.symbol_name(instance).name)
        .filter_map(|instance| prepare_covfun_record(tcx, instance, true))
        .collect::<Vec<_>>();

    // In a single designated CGU, also prepare covfun records for functions
    // in this crate that were instrumented for coverage, but are unused.
    if cx.codegen_unit.is_code_coverage_dead_code_cgu() {
        unused::prepare_covfun_records_for_unused_functions(cx, &mut covfun_records);
    }

    // If there are no covfun records for this CGU, don't generate a covmap record.
    // Emitting a covmap record without any covfun records causes `llvm-cov` to
    // fail when generating coverage reports, and if there are no covfun records
    // then the covmap record isn't useful anyway.
    // This should prevent a repeat of <https://github.com/rust-lang/rust/issues/133606>.
    if covfun_records.is_empty() {
        return;
    }

    // Prepare the global file table for this CGU, containing all paths needed
    // by one or more covfun records.
    let global_file_table =
        GlobalFileTable::build(tcx, covfun_records.iter().flat_map(|c| c.all_source_files()));

    for covfun in &covfun_records {
        covfun::generate_covfun_record(cx, &global_file_table, covfun)
    }

    // Generate the coverage map header, which contains the filenames used by
    // this CGU's coverage mappings, and store it in a well-known global.
    // (This is skipped if we returned early due to having no covfun records.)
    generate_covmap_record(cx, covmap_version, &global_file_table.filenames_buffer);
}

/// Maps "global" (per-CGU) file ID numbers to their underlying source file paths.
#[derive(Debug)]
struct GlobalFileTable {
    /// This "raw" table doesn't include the working dir, so a file's
    /// global ID is its index in this set **plus one**.
    raw_file_table: FxIndexMap<StableSourceFileId, String>,

    /// The file table in encoded form (possibly compressed), which can be
    /// included directly in this CGU's `__llvm_covmap` record.
    filenames_buffer: Vec<u8>,

    /// Truncated hash of the bytes in `filenames_buffer`.
    ///
    /// The `llvm-cov` tool uses this hash to associate each covfun record with
    /// its corresponding filenames table, since the final binary will typically
    /// contain multiple covmap records from different compilation units.
    filenames_hash: u64,
}

impl GlobalFileTable {
    /// Builds a "global file table" for this CGU, mapping numeric IDs to
    /// path strings.
    fn build<'a>(tcx: TyCtxt<'_>, all_files: impl Iterator<Item = &'a SourceFile>) -> Self {
        let mut raw_file_table = FxIndexMap::default();

        for file in all_files {
            raw_file_table.entry(file.stable_id).or_insert_with(|| {
                file.name
                    .for_scope(tcx.sess, RemapPathScopeComponents::MACRO)
                    .to_string_lossy()
                    .into_owned()
            });
        }

        // FIXME(Zalathar): Consider sorting the file table here, but maybe
        // only after adding filename support to coverage-dump, so that the
        // table order isn't directly visible in `.coverage-map` snapshots.

        let mut table = Vec::with_capacity(raw_file_table.len() + 1);

        // Since version 6 of the LLVM coverage mapping format, the first entry
        // in the global file table is treated as a base directory, used to
        // resolve any other entries that are stored as relative paths.
        let base_dir = tcx
            .sess
            .opts
            .working_dir
            .for_scope(tcx.sess, RemapPathScopeComponents::MACRO)
            .to_string_lossy();
        table.push(base_dir.as_ref());

        // Add the regular entries after the base directory.
        table.extend(raw_file_table.values().map(|name| name.as_str()));

        // Encode the file table into a buffer, and get the hash of its encoded
        // bytes, so that we can embed that hash in `__llvm_covfun` records.
        let filenames_buffer = llvm_cov::write_filenames_to_buffer(&table);
        let filenames_hash = llvm_cov::hash_bytes(&filenames_buffer);

        Self { raw_file_table, filenames_buffer, filenames_hash }
    }

    fn get_existing_id(&self, file: &SourceFile) -> Option<GlobalFileId> {
        let raw_id = self.raw_file_table.get_index_of(&file.stable_id)?;
        // The raw file table doesn't include an entry for the base dir
        // (which has ID 0), so add 1 to get the correct ID.
        Some(GlobalFileId::from_usize(raw_id + 1))
    }
}

rustc_index::newtype_index! {
    /// An index into the CGU's overall list of file paths. The underlying paths
    /// will be embedded in the `__llvm_covmap` linker section.
    struct GlobalFileId {}
}
rustc_index::newtype_index! {
    /// An index into a function's list of global file IDs. That underlying list
    /// of local-to-global mappings will be embedded in the function's record in
    /// the `__llvm_covfun` linker section.
    struct LocalFileId {}
}

/// Holds a mapping from "local" (per-function) file IDs to their corresponding
/// source files.
#[derive(Debug, Default)]
struct VirtualFileMapping {
    local_file_table: IndexVec<LocalFileId, Arc<SourceFile>>,
}

impl VirtualFileMapping {
    fn push_file(&mut self, source_file: &Arc<SourceFile>) -> LocalFileId {
        self.local_file_table.push(Arc::clone(source_file))
    }

    /// Resolves all of the filenames in this local file mapping to a list of
    /// global file IDs in its CGU, for inclusion in this function's
    /// `__llvm_covfun` record.
    ///
    /// The global file IDs are returned as `u32` to make FFI easier.
    fn resolve_all(&self, global_file_table: &GlobalFileTable) -> Option<Vec<u32>> {
        self.local_file_table
            .iter()
            .map(|file| try {
                let id = global_file_table.get_existing_id(file)?;
                GlobalFileId::as_u32(id)
            })
            .collect::<Option<Vec<_>>>()
    }
}

/// Generates the contents of the covmap record for this CGU, which mostly
/// consists of a header and a list of filenames. The record is then stored
/// as a global variable in the `__llvm_covmap` section.
fn generate_covmap_record<'ll>(cx: &CodegenCx<'ll, '_>, version: u32, filenames_buffer: &[u8]) {
    // A covmap record consists of four target-endian u32 values, followed by
    // the encoded filenames table. Two of the header fields are unused in
    // modern versions of the LLVM coverage mapping format, and are always 0.
    // <https://llvm.org/docs/CoverageMappingFormat.html#llvm-ir-representation>
    // See also `src/llvm-project/clang/lib/CodeGen/CoverageMappingGen.cpp`.
    let covmap_header = cx.const_struct(
        &[
            cx.const_u32(0), // (unused)
            cx.const_u32(filenames_buffer.len() as u32),
            cx.const_u32(0), // (unused)
            cx.const_u32(version),
        ],
        /* packed */ false,
    );
    let covmap_record = cx
        .const_struct(&[covmap_header, cx.const_bytes(filenames_buffer)], /* packed */ false);

    let covmap_global =
        llvm::add_global(cx.llmod, cx.val_ty(covmap_record), &llvm_cov::covmap_var_name());
    llvm::set_initializer(covmap_global, covmap_record);
    llvm::set_global_constant(covmap_global, true);
    llvm::set_linkage(covmap_global, llvm::Linkage::PrivateLinkage);
    llvm::set_section(covmap_global, &llvm_cov::covmap_section_name(cx.llmod));
    // LLVM's coverage mapping format specifies 8-byte alignment for items in this section.
    // <https://llvm.org/docs/CoverageMappingFormat.html>
    llvm::set_alignment(covmap_global, Align::EIGHT);

    cx.add_used_global(covmap_global);
}