3 files changed, 87 insertions, 3 deletions
diff --git a/src/tools/coverage-dump/src/covfun.rs b/src/tools/coverage-dump/src/covfun.rs
index 4c0ce205822..d3744d3bf80 100644
--- a/src/tools/coverage-dump/src/covfun.rs
+++ b/src/tools/coverage-dump/src/covfun.rs
@@ -2,10 +2,11 @@ use std::collections::HashMap;
 use std::fmt::{self, Debug, Write as _};
 use std::sync::LazyLock;
 
-use anyhow::{Context, anyhow, ensure};
+use anyhow::{Context, anyhow, bail, ensure};
 use itertools::Itertools;
 use regex::Regex;
 
+use crate::covmap::FilenameTables;
 use crate::llvm_utils::unescape_llvm_string_contents;
 use crate::parser::Parser;
 
@@ -14,6 +15,7 @@ mod tests;
 
 pub(crate) fn dump_covfun_mappings(
     llvm_ir: &str,
+    filename_tables: &FilenameTables,
     function_names: &HashMap<u64, String>,
 ) -> anyhow::Result<()> {
     // Extract function coverage entries from the LLVM IR assembly, and associate
@@ -49,7 +51,12 @@ pub(crate) fn dump_covfun_mappings(
         println!("Number of files: {num_files}");
 
         for i in 0..num_files {
-            let global_file_id = parser.read_uleb128_u32()?;
+            let global_file_id = parser.read_uleb128_usize()?;
+            let &CovfunLineData { filenames_hash, .. } = line_data;
+            #[expect(unused)] // Removed later in this PR.
+            let Some(filename) = filename_tables.lookup(filenames_hash, global_file_id) else {
+                bail!("couldn't resolve global file: {filenames_hash}, {global_file_id}");
+            };
             println!("- file {i} => global file {global_file_id}");
         }
 
diff --git a/src/tools/coverage-dump/src/covmap.rs b/src/tools/coverage-dump/src/covmap.rs
new file mode 100644
index 00000000000..2246ca2d575
--- /dev/null
+++ b/src/tools/coverage-dump/src/covmap.rs
@@ -0,0 +1,75 @@
+use std::collections::HashMap;
+use std::sync::LazyLock;
+
+use anyhow::{Context, ensure};
+use regex::Regex;
+
+use crate::llvm_utils::{truncated_md5, unescape_llvm_string_contents};
+use crate::parser::Parser;
+
+#[derive(Debug, Default)]
+pub(crate) struct FilenameTables {
+    map: HashMap<u64, Vec<String>>,
+}
+
+impl FilenameTables {
+    pub(crate) fn lookup(&self, filenames_hash: u64, global_file_id: usize) -> Option<&str> {
+        let table = self.map.get(&filenames_hash)?;
+        let filename = table.get(global_file_id)?;
+        Some(filename)
+    }
+}
+
+struct CovmapLineData {
+    payload: Vec<u8>,
+}
+
+pub(crate) fn make_filename_tables(llvm_ir: &str) -> anyhow::Result<FilenameTables> {
+    let mut map = HashMap::default();
+
+    for line in llvm_ir.lines().filter(|line| is_covmap_line(line)) {
+        let CovmapLineData { payload } = parse_covmap_line(line)?;
+
+        let mut parser = Parser::new(&payload);
+        let n_filenames = parser.read_uleb128_usize()?;
+        let uncompressed_bytes = parser.read_chunk_to_uncompressed_bytes()?;
+        parser.ensure_empty()?;
+
+        let mut filenames_table = vec![];
+
+        let mut parser = Parser::new(&uncompressed_bytes);
+        for _ in 0..n_filenames {
+            let len = parser.read_uleb128_usize()?;
+            let bytes = parser.read_n_bytes(len)?;
+            let filename = str::from_utf8(bytes)?;
+            filenames_table.push(filename.to_owned());
+        }
+
+        let filenames_hash = truncated_md5(&payload);
+        map.insert(filenames_hash, filenames_table);
+    }
+
+    Ok(FilenameTables { map })
+}
+
+fn is_covmap_line(line: &str) -> bool {
+    line.starts_with("@__llvm_coverage_mapping ")
+}
+
+fn parse_covmap_line(line: &str) -> anyhow::Result<CovmapLineData> {
+    ensure!(is_covmap_line(line));
+
+    const RE_STRING: &str = r#"(?x)^
+        @__llvm_coverage_mapping \ =
+        .*
+        \[ [0-9]+ \ x \ i8 \] \ c"(?<payload>[^"]*)"
+        .*$
+    "#;
+    static RE: LazyLock<Regex> = LazyLock::new(|| Regex::new(RE_STRING).unwrap());
+
+    let captures =
+        RE.captures(line).with_context(|| format!("couldn't parse covmap line: {line:?}"))?;
+    let payload = unescape_llvm_string_contents(&captures["payload"]);
+
+    Ok(CovmapLineData { payload })
+}
diff --git a/src/tools/coverage-dump/src/main.rs b/src/tools/coverage-dump/src/main.rs
index 6408b97a06f..2c76d2f2460 100644
--- a/src/tools/coverage-dump/src/main.rs
+++ b/src/tools/coverage-dump/src/main.rs
@@ -1,4 +1,5 @@
 mod covfun;
+mod covmap;
 mod llvm_utils;
 mod parser;
 mod prf_names;
@@ -18,8 +19,9 @@ fn main() -> anyhow::Result<()> {
     let llvm_ir_path = args.get(1).context("LLVM IR file not specified")?;
     let llvm_ir = std::fs::read_to_string(llvm_ir_path).context("couldn't read LLVM IR file")?;
 
+    let filename_tables = covmap::make_filename_tables(&llvm_ir)?;
     let function_names = crate::prf_names::make_function_names_table(&llvm_ir)?;
-    crate::covfun::dump_covfun_mappings(&llvm_ir, &function_names)?;
+    crate::covfun::dump_covfun_mappings(&llvm_ir, &filename_tables, &function_names)?;
 
     Ok(())
 }