Merge pull request #700 from FractalFir/fuzz_support

Add support for automatically reducing found fuzz cases.

2 files changed, 507 insertions, 23 deletions

diff --git a/build_system/src/fuzz.rs b/build_system/src/fuzz.rs
index 05a87412b36..f170453bfe4 100644
--- a/build_system/src/fuzz.rs
+++ b/build_system/src/fuzz.rs
@@ -1,13 +1,19 @@
 use std::ffi::OsStr;
 use std::path::Path;
 
+mod reduce;
+
 use crate::utils::run_command_with_output;
 
 fn show_usage() {
     println!(
         r#"
 `fuzz` command help:
-    --help                 : Show this help"#
+    --reduce               : Reduces a file generated by rustlantis
+    --help                 : Show this help
+    --start                : Start of the fuzzed range
+    --count                : The number of cases to fuzz
+    -j --jobs              : The number of threads to use during fuzzing"#
     );
 }
 
@@ -20,6 +26,16 @@ pub fn run() -> Result<(), String> {
         std::thread::available_parallelism().map(|threads| threads.get()).unwrap_or(1);
     while let Some(arg) = args.next() {
         match arg.as_str() {
+            "--reduce" => {
+                let Some(path) = args.next() else {
+                    return Err("--reduce must be provided with a path".into());
+                };
+                if !std::fs::exists(&path).unwrap_or(false) {
+                    return Err("--reduce must be provided with a valid path".into());
+                }
+                reduce::reduce(&path);
+                return Ok(());
+            }
             "--help" => {
                 show_usage();
                 return Ok(());
@@ -75,16 +91,17 @@ fn fuzz_range(start: u64, end: u64, threads: usize) {
     let start = Arc::new(AtomicU64::new(start));
     // Count time during fuzzing
     let start_time = Instant::now();
+    let mut workers = Vec::with_capacity(threads);
     // Spawn `threads`..
     for _ in 0..threads {
         let start = start.clone();
         // .. which each will ..
-        std::thread::spawn(move || {
+        workers.push(std::thread::spawn(move || {
             // ... grab the next fuzz seed ...
             while start.load(Ordering::Relaxed) < end {
                 let next = start.fetch_add(1, Ordering::Relaxed);
                 // .. test that seed .
-                match test(next) {
+                match test(next, false) {
                     Err(err) => {
                         // If the test failed at compile-time...
                         println!("test({}) failed because {err:?}", next);
@@ -99,21 +116,30 @@ fn fuzz_range(start: u64, end: u64, threads: usize) {
                     Ok(Err(err)) => {
                         // If the test failed at run-time...
                         println!("The LLVM and GCC results don't match for {err:?}");
-                        // ... copy that file to the directory `target/fuzz/runtime_error`...
+                        // ... generate a new file, which prints temporaries(instead of hashing them)...
                         let mut out_path: std::path::PathBuf = "target/fuzz/runtime_error".into();
                         std::fs::create_dir_all(&out_path).unwrap();
-                        // .. into a file named `fuzz{seed}.rs`.
+                        let Ok(Err(tmp_print_err)) = test(next, true) else {
+                            // ... if that file does not reproduce the issue...
+                            // ... save the original sample in a file named `fuzz{seed}.rs`...
+                            out_path.push(&format!("fuzz{next}.rs"));
+                            std::fs::copy(err, &out_path).unwrap();
+                            continue;
+                        };
+                        // ... if that new file still produces the issue, copy it to `fuzz{seed}.rs`..
                         out_path.push(&format!("fuzz{next}.rs"));
-                        std::fs::copy(err, out_path).unwrap();
+                        std::fs::copy(tmp_print_err, &out_path).unwrap();
+                        // ... and start reducing it, using some properties of `rustlantis` to speed up the process.
+                        reduce::reduce(&out_path);
                     }
                     // If the test passed, do nothing
                     Ok(Ok(())) => (),
                 }
             }
-        });
+        }));
     }
     // The "manager" thread loop.
-    while start.load(Ordering::Relaxed) < end {
+    while start.load(Ordering::Relaxed) < end || !workers.iter().all(|t| t.is_finished()) {
         // Every 500 ms...
         let five_hundred_millis = Duration::from_millis(500);
         std::thread::sleep(five_hundred_millis);
@@ -121,7 +147,7 @@ fn fuzz_range(start: u64, end: u64, threads: usize) {
         let remaining = end - start.load(Ordering::Relaxed);
         // ... fix the count(the start counter counts the cases that
         // begun fuzzing, and not only the ones that are done)...
-        let fuzzed = (total - remaining) - threads as u64;
+        let fuzzed = (total - remaining).saturating_sub(threads as u64);
         // ... and the fuzz speed ...
         let iter_per_sec = fuzzed as f64 / start_time.elapsed().as_secs_f64();
         // .. and use them to display fuzzing stats.
@@ -131,6 +157,7 @@ fn fuzz_range(start: u64, end: u64, threads: usize) {
             (remaining as f64) / iter_per_sec
         )
     }
+    drop(workers);
 }
 
 /// Builds & runs a file with LLVM.
@@ -198,37 +225,61 @@ fn release_gcc(path: &std::path::Path) -> Result<Vec<u8>, String> {
     res.extend(output.stderr);
     Ok(res)
 }
-
+type ResultCache = Option<(Vec<u8>, Vec<u8>)>;
 /// Generates a new rustlantis file, & compares the result of running it with GCC and LLVM.
-fn test(seed: u64) -> Result<Result<(), std::path::PathBuf>, String> {
+fn test(seed: u64, print_tmp_vars: bool) -> Result<Result<(), std::path::PathBuf>, String> {
     // Generate a Rust source...
-    let source_file = generate(seed)?;
-    // ... test it with debug LLVM ...
-    let llvm_res = debug_llvm(&source_file)?;
-    // ... test it with release GCC ...
+    let source_file = generate(seed, print_tmp_vars)?;
+    test_file(&source_file, true)
+}
+/// Tests a file with a cached LLVM result. Used for reduction, when it is known
+/// that a given transformation should not change the execution result.
+fn test_cached(
+    source_file: &Path,
+    remove_tmps: bool,
+    cache: &mut ResultCache,
+) -> Result<Result<(), std::path::PathBuf>, String> {
+    //  Test `source_file` with release GCC ...
     let gcc_res = release_gcc(&source_file)?;
+    if cache.is_none() {
+        // ...test `source_file` with debug LLVM ...
+        *cache = Some((debug_llvm(&source_file)?, gcc_res.clone()));
+    }
+    let (llvm_res, old_gcc) = cache.as_ref().unwrap();
     // ... compare the results ...
-    if llvm_res != gcc_res {
+    if *llvm_res != gcc_res && gcc_res == *old_gcc {
         // .. if they don't match, report an error.
-        Ok(Err(source_file))
+        Ok(Err(source_file.to_path_buf()))
     } else {
-        std::fs::remove_file(source_file).map_err(|err| format!("{err:?}"))?;
+        if remove_tmps {
+            std::fs::remove_file(source_file).map_err(|err| format!("{err:?}"))?;
+        }
         Ok(Ok(()))
     }
 }
+fn test_file(
+    source_file: &Path,
+    remove_tmps: bool,
+) -> Result<Result<(), std::path::PathBuf>, String> {
+    let mut uncached = None;
+    test_cached(source_file, remove_tmps, &mut uncached)
+}
 
 /// Generates a new rustlantis file for us to run tests on.
-fn generate(seed: u64) -> Result<std::path::PathBuf, String> {
+fn generate(seed: u64, print_tmp_vars: bool) -> Result<std::path::PathBuf, String> {
     use std::io::Write;
     let mut out_path = std::env::temp_dir();
     out_path.push(&format!("fuzz{seed}.rs"));
     // We need to get the command output here.
-    let out = std::process::Command::new("cargo")
+    let mut generate = std::process::Command::new("cargo");
+    generate
         .args(["run", "--release", "--bin", "generate"])
         .arg(&format!("{seed}"))
-        .current_dir("clones/rustlantis")
-        .output()
-        .map_err(|err| format!("{err:?}"))?;
+        .current_dir("clones/rustlantis");
+    if print_tmp_vars {
+        generate.arg("--debug");
+    }
+    let out = generate.output().map_err(|err| format!("{err:?}"))?;
     // Stuff the rustlantis output in a source file.
     std::fs::File::create(&out_path)
         .map_err(|err| format!("{err:?}"))?
diff --git a/build_system/src/fuzz/reduce.rs b/build_system/src/fuzz/reduce.rs
new file mode 100644
index 00000000000..3c18c9555bd
--- /dev/null
+++ b/build_system/src/fuzz/reduce.rs
@@ -0,0 +1,433 @@
+use std::io::Write;
+use std::path::{Path, PathBuf};
+
+use super::ResultCache;
+
+/// Saves a reduced file for a given `stage`
+fn save_reduction(lines: &[String], path: &PathBuf, stage: &str) {
+    let mut path = path.clone();
+    path.set_extension(&format!("rs.{stage}"));
+    let mut file = std::fs::File::create(&path).expect("Could not create the reduced example file");
+    for line in lines {
+        file.write_all(line.as_bytes()).expect("Could not save the reduced example");
+    }
+}
+
+/// Checks if a given reduction is valid.
+fn test_reduction(lines: &[String], path: &PathBuf, cache: &mut ResultCache) -> bool {
+    let mut path = path.clone();
+    path.set_extension("rs_reduced");
+    let mut file = std::fs::File::create(&path).expect("Could not create the reduced example file");
+    for line in lines {
+        file.write_all(line.as_bytes()).expect("Could not save the reduced example");
+    }
+    let res = super::test_cached(&path, false, cache);
+    let Ok(Err(_)) = res else {
+        return false;
+    };
+    return true;
+}
+
+/// Removes duplicate assignments in bulk.
+/// If a line A = B is followed directly by A = C,
+/// then removing the first line ought to be fully sound,
+/// and not change the behaviour of the program at all. Detect & remove such lines.
+fn remove_dup_assign(
+    file: &mut Vec<String>,
+    path: &PathBuf,
+    starts: usize,
+    ends: usize,
+    cache: &mut ResultCache,
+) {
+    let mut curr = 0;
+    let mut file_copy = file.clone();
+    let mut reduction_count = 0;
+    // Not worth it.
+    if ends - starts < 8 {
+        return;
+    }
+    for index in starts..ends {
+        let Some((prefix, _)) = file_copy[index].split_once('=') else {
+            continue;
+        };
+        let Some((prefix2, postifx2)) = file_copy[index + 1].split_once('=') else {
+            continue;
+        };
+        let prefix = prefix.trim();
+        let prefix2 = prefix2.trim();
+        // FIXME: Right now, remove_dup_assign cares about assignments to the exact same place.
+        // However, given an assigemnt like this:
+        // ```
+        // A.0 = 1_u32;
+        // A = (2_u32, 3.0);
+        // ```
+        // The first assignment could be safely omitted.
+        // Additionally, we try to check if the second assignment could depend on the first one.
+        // In such cases, the result is likely to change, so we bail.
+        if prefix == prefix2 && !postifx2.contains(prefix) {
+            file_copy[index] = "".into();
+            reduction_count += 1;
+        }
+    }
+    // We have removed no lines - no point in testing.
+    if reduction_count == 0 {
+        return;
+    }
+    // Check if the removed lines affected the execution result in any way, shape or form.
+    if test_reduction(&file_copy, &path, cache) {
+        println!("Reduced {path:?} by {} lines `remove_dup_assign`", reduction_count);
+        *file = file_copy;
+    } else {
+        // The execution result changed.
+        // This can occur if the second assignment depended on the first one.
+        // Eg.
+        // ```
+        // a = b + c;
+        // a = a + d;
+        // ```
+        remove_dup_assign(file, path, starts, (starts + ends) / 2, cache);
+        remove_dup_assign(file, path, (starts + ends) / 2, ends, cache);
+    }
+    save_reduction(file, path, "remove_dup_assign");
+}
+
+/// Removes all the unneeded calls to `dump_var`. This is not something tools like `cvise` can do,
+/// but it greately speeds up MIR interpretation + native execution.
+fn remove_dump_var(file: &mut Vec<String>, path: &PathBuf) {
+    let mut curr = 0;
+    // ... try disabling `dump_vars` one by one, until only the necessary ones are left.
+    while curr < file.len() {
+        let Some(line) = file[curr..].iter().position(|line| line.contains("dump_var")) else {
+            // No more `dump_var`s to remove - exit early.
+            break;
+        };
+        // Make the line absolute again.
+        let line = line + curr;
+        let mut file_copy = file.clone();
+        // Try removing 3 consecutive lines(the call, block end and block beginning). This effectively removes a `dump_var`.
+        file_copy.remove(line);
+        file_copy.remove(line);
+        file_copy.remove(line);
+        // Not cached - the execution result can change.
+        let mut uncached = None;
+        // Check if this reduction is valid.
+        if test_reduction(&file_copy, &path, &mut uncached) {
+            println!("Reduced {path:?} by 3 lines `remove_dump_var`");
+            *file = file_copy;
+            curr = line;
+        } else {
+            curr = line + 1;
+        }
+    }
+    save_reduction(file, path, "remove_dump_var");
+}
+
+/// Replaces matches with gotos where possible.
+/// This exploits some properties of rustlantis(match arm order),
+/// and is only soundly applicable to MIR generated by it.
+/// Still, it is not something `cvise` can do, but it simplifies the code a ton.
+fn match_to_goto(file: &mut Vec<String>, path: &PathBuf, cache: &mut ResultCache) {
+    let mut curr = 0;
+
+    while curr < file.len() {
+        let Some(match_starts) = file[curr..].iter().position(|line| line.contains("match")) else {
+            // No more `match`es to remove - exit early.
+            break;
+        };
+        let match_starts = match_starts + curr;
+        // Find the end of the match
+        let Some(match_ends) = file[match_starts..].iter().position(|line| line.contains('}'))
+        else {
+            // Can't find match end - exit early.
+            break;
+        };
+        let match_ends = match_ends + match_starts;
+        let match_body = &file[match_starts..match_ends];
+
+        // Find where this match should normally jump to.
+        // This *should* be the second-last arm of the match, as per the paper(the remaining blocks are decoys).
+        // If this ever changes, this reduction may not always be sound.
+        // This is not a problem, however: we NEED to use MIRI for reduction anwyway,
+        // and it will catch this issue.
+        let jumps_to = &match_body[match_body.len() - 2].trim();
+        let Some((_, bb_ident)) = jumps_to.split_once("bb") else {
+            break;
+        };
+        // We now have the number of the block we jump to at runtime.
+        let bb_ident = bb_ident.trim_matches(',');
+        // Try replacing this match with an unconditional jump.
+        let mut file_copy = file.clone();
+        for _ in match_starts..(match_ends + 1) {
+            file_copy.remove(match_starts);
+        }
+        file_copy.insert(match_starts, format!("Goto(bb{bb_ident})\n"));
+        if test_reduction(&file_copy, &path, cache) {
+            println!("Reduced {path:?} by {} lines `match_to_goto`", match_ends - match_starts);
+            *file = file_copy;
+            curr = match_starts;
+        } else {
+            curr = match_ends;
+        }
+    }
+    save_reduction(file, path, "match_to_goto");
+}
+
+/// At this point, we can try "killing" blocks, by replacing their bodies with calls to `abort`.
+/// This is always sound(the program aborts, so no UB can occur after the block),
+/// and allows us to safely remove *a lot* of unneeded blocks.
+fn block_abort(file: &mut Vec<String>, path: &PathBuf, cache: &mut ResultCache) {
+    let mut curr = 0;
+    while curr < file.len() {
+        let Some(block_starts) = file[curr..]
+            .iter()
+            .position(|line| line.starts_with("bb") && line.trim_end().ends_with(" = {"))
+        else {
+            // No more `block`s to kill - exit early.
+            break;
+        };
+        let block_starts = block_starts + curr;
+        // Find the beginning of the next block to find the end of this block.
+        let Some(block_ends) = file[(block_starts + 1)..]
+            .iter()
+            .position(|line| line.starts_with("bb") && line.trim_end().ends_with(" = {"))
+        else {
+            // No more `block`s to kill - exit early.
+            break;
+        };
+        let block_ends = block_starts + block_ends;
+        let block_starts = block_starts + 1;
+        let mut file_copy = file.clone();
+        // Remove the block body...
+        for _ in block_starts..(block_ends) {
+            file_copy.remove(block_starts);
+        }
+        // ..and insert an unconditional call to abort.
+        file_copy.insert(
+            block_starts,
+            format!("Call(tmp = core::intrinsics::abort(), ReturnTo(bb1), UnwindUnreachable())\n"),
+        );
+        file_copy.insert(block_starts, format!("let tmp = ();\n"));
+
+        if test_reduction(&file_copy, &path, cache) {
+            println!("Reduced {path:?} by {} lines `block_abort`", block_ends - block_starts - 2);
+            *file = file_copy;
+            curr = block_starts;
+        } else {
+            curr = block_ends;
+        }
+    }
+    save_reduction(file, path, "block_abort");
+}
+
+/// Removes unreachable basic blocks
+fn remove_block(file: &mut Vec<String>, path: &PathBuf, cache: &mut ResultCache) {
+    let mut curr = 0;
+
+    // Next, we try to outright remove blocks.
+    while curr < file.len() {
+        let Some(block_starts) = file[curr..]
+            .iter()
+            .position(|line| line.starts_with("bb") && line.trim_end().ends_with(" = {"))
+        else {
+            // No more `block`s to remove - exit early.
+            break;
+        };
+        let block_starts = block_starts + curr;
+        // Find the beginning of the next block to find the end of this block.
+        let Some(block_ends) = file[(block_starts + 1)..]
+            .iter()
+            .position(|line| line.starts_with("bb") && line.trim_end().ends_with(" = {"))
+        else {
+            // No more `block`s to remove - exit early.
+            break;
+        };
+        let block_ends = block_starts + block_ends + 1;
+        // Large blocks are likely to be necessary.
+        if block_ends - block_starts > 6 {
+            curr = block_starts + 1;
+            continue;
+        }
+        let mut file_copy = file.clone();
+        file_copy.drain(block_starts..block_ends);
+        if test_reduction(&file_copy, &path, cache) {
+            println!("Reduced {path:?} by {} lines `remove_blocks`", block_ends - block_starts);
+            *file = file_copy;
+            curr = block_starts;
+        } else {
+            curr = block_starts + 1;
+        }
+    }
+    save_reduction(file, path, "remove_block");
+}
+
+/// Merges blocks ending with unconditional jumps.
+fn linearize_cf(file: &mut Vec<String>, path: &PathBuf, cache: &mut ResultCache) {
+    let mut curr = 0;
+
+    // Next, we try to linearize the control flow. What the does that mean?
+    // Given a sequence like this:
+    // Goto(bb22)
+    // }
+    // bb22 = {
+    // We remove those 3 lines, merging the blocks together. This is not something `cvise` can do,
+    // and it makes other transformations easier.
+    while curr < file.len() {
+        let Some(block_starts) = file[curr..]
+            .iter()
+            .position(|line| line.starts_with("bb") && line.trim_end().ends_with(" = {"))
+        else {
+            // No more `block`s to remove - exit early.
+            break;
+        };
+        let block_starts = block_starts + curr;
+        // Extract the block id.
+        let Some((block, _)) = file[block_starts].split_once('=') else {
+            curr = block_starts + 1;
+            continue;
+        };
+        let block = block.trim();
+        if file[block_starts - 2].trim() != format!("Goto({block})") {
+            curr = block_starts + 1;
+            continue;
+        }
+        let mut file_copy = file.clone();
+        // Try removing 3 consecutive lines(the goto, block end and block beginning). This effectively removes a `Goto(next)`.
+        file_copy.remove(block_starts - 2);
+        file_copy.remove(block_starts - 2);
+        file_copy.remove(block_starts - 2);
+        // Check if this reduction is valid.
+        if test_reduction(&file_copy, &path, cache) {
+            println!("Reduced {path:?} by 3 lines `linearize_cf`");
+            *file = file_copy;
+            curr = block_starts;
+        } else {
+            curr = block_starts + 1;
+        }
+    }
+    save_reduction(file, path, "linearize_cf");
+}
+
+/// Replaces a call to a given function with a 0 assignment to the destination place, and a Goto.
+/// This is always sound, because:
+/// 1. All the functions arguments are always initialized
+/// 2. and point to initialized  memory(the operand of &raw must be an initialized place in rustlantis).
+fn remove_fn_calls(file: &mut Vec<String>, path: &PathBuf, cache: &mut ResultCache) {
+    let mut curr = 0;
+
+    while curr < file.len() {
+        let Some(fn_call) =
+            file[curr..].iter().position(|line| line.contains("Call(") && line.contains(" = fn"))
+        else {
+            // No more calls to remove - exit early.
+            break;
+        };
+        let fn_call = fn_call + curr;
+        let line = file[fn_call].trim();
+        // Skip the Call(
+        let line = &line["Call(".len()..];
+        // Extract the destination place
+        let Some((place, line)) = line.split_once('=') else {
+            curr = fn_call + 1;
+            continue;
+        };
+        // Skip till the return block id.
+        let Some((_, line)) = line.split_once("ReturnTo(") else {
+            curr = fn_call + 1;
+            continue;
+        };
+        // Extract the full return block
+        let Some((block, _)) = line.split_once(')') else {
+            curr = fn_call + 1;
+            continue;
+        };
+        let mut file_copy = file.clone();
+        // Remove the call.
+        file_copy.remove(fn_call);
+        file_copy.insert(fn_call, format!("Goto({block})\n"));
+        file_copy.insert(fn_call, format!("{place} = 0;\n"));
+        // Check if this reduction is valid.
+        if test_reduction(&file_copy, &path, cache) {
+            println!("Reduced {path:?} using `remove_fn_calls` {cache:?}");
+            *file = file_copy;
+            curr = fn_call;
+        } else {
+            curr = fn_call + 1;
+        }
+    }
+    save_reduction(file, path, "remove_fn_calls");
+}
+
+/// Fully removes unreachable functions.
+fn remove_fns(file: &mut Vec<String>, path: &PathBuf, cache: &mut ResultCache) {
+    let mut curr = 0;
+
+    while curr < file.len() {
+        // Find a function start
+        let Some(fn_start) = file[curr..].iter().position(|line| {
+            line.contains("#[custom_mir(dialect = \"runtime\", phase = \"initial\")]")
+        }) else {
+            // No more functions to remove - exit early.
+            break;
+        };
+        // Find the next function(and use that to find the end of this one).
+        // FIXME: this check is flawed: it will never remove the very last function(the one before main).
+        // The other checks will turn that function into a single call to abort, but it is still annoying that it is kept.
+        let fn_start = fn_start + curr;
+        let Some(fn_end) = file[(fn_start + 3)..].iter().position(|line| line.contains("fn fn"))
+        else {
+            // No more functions to remove - exit early.
+            break;
+        };
+        let fn_end = fn_start + 2 + fn_end;
+        let mut file_copy = file.clone();
+        // Remove the function.\\
+        file_copy.drain(fn_start..fn_end);
+        // Check if this reduction is valid.
+        if test_reduction(&file_copy, &path, cache) {
+            println!("Reduced {path:?} by {} lines `remove_fns`", fn_end - fn_start);
+            *file = file_copy;
+        } else {
+            curr = fn_start + 1;
+        }
+    }
+    save_reduction(file, path, "remove_fns");
+}
+
+pub(super) fn reduce(path: impl AsRef<Path>) {
+    let path = path.as_ref().to_owned();
+    // ... read the file to a buffer ..
+    let file = std::fs::read_to_string(&path).expect("Could not open the file to reduce");
+    let mut file: Vec<_> = file.split_inclusive('\n').map(|s| s.to_string()).collect();
+
+    // ... and run reduction passes.
+    println!("running `remove_dump_var` on {path:?}.");
+    remove_dump_var(&mut file, &path);
+    // After `dump_var`, the execution results ought not to change. Cache them.
+    let mut cache = None;
+    // Fill the cache
+    assert!(
+        test_reduction(&file, &path, &mut cache),
+        "Reduction error: check that the input file is a valid reproducer."
+    );
+    println!("cache:{cache:?}");
+    println!("running `remove_fn_calls` on {path:?}.");
+    remove_fn_calls(&mut file, &path, &mut cache);
+    println!("running `remove_fns` on {path:?}.");
+    remove_fns(&mut file, &path, &mut cache);
+    let len = file.len();
+    println!("running `remove_dup_assign` on {path:?}.");
+    remove_dup_assign(&mut file, &path, 0, len, &mut cache);
+    file.retain(|line| !line.is_empty());
+    println!("running `match_to_goto` on {path:?}.");
+    match_to_goto(&mut file, &path, &mut cache);
+    println!("running `block_abort` on {path:?}.");
+    block_abort(&mut file, &path, &mut cache);
+    println!("running `remove_block` on {path:?}.");
+    remove_block(&mut file, &path, &mut cache);
+    println!("running `linearize_cf` on {path:?}.");
+    linearize_cf(&mut file, &path, &mut cache);
+    let mut out = std::fs::File::create(&path).expect("Could not save the reduction result.");
+    for line in file {
+        out.write_all(line.as_bytes());
+    }
+}