Chaos mode MVP: Skip branch optimization in MachBuffer (#6039)

* fuzz: Add chaos mode control plane

Co-authored-by: Falk Zwimpfer <24669719+FalkZ@users.noreply.github.com>
Co-authored-by: Moritz Waser <mzrw.dev@pm.me>

* fuzz: Skip branch optimization with chaos mode

Co-authored-by: Falk Zwimpfer <24669719+FalkZ@users.noreply.github.com>
Co-authored-by: Moritz Waser <mzrw.dev@pm.me>

* fuzz: Rename chaos engine -> control plane

Co-authored-by: Falk Zwimpfer <24669719+FalkZ@users.noreply.github.com>
Co-authored-by: Moritz Waser <mzrw.dev@pm.me>

* chaos mode: refactoring ControlPlane to be passed through the call stack by reference

Co-authored-by: Falk Zwimpfer <24669719+FalkZ@users.noreply.github.com>
Co-authored-by: Remo Senekowitsch <contact@remsle.dev>

* fuzz: annotate chaos todos

Co-authored-by: Falk Zwimpfer <24669719+FalkZ@users.noreply.github.com>
Co-authored-by: Moritz Waser <mzrw.dev@pm.me>

* fuzz: cleanup control plane

Co-authored-by: Falk Zwimpfer <24669719+FalkZ@users.noreply.github.com>
Co-authored-by: Moritz Waser <mzrw.dev@pm.me>

* fuzz: remove control plane from compiler context

Co-authored-by: Falk Zwimpfer <24669719+FalkZ@users.noreply.github.com>
Co-authored-by: Moritz Waser <mzrw.dev@pm.me>

* fuzz: move control plane into emit state

Co-authored-by: Falk Zwimpfer <24669719+FalkZ@users.noreply.github.com>
Co-authored-by: Moritz Waser <mzrw.dev@pm.me>

* fuzz: fix remaining compiler errors

Co-authored-by: Falk Zwimpfer <24669719+FalkZ@users.noreply.github.com>
Co-authored-by: Moritz Waser <mzrw.dev@pm.me>

* fix tests

* refactor emission state ctrl plane accessors

Co-authored-by: Falk Zwimpfer <24669719+FalkZ@users.noreply.github.com>
Co-authored-by: Moritz Waser <mzrw.dev@pm.me>

* centralize conditional compilation of chaos mode

Also cleanup a few straggling dependencies on cranelift-control
that aren't needed anymore.

Co-authored-by: Falk Zwimpfer <24669719+FalkZ@users.noreply.github.com>
Co-authored-by: Moritz Waser <mzrw.dev@pm.me>

* add cranelift-control to published crates

prtest:full

Co-authored-by: Falk Zwimpfer <24669719+FalkZ@users.noreply.github.com>
Co-authored-by: Moritz Waser <mzrw.dev@pm.me>

* add cranelift-control to public crates

Co-authored-by: Falk Zwimpfer <24669719+FalkZ@users.noreply.github.com>
Co-authored-by: Moritz Waser <mzrw.dev@pm.me>

---------

Co-authored-by: Falk Zwimpfer <24669719+FalkZ@users.noreply.github.com>
Co-authored-by: Moritz Waser <mzrw.dev@pm.me>
Co-authored-by: Remo Senekowitsch <contact@remsle.dev>

cranelift/control/src/chaos.rs

@@ -0,0 +1,28 @@
+use arbitrary::Arbitrary;
+
+/// The control plane of chaos mode.
+/// Please see the [crate-level documentation](crate).
+#[derive(Debug, Clone, Default)]
+pub struct ControlPlane {
+    data: Vec<bool>,
+}
+
+impl Arbitrary<'_> for ControlPlane {
+    fn arbitrary<'a>(u: &mut arbitrary::Unstructured<'a>) -> arbitrary::Result<Self> {
+        Ok(Self {
+            data: u.arbitrary()?,
+        })
+    }
+}
+
+impl ControlPlane {
+    /// Returns a pseudo-random boolean if the control plane was constructed
+    /// with `arbitrary`.
+    ///
+    /// The default value `false` will always be returned if the
+    /// pseudo-random data is exhausted or the control plane was constructed
+    /// with `default`.
+    pub fn get_decision(&mut self) -> bool {
+        self.data.pop().unwrap_or_default()
+    }
+}

cranelift/control/src/lib.rs

@@ -0,0 +1,30 @@
+//! # Cranelift Control
+//!
+//! This is the home of the control plane of chaos mode, a compilation feature
+//! intended to be turned on for certain fuzz targets. When the feature is
+//! turned off, as is normally the case, [ControlPlane] will be a zero-sized
+//! type and optimized away.
+//!
+//! While the feature is turned on, the struct [ControlPlane]
+//! provides functionality to tap into pseudo-randomness at specific locations
+//! in the code. It may be used for targeted fuzzing of compiler internals,
+//! e.g. manipulate heuristic optimizations, clobber undefined register bits
+//! etc.
+//!
+//! There are two ways to acquire a [ControlPlane]:
+//! - [arbitrary] for the real deal
+//! - [default] for an "empty" control plane which always returns default
+//!   values
+//!
+//! [arbitrary]: ControlPlane#method.arbitrary
+//! [default]: ControlPlane#method.default
+
+#[cfg(not(feature = "chaos"))]
+mod zero_sized;
+#[cfg(not(feature = "chaos"))]
+pub use zero_sized::*;
+
+#[cfg(feature = "chaos")]
+mod chaos;
+#[cfg(feature = "chaos")]
+pub use chaos::*;

cranelift/control/src/zero_sized.rs

@@ -0,0 +1,28 @@
+//! Shims for ControlPlane when chaos mode is disabled. Enables
+//! unconditional use of the type and its methods throughout cranelift.
+
+/// A shim for ControlPlane when chaos mode is disabled.
+/// Please see the [crate-level documentation](crate).
+#[derive(Debug, Clone, Default)]
+pub struct ControlPlane {
+    /// prevent direct instantiation (use `default` instead)
+    _private: (),
+}
+
+/// A shim for ControlPlane's `Arbitrary` implementation when chaos mode is
+/// disabled. It doesn't consume any bytes and always returns a default
+/// control plane.
+impl arbitrary::Arbitrary<'_> for ControlPlane {
+    fn arbitrary<'a>(_u: &mut arbitrary::Unstructured<'a>) -> arbitrary::Result<Self> {
+        Ok(Self::default())
+    }
+}
+
+impl ControlPlane {
+    /// Returns a pseudo-random boolean. This variant is used when chaos
+    /// mode is disabled. It always returns `false`.
+    #[inline]
+    pub fn get_decision(&mut self) -> bool {
+        false
+    }
+}