Various preparatory changes.

executor/src/witgen/data_structures/mutable_state.rs

@@ -1,12 +1,12 @@
 use std::{
-    cell::RefCell,
+    cell::{RefCell, RefMut},
     collections::{BTreeMap, HashMap},
 };
 
 use powdr_number::FieldElement;
 
 use crate::witgen::{
-    machines::{KnownMachine, Machine},
+    machines::{KnownMachine, LookupCell, Machine},
     rows::RowPair,
     EvalError, EvalResult, QueryCallback,
 };
@@ -52,19 +52,35 @@ impl<'a, T: FieldElement, Q: QueryCallback<T>> MutableState<'a, T, Q> {
     /// Call the machine responsible for the right-hand-side of an identity given its ID
     /// and the row pair of the caller.
     pub fn call(&self, identity_id: u64, caller_rows: &RowPair<'_, 'a, T>) -> EvalResult<'a, T> {
+        self.responsible_machine(identity_id)?
+            .process_plookup_timed(self, identity_id, caller_rows)
+    }
+
+    /// Call the machine responsible for the right-hand-side of an identity given its ID,
+    /// use the direct interface.
+    #[allow(unused)]
+    pub fn call_direct(
+        &self,
+        identity_id: u64,
+        values: &mut [LookupCell<'_, T>],
+    ) -> Result<bool, EvalError<T>> {
+        self.responsible_machine(identity_id)?
+            .process_lookup_direct_timed(self, identity_id, values)
+    }
+
+    fn responsible_machine(
+        &self,
+        identity_id: u64,
+    ) -> Result<RefMut<KnownMachine<'a, T>>, EvalError<T>> {
         let machine_index = *self
             .identity_to_machine_index
             .get(&identity_id)
             .unwrap_or_else(|| panic!("No executor machine matched identity ID: {identity_id}"));
-
-        self.machines[machine_index]
-            .try_borrow_mut()
-            .map_err(|_| {
-                EvalError::RecursiveMachineCalls(format!(
-                    "Detected when processing identity with ID {identity_id}"
-                ))
-            })?
-            .process_plookup_timed(self, identity_id, caller_rows)
+        self.machines[machine_index].try_borrow_mut().map_err(|_| {
+            EvalError::RecursiveMachineCalls(format!(
+                "Detected when processing identity with ID {identity_id}"
+            ))
+        })
     }
 
     /// Extracts the witness column values from the machines.

executor/src/witgen/machines/mod.rs

@@ -66,6 +66,19 @@ pub trait Machine<'a, T: FieldElement>: Send + Sync {
         result
     }
 
+    /// Like 'process_lookup_direct', but also records the time spent in this machine.
+    fn process_lookup_direct_timed<'b, 'c, Q: QueryCallback<T>>(
+        &mut self,
+        mutable_state: &'b MutableState<'a, T, Q>,
+        identity_id: u64,
+        values: &mut [LookupCell<'c, T>],
+    ) -> Result<bool, EvalError<T>> {
+        record_start(self.name());
+        let result = self.process_lookup_direct(mutable_state, identity_id, values);
+        record_end(self.name());
+        result
+    }
+
     /// Returns a unique name for this machine.
     fn name(&self) -> &str;
 
@@ -106,6 +119,7 @@ pub trait Machine<'a, T: FieldElement>: Send + Sync {
     fn identity_ids(&self) -> Vec<u64>;
 }
 
+#[repr(C)]
 pub enum LookupCell<'a, T> {
     /// Value is known (i.e. an input)
     Input(&'a T),

jit-compiler/src/compiler.rs

@@ -218,10 +218,10 @@ pub fn call_cargo(code: &str) -> Result<PathInTempDir, String> {
         if log::log_enabled!(log::Level::Debug) {
             let stderr = from_utf8(&out.stderr).unwrap_or("UTF-8 error in error message.");
             return Err(format!(
-                "Rust compiler error when JIT-compiling. Will use evaluator for all symbols. Error message:\n{stderr}."
+                "Rust compiler error when JIT-compiling. Will use interpreter instead. Error message:\n{stderr}."
             ));
         } else {
-            return Err("Rust compiler error when JIT-compiling. Will use evaluator for all symbols. Set log level to DEBUG for reason.".to_string());
+            return Err("Rust compiler error when JIT-compiling. Will use interpreter instead. Set log level to DEBUG for reason.".to_string());
         }
     }
     let extension = if cfg!(target_os = "windows") {

jit-compiler/src/lib.rs

@@ -73,7 +73,7 @@ pub fn compile<T: FieldElement>(
         let successful_hash = successful_symbol_names.iter().collect::<HashSet<_>>();
         // TODO this should be changed back to Info after the introduction of the ToCol trait.
         log::debug!(
-            "Unable to generate code during JIT-compilation for the following symbols. Will use evaluator instead.\n{}",
+            "Unable to generate code during JIT-compilation for the following symbols. Will use interpreter instead.\n{}",
             requested_symbols
                 .iter()
                 .filter(|&sym| !successful_hash.contains(sym))

number/src/goldilocks.rs

@@ -398,6 +398,10 @@ impl FieldElement for GoldilocksField {
         // Undo the shift
         Some(v.wrapping_sub(SHIFT as u32) as i32)
     }
+
+    fn has_direct_repr() -> bool {
+        true
+    }
 }
 
 impl LowerHex for GoldilocksField {

number/src/macros.rs

@@ -400,6 +400,10 @@ macro_rules! powdr_field {
                 // Undo the shift
                 Some(v.wrapping_sub(SHIFT as u32) as i32)
             }
+
+            fn has_direct_repr() -> bool {
+                false
+            }
         }
 
         impl From<$ark_type> for $name {

number/src/plonky3_macros.rs

@@ -117,6 +117,11 @@ macro_rules! powdr_field_plonky3 {
             fn try_into_i32(&self) -> Option<i32> {
                 Some(self.to_canonical_u32() as i32)
             }
+
+            fn has_direct_repr() -> bool {
+                // No direct repr, because 'mod' is not always applied.
+                false
+            }
         }
 
         impl LowerHex for $name {

number/src/traits.rs

@@ -185,6 +185,12 @@ pub trait FieldElement:
     /// As conventional, negative values are in relation to 0 in the field.
     /// Returns None if out of the range [0 - 2^31, 2^31).
     fn try_into_i32(&self) -> Option<i32>;
+
+    /// Returns `true` if values of this type are directly stored as their integer
+    /// value (i.e. not in montgomery representation and there are also no
+    /// additional fields), i.e. the `to_integer` function can be implemented as
+    /// a mem::transmute operation on pointers.
+    fn has_direct_repr() -> bool;
 }
 
 #[cfg(test)]