perf: parallel build_lookup_fractions + drop cycle_lookups

CHANGELOG.md

@@ -81,6 +81,7 @@
 - Aligned `core_lib::math::u256` user docs with unified LE stack limb ordering (`a0/b0` on top), removing conflicting `[b7..b0, a7..a0]` notation ([#3066](https://github.com/0xMiden/miden-vm/pull/3066)).
 - Made all internal `core::math` procedures natively little-endian ([#3084](https://github.com/0xMiden/miden-vm/pull/3084)).
 - [BREAKING] Updated the Miden crypto stack to `miden-crypto` v0.25, and switched SMT leaf hashing to use Poseidon2 domain separation so masm-side leaf digests match `SmtLeaf::hash()` ([#3095](https://github.com/0xMiden/miden-vm/pull/3095)).
+- Improved performances of auxiliary trace generation ([#3119](https://github.com/0xMiden/miden-vm/pull/3119)).
 
 ## 0.22.3 (2026-05-01)
 

air/src/lookup/aux_builder.rs

@@ -54,7 +54,7 @@ use super::{Challenges, LookupAir, ProverLookupBuilder, prover::build_lookup_fra
 /// [`crate::trace::main_trace::ROW_MAJOR_CHUNK_SIZE`] so we stay consistent with the
 /// repo's row-major tuning: ~512 rows × avg shape ~3 ≈ 1.5 K fractions per chunk and
 /// ~24 KiB of chunk-local scratch, comfortably L1-resident on any modern x86/arm core.
-const ACCUMULATE_ROWS_PER_CHUNK: usize = 512;
+pub(crate) const ACCUMULATE_ROWS_PER_CHUNK: usize = 512;
 
 // TOP-LEVEL DRIVER
 // ================================================================================================
@@ -174,6 +174,26 @@ where
         }
     }
 
+    #[cfg(feature = "concurrent")]
+    /// Build a `LookupFractions` from already-populated `fractions` and `counts` buffers.
+    pub(super) fn from_parts(
+        shape: Vec<usize>,
+        num_rows: usize,
+        fractions: Vec<(F, EF)>,
+        counts: Vec<usize>,
+    ) -> Self {
+        let num_cols = shape.len();
+        debug_assert_eq!(counts.len(), num_rows * num_cols);
+
+        Self {
+            fractions,
+            counts,
+            shape,
+            num_rows,
+            num_cols,
+        }
+    }
+
     /// Number of permutation columns.
     pub fn num_columns(&self) -> usize {
         self.num_cols

air/src/lookup/prover.rs

@@ -156,34 +156,69 @@ pub fn build_lookup_fractions<A, F, EF>(
     challenges: &Challenges<EF>,
 ) -> LookupFractions<F, EF>
 where
-    F: Field,
-    EF: ExtensionField<F>,
+    F: Field + Sync,
+    EF: ExtensionField<F> + Sync,
+    A: Sync,
     for<'a> A: LookupAir<ProverLookupBuilder<'a, F, EF>>,
 {
     let num_rows = main_trace.height();
     let width = main_trace.width();
     let flat: &[F] = main_trace.values.borrow();
 
     let shape = air.column_shape().to_vec();
-    let mut fractions = LookupFractions::from_shape(shape, num_rows);
 
-    // Per-row periodic slice, filled in place each row — no per-iteration allocation.
-    let mut periodic_row: Vec<F> = vec![F::ZERO; periodic_columns.len()];
-
-    for r in 0..num_rows {
-        let curr = &flat[r * width..(r + 1) * width];
-        let nxt_idx = (r + 1) % num_rows;
-        let next = &flat[nxt_idx * width..(nxt_idx + 1) * width];
-        let window = RowWindow::from_two_rows(curr, next);
+    // Fill one chunk of rows into a fresh per-chunk `LookupFractions`.
+    let process_chunk = |row_lo: usize, row_hi: usize| -> LookupFractions<F, EF> {
+        let mut chunk = LookupFractions::from_shape(shape.clone(), row_hi - row_lo);
+        let mut periodic_row: Vec<F> = vec![F::ZERO; periodic_columns.len()];
+        for r in row_lo..row_hi {
+            let curr = &flat[r * width..(r + 1) * width];
+            let nxt_idx = (r + 1) % num_rows;
+            let next = &flat[nxt_idx * width..(nxt_idx + 1) * width];
+            let window = RowWindow::from_two_rows(curr, next);
+            for (i, col) in periodic_columns.iter().enumerate() {
+                periodic_row[i] = col[r % col.len()];
+            }
+            let mut lb =
+                ProverLookupBuilder::new(window, &periodic_row, challenges, air, &mut chunk);
+            air.eval(&mut lb);
+        }
+        chunk
+    };
 
-        for (i, col) in periodic_columns.iter().enumerate() {
-            periodic_row[i] = col[r % col.len()];
+    #[cfg(not(feature = "concurrent"))]
+    let fractions = process_chunk(0, num_rows);
+
+    // Concatenation after parallel processing preserves global row order because chunks
+    // tile `0..num_rows` contiguously and each chunk's `fractions` / `counts` are
+    // row-major within the chunk.
+    #[cfg(feature = "concurrent")]
+    let fractions = {
+        use miden_crypto::parallel::*;
+
+        let num_cols = shape.len();
+        let rows_per_chunk = crate::lookup::aux_builder::ACCUMULATE_ROWS_PER_CHUNK;
+        let num_chunks = num_rows.div_ceil(rows_per_chunk);
+
+        let chunks: Vec<LookupFractions<F, EF>> = (0..num_chunks)
+            .into_par_iter()
+            .map(|chunk_idx| {
+                let row_lo = chunk_idx * rows_per_chunk;
+                let row_hi = (row_lo + rows_per_chunk).min(num_rows);
+                process_chunk(row_lo, row_hi)
+            })
+            .collect();
+
+        let total_fractions: usize = chunks.iter().map(|c| c.fractions.len()).sum();
+        let mut fractions_vec: Vec<(F, EF)> = Vec::with_capacity(total_fractions);
+        let mut counts_vec: Vec<usize> = Vec::with_capacity(num_rows * num_cols);
+        for chunk in chunks {
+            fractions_vec.extend(chunk.fractions);
+            counts_vec.extend(chunk.counts);
         }
 
-        let mut lb =
-            ProverLookupBuilder::new(window, &periodic_row, challenges, air, &mut fractions);
-        air.eval(&mut lb);
-    }
+        LookupFractions::from_parts(shape, num_rows, fractions_vec, counts_vec)
+    };
 
     debug_assert_eq!(
         fractions.counts().len(),

processor/src/trace/chiplets/memory/mod.rs

@@ -280,7 +280,7 @@ impl Memory {
                     };
 
                     let (delta_hi, delta_lo) = split_u32_into_u16(delta);
-                    range.add_range_checks(row, &[delta_lo, delta_hi]);
+                    range.add_range_checks(&[delta_lo, delta_hi]);
 
                     // word index decomposition range checks: prove addr is a valid 32-bit value
                     // by checking w0, w1, and 4*w1 are all in [0, 2^16).

processor/src/trace/parallel/mod.rs

@@ -422,8 +422,8 @@ fn initialize_range_checker(
     let mut range_checker = RangeChecker::new();
 
     // Add all u32 range checks recorded during execution
-    for (clk, values) in range_checker_replay.into_iter() {
-        range_checker.add_range_checks(clk, &values);
+    for (_clk, values) in range_checker_replay.into_iter() {
+        range_checker.add_range_checks(&values);
     }
 
     // Add all memory-related range checks

processor/src/trace/range/mod.rs

@@ -3,7 +3,6 @@ use core::mem::MaybeUninit;
 
 use miden_air::trace::RANGE_CHECK_TRACE_WIDTH;
 
-use super::RowIndex;
 use crate::{
     Felt, ZERO,
     utils::{assume_init_vec, uninit_vector},
@@ -50,10 +49,6 @@ pub struct RangeCheckTrace {
 pub struct RangeChecker {
     /// Tracks lookup count for each checked value.
     lookups: BTreeMap<u16, usize>,
-    /// Range check lookups performed by all user operations, grouped and sorted by clock cycle.
-    /// Each cycle is mapped to a vector of the range checks requested at that cycle, which can
-    /// come from the stack, memory, or both.
-    cycle_lookups: BTreeMap<RowIndex, Vec<u16>>,
 }
 
 impl RangeChecker {
@@ -66,7 +61,7 @@ impl RangeChecker {
         // range checker table are initialized. this simplifies trace table building later on.
         lookups.insert(0, 0);
         lookups.insert(u16::MAX, 0);
-        Self { lookups, cycle_lookups: BTreeMap::new() }
+        Self { lookups }
     }
 
     // TRACE MUTATORS
@@ -78,7 +73,7 @@ impl RangeChecker {
     }
 
     /// Adds range check lookups from the stack or memory to this [RangeChecker] instance.
-    pub fn add_range_checks(&mut self, clk: RowIndex, values: &[u16]) {
+    pub fn add_range_checks(&mut self, values: &[u16]) {
         // range checks requests only come from memory or from the stack, which always request 2 or
         // 4 lookups respectively.
         debug_assert!(values.len() == 2 || values.len() == 4);
@@ -87,17 +82,6 @@ impl RangeChecker {
             // add the specified value to the trace of this range checker's lookups.
             self.add_value(*value);
         }
-
-        // track the range check requests at each cycle
-        // TODO: optimize this to use a struct instead of vectors, e.g. (#2793):
-        // struct MemoryLookupValues {
-        //   num_lookups: u8,
-        //   lookup_values: [u16; 6],
-        // }
-        self.cycle_lookups
-            .entry(clk)
-            .and_modify(|entry| entry.extend_from_slice(values))
-            .or_insert_with(|| values.to_vec());
     }
 
     // EXECUTION TRACE GENERATION (INTERNAL)