From b352e15b9ee735f5282678ca5f0a7e800d5694b4 Mon Sep 17 00:00:00 2001
From: Georg Wiese <georgwiese@gmail.com>
Date: Thu, 9 Jan 2025 04:53:04 +0100
Subject: [PATCH] Parallelize index lookup => 30% faster

---
 executor/src/witgen/multiplicity_column_generator.rs | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/executor/src/witgen/multiplicity_column_generator.rs b/executor/src/witgen/multiplicity_column_generator.rs
index 1d8149ff5..d7e8a6f91 100644
--- a/executor/src/witgen/multiplicity_column_generator.rs
+++ b/executor/src/witgen/multiplicity_column_generator.rs
@@ -123,8 +123,14 @@ impl<'a, T: FieldElement> MultiplicityColumnGenerator<'a, T> {
                 .or_insert_with(|| vec![0; rhs_machine_size]);
             assert_eq!(multiplicities.len(), rhs_machine_size);
 
-            for (_, tuple) in lhs_tuples {
-                multiplicities[index[&tuple]] += 1;
+            // Looking up the index is slow, so we do it in parallel.
+            let indices = lhs_tuples
+                .into_par_iter()
+                .map(|(_, tuple)| index[&tuple])
+                .collect::<Vec<_>>();
+
+            for index in indices {
+                multiplicities[index] += 1;
             }
             log::trace!(
                 "    Done updating multiplicities, took: {}s",