From c3130f2783579c4366207d0efc4183651e106583 Mon Sep 17 00:00:00 2001
From: Yuval Goldberg <yuval@starkware.co>
Date: Fri, 28 Feb 2025 00:20:48 +0200
Subject: [PATCH] Limited padding of segments to >=16

---
 CHANGELOG.md                            |  2 ++
 vm/src/vm/runners/builtin_runner/mod.rs | 20 ++++++++++++++++++--
 2 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 740194f41d..fde5e93d9b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,8 @@
 
 #### Upcoming Changes
 
+* feat: Limited padding of builtin segments to >=16 [#1981](https://github.com/lambdaclass/cairo-vm/pull/1981)
+
 #### [2.0.0] - 2025-02-26
 
 * fix: Check overflow in cairo pie address calculation [#1945](https://github.com/lambdaclass/cairo-vm/pull/1945)
diff --git a/vm/src/vm/runners/builtin_runner/mod.rs b/vm/src/vm/runners/builtin_runner/mod.rs
index 9e2fa53d54..b4870ba587 100644
--- a/vm/src/vm/runners/builtin_runner/mod.rs
+++ b/vm/src/vm/runners/builtin_runner/mod.rs
@@ -56,6 +56,8 @@ pub use signature::SignatureBuiltinRunner;
 
 use super::cairo_pie::BuiltinAdditionalData;
 
+const MIN_N_INSTANCES_IN_BUILTIN_SEGMENT: usize = 0;
+
 /* NB: this enum is no accident: we may need (and cairo-vm-py *does* need)
  * structs containing this to be `Send`. The only two ways to achieve that
  * are either storing a `dyn Trait` inside an `Arc<Mutex<&dyn Trait>>` or
@@ -535,10 +537,15 @@ impl BuiltinRunner {
                 let used_cells = self.get_used_cells(&vm.segments)?;
                 if vm.disable_trace_padding {
                     // If trace padding is disabled, we pad the used cells to still ensure that the
-                    // number of instances is a power of 2.
+                    // number of instances is a power of 2, and at least
+                    // MIN_N_INSTANCES_IN_BUILTIN_SEGMENT.
                     let num_instances = self.get_used_instances(&vm.segments)?;
                     let padded_used_cells = if num_instances > 0 {
-                        num_instances.next_power_of_two() * self.cells_per_instance() as usize
+                        let padded_num_instances = std::cmp::max(
+                            MIN_N_INSTANCES_IN_BUILTIN_SEGMENT,
+                            num_instances.next_power_of_two(),
+                        );
+                        padded_num_instances * self.cells_per_instance() as usize
                     } else {
                         0
                     };
@@ -971,6 +978,15 @@ mod tests {
                 assert!(
                     n_allocated_instances_true.is_power_of_two() || n_allocated_instances_true == 0
                 );
+                // Assert the builtin segment is padded to at least
+                // `MIN_N_INSTANCES_IN_BUILTIN_SEGMENT`.
+                // Pedersen proof has exactly one pedersen builtin, so this indeed tests the padding
+                // to at least `MIN_N_INSTANCES_IN_BUILTIN_SEGMENT`.
+                assert!(
+                    n_allocated_instances_true >= MIN_N_INSTANCES_IN_BUILTIN_SEGMENT
+                        || n_allocated_instances_true == 0
+                );
+
                 // Checks that the number of allocated instances is different when trace padding is
                 // enabled/disabled. Holds for this specific program, not always (that is, in other
                 // programs, padding may be of size 0, or the same).