Limited padding of segments to >=16

Author: yuvalsw

vm/src/vm/runners/builtin_runner/mod.rs

@@ -522,6 +522,8 @@ impl BuiltinRunner {
         Ok(())
     }
 
+    // TODO(yg): add tests for 16.
+    const MIN_N_INSTANCES_IN_BUILTIN_SEGMENT: usize = 16;
     pub fn get_used_cells_and_allocated_size(
         &self,
         vm: &VirtualMachine,
@@ -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(
+                            Self::MIN_N_INSTANCES_IN_BUILTIN_SEGMENT,
+                            num_instances.next_power_of_two(),
+                        );
+                        padded_num_instances * self.cells_per_instance() as usize
                     } else {
                         0
                     };