fix(integer): fix parallel carry propagation on empty input

tfhe/src/integer/server_key/radix_parallel/add.rs

@@ -545,6 +545,10 @@ impl ServerKey {
         &self,
         generates_or_propagates: Vec<Ciphertext>,
     ) -> (Vec<Ciphertext>, Ciphertext) {
+        if generates_or_propagates.is_empty() {
+            return (vec![], self.key.create_trivial(0));
+        }
+
         let lut_carry_propagation_sum = self
             .key
             .generate_lookup_table_bivariate(prefix_sum_carry_propagation);
@@ -581,6 +585,10 @@ impl ServerKey {
     {
         debug_assert!(self.key.message_modulus.0 * self.key.carry_modulus.0 >= (1 << 4));
 
+        if blocks.is_empty() {
+            return vec![];
+        }
+
         let num_blocks = blocks.len();
         let num_steps = blocks.len().ceil_ilog2() as usize;
 
@@ -1274,6 +1282,19 @@ impl ServerKey {
 #[cfg(test)]
 mod tests {
     use super::should_hillis_steele_propagation_be_faster;
+    use crate::integer::gen_keys_radix;
+    use crate::shortint::prelude::PARAM_MESSAGE_2_CARRY_2_KS_PBS;
+
+    #[test]
+    fn test_propagate_single_carry_on_empty_input_ci_run_filter() {
+        // Parameters and num blocks do not matter here
+        let (_, sks) = gen_keys_radix(PARAM_MESSAGE_2_CARRY_2_KS_PBS, 4);
+
+        let carry = sks.propagate_single_carry_parallelized_low_latency([].as_mut_slice());
+
+        // The most interesting part we test is that the code does not panic
+        assert!(carry.is_trivial());
+    }
 
     #[test]
     fn test_hillis_steele_choice_128_threads() {

tfhe/src/integer/server_key/radix_parallel/mod.rs

@@ -177,6 +177,7 @@ impl ServerKey {
             }
 
             let len = ctxt.blocks().len();
+            // If start_index >= len, the range is considered empty
             for i in start_index..len {
                 let _ = self.propagate_parallelized(ctxt, i);
             }