fix(integer): StaticSignedBigInt right shift

tfhe/src/integer/bigint/algorithms.rs

@@ -327,7 +327,11 @@ pub(crate) fn shr_assign(lhs: &mut [u64], shift: u32, shift_type: ShiftType) {
     let value_mask = u64::MAX >> shift_in_words;
     let carry_mask = ((1u64 << shift_in_words) - 1u64).rotate_right(shift_in_words);
 
-    let mut carry = if sign_bit == 1 { u64::MAX } else { 0 };
+    let mut carry = if sign_bit == 1 {
+        u64::MAX & carry_mask
+    } else {
+        0
+    };
     for word in &mut head.iter_mut().rev() {
         let rotated = word.rotate_right(shift_in_words);
         let value = (rotated & value_mask) | carry;

tfhe/src/integer/bigint/i256.rs

@@ -98,6 +98,21 @@ mod tests {
         assert_eq!(a << 1u32, I256::from(input << 1));
     }
 
+    #[test]
+    fn test_shr() {
+        let input = i128::MIN;
+        let a = I256::from(input);
+        assert_eq!(a >> 1u32, I256::from(input >> 1));
+
+        let a = I256::MIN;
+        // We expect (MSB) 110............0 (LSB)
+        assert_eq!(
+            a >> 1u32,
+            // 3 is '11'
+            I256::from([0, 0, 0, 3 << 62])
+        );
+    }
+
     #[test]
     fn test_div_rem() {
         let i64_max = I256::from(i64::MAX);