diff --git a/halo2-base/src/gates/flex_gate.rs b/halo2-base/src/gates/flex_gate.rs
index d5292d90..05317338 100644
--- a/halo2-base/src/gates/flex_gate.rs
+++ b/halo2-base/src/gates/flex_gate.rs
@@ -896,7 +896,7 @@ impl<F: ScalarField> GateInstructions<F> for GateChip<F> {
         let bits = a_bytes
             .as_ref()
             .iter()
-            .flat_map(|byte| (0..8).map(|i| (*byte as u64 >> i) & 1))
+            .flat_map(|byte| (0..8u32).map(|i| (*byte as u64 >> i) & 1))
             .map(|x| Witness(F::from(x)))
             .take(range_bits);
 
diff --git a/halo2-base/src/gates/tests.rs b/halo2-base/src/gates/tests.rs
index cf6a3cb6..06406043 100644
--- a/halo2-base/src/gates/tests.rs
+++ b/halo2-base/src/gates/tests.rs
@@ -54,7 +54,7 @@ fn test_multithread_gates() {
     let mut builder = GateThreadBuilder::mock();
     gate_tests(builder.main(0), inputs);
 
-    let thread_ids = (0..4).map(|_| builder.get_new_thread_id()).collect::<Vec<_>>();
+    let thread_ids = (0..4usize).map(|_| builder.get_new_thread_id()).collect::<Vec<_>>();
     let new_threads = thread_ids
         .into_par_iter()
         .map(|id| {
diff --git a/halo2-base/src/lib.rs b/halo2-base/src/lib.rs
index ccf4f973..3b7523ae 100644
--- a/halo2-base/src/lib.rs
+++ b/halo2-base/src/lib.rs
@@ -2,6 +2,7 @@
 #![feature(trait_alias)]
 #![deny(clippy::perf)]
 #![allow(clippy::too_many_arguments)]
+#![warn(clippy::default_numeric_fallback)]
 
 // different memory allocator options:
 // mimalloc is fastest on Mac M2
diff --git a/halo2-base/src/utils.rs b/halo2-base/src/utils.rs
index 6802b71c..152971ac 100644
--- a/halo2-base/src/utils.rs
+++ b/halo2-base/src/utils.rs
@@ -62,7 +62,7 @@ pub(crate) fn decompose_u64_digits_to_limbs(
     number_of_limbs: usize,
     bit_len: usize,
 ) -> Vec<u64> {
-    debug_assert!(bit_len <= 64);
+    debug_assert!(bit_len < 64);
 
     let mut e = e.into_iter();
     let mask: u64 = (1u64 << bit_len) - 1u64;
@@ -196,22 +196,22 @@ pub fn decompose_biguint<F: BigPrimeField>(
     num_limbs: usize,
     bit_len: usize,
 ) -> Vec<F> {
-    debug_assert!(bit_len > 64 && bit_len <= 128);
+    debug_assert!((64..128).contains(&bit_len));
     let mut e = e.iter_u64_digits();
 
     let mut limb0 = e.next().unwrap_or(0) as u128;
     let mut rem = bit_len - 64;
     let mut u64_digit = e.next().unwrap_or(0);
-    limb0 |= ((u64_digit & ((1 << rem) - 1)) as u128) << 64;
+    limb0 |= ((u64_digit & ((1 << rem) - 1u64)) as u128) << 64u32;
     u64_digit >>= rem;
     rem = 64 - rem;
 
     core::iter::once(F::from_u128(limb0))
         .chain((1..num_limbs).map(|_| {
-            let mut limb: u128 = u64_digit.into();
+            let mut limb = u64_digit as u128;
             let mut bits = rem;
             u64_digit = e.next().unwrap_or(0);
-            if bit_len - bits >= 64 {
+            if bit_len >= 64 + bits {
                 limb |= (u64_digit as u128) << bits;
                 u64_digit = e.next().unwrap_or(0);
                 bits += 64;
@@ -258,13 +258,6 @@ pub fn compose(input: Vec<BigUint>, bit_len: usize) -> BigUint {
     input.iter().rev().fold(BigUint::zero(), |acc, val| (acc << bit_len) + val)
 }
 
-#[cfg(test)]
-#[test]
-fn test_signed_roundtrip() {
-    use crate::halo2_proofs::halo2curves::bn256::Fr;
-    assert_eq!(fe_to_bigint(&bigint_to_fe::<Fr>(&-BigInt::one())), -BigInt::one());
-}
-
 #[cfg(feature = "halo2-axiom")]
 pub use halo2_proofs_axiom::halo2curves::CurveAffineExt;
 
@@ -337,3 +330,76 @@ pub mod fs {
         })
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use crate::halo2_proofs::halo2curves::bn256::Fr;
+    use num_bigint::RandomBits;
+    use rand::{rngs::OsRng, Rng};
+    use std::ops::Shl;
+
+    use super::*;
+
+    #[test]
+    fn test_signed_roundtrip() {
+        use crate::halo2_proofs::halo2curves::bn256::Fr;
+        assert_eq!(fe_to_bigint(&bigint_to_fe::<Fr>(&-BigInt::one())), -BigInt::one());
+    }
+
+    #[test]
+    fn test_decompose_biguint() {
+        let mut rng = OsRng;
+        const MAX_LIMBS: u64 = 5;
+        for bit_len in 64..128usize {
+            for num_limbs in 1..=MAX_LIMBS {
+                for _ in 0..10_000usize {
+                    let mut e: BigUint = rng.sample(RandomBits::new(num_limbs * bit_len as u64));
+                    let limbs = decompose_biguint::<Fr>(&e, num_limbs as usize, bit_len);
+
+                    let limbs2 = {
+                        let mut limbs = vec![];
+                        let mask = BigUint::one().shl(bit_len) - 1usize;
+                        for _ in 0..num_limbs {
+                            let limb = &e & &mask;
+                            let mut bytes_le = limb.to_bytes_le();
+                            bytes_le.resize(32, 0u8);
+                            limbs.push(Fr::from_bytes(&bytes_le.try_into().unwrap()).unwrap());
+                            e >>= bit_len;
+                        }
+                        limbs
+                    };
+                    assert_eq!(limbs, limbs2);
+                }
+            }
+        }
+    }
+
+    #[test]
+    fn test_decompose_u64_digits_to_limbs() {
+        let mut rng = OsRng;
+        const MAX_LIMBS: u64 = 5;
+        for bit_len in 0..64usize {
+            for num_limbs in 1..=MAX_LIMBS {
+                for _ in 0..10_000usize {
+                    let mut e: BigUint = rng.sample(RandomBits::new(num_limbs * bit_len as u64));
+                    let limbs = decompose_u64_digits_to_limbs(
+                        e.to_u64_digits(),
+                        num_limbs as usize,
+                        bit_len,
+                    );
+                    let limbs2 = {
+                        let mut limbs = vec![];
+                        let mask = BigUint::one().shl(bit_len) - 1usize;
+                        for _ in 0..num_limbs {
+                            let limb = &e & &mask;
+                            limbs.push(u64::try_from(limb).unwrap());
+                            e >>= bit_len;
+                        }
+                        limbs
+                    };
+                    assert_eq!(limbs, limbs2);
+                }
+            }
+        }
+    }
+}