diff --git a/libc/src/__support/FPUtil/dyadic_float.h b/libc/src/__support/FPUtil/dyadic_float.h
index 49fb11971104b3..12a69228d36c7b 100644
--- a/libc/src/__support/FPUtil/dyadic_float.h
+++ b/libc/src/__support/FPUtil/dyadic_float.h
@@ -122,7 +122,8 @@ template <size_t Bits> struct DyadicFloat {
 
     int exp_lo = exp_hi - static_cast<int>(PRECISION) - 1;
 
-    MantissaType m_hi(mantissa >> shift);
+    MantissaType m_hi =
+        shift >= MantissaType::BITS ? MantissaType(0) : mantissa >> shift;
 
     T d_hi = FPBits<T>::create_value(
                  sign, exp_hi,
@@ -130,7 +131,8 @@ template <size_t Bits> struct DyadicFloat {
                      IMPLICIT_MASK)
                  .get_val();
 
-    MantissaType round_mask = MantissaType(1) << (shift - 1);
+    MantissaType round_mask =
+        shift > MantissaType::BITS ? 0 : MantissaType(1) << (shift - 1);
     MantissaType sticky_mask = round_mask - MantissaType(1);
 
     bool round_bit = !(mantissa & round_mask).is_zero();
diff --git a/libc/src/__support/big_int.h b/libc/src/__support/big_int.h
index c1e55ceef21113..f722a81d357d4f 100644
--- a/libc/src/__support/big_int.h
+++ b/libc/src/__support/big_int.h
@@ -249,18 +249,14 @@ LIBC_INLINE constexpr bool is_negative(cpp::array<word, N> &array) {
 enum Direction { LEFT, RIGHT };
 
 // A bitwise shift on an array of elements.
-// TODO: Make the result UB when 'offset' is greater or equal to the number of
-// bits in 'array'. This will allow for better code performance.
+// 'offset' must be less than TOTAL_BITS (i.e., sizeof(word) * CHAR_BIT * N)
+// otherwise the behavior is undefined.
 template <Direction direction, bool is_signed, typename word, size_t N>
 LIBC_INLINE constexpr cpp::array<word, N> shift(cpp::array<word, N> array,
                                                 size_t offset) {
   static_assert(direction == LEFT || direction == RIGHT);
   constexpr size_t WORD_BITS = cpp::numeric_limits<word>::digits;
   constexpr size_t TOTAL_BITS = N * WORD_BITS;
-  if (LIBC_UNLIKELY(offset == 0))
-    return array;
-  if (LIBC_UNLIKELY(offset >= TOTAL_BITS))
-    return {};
 #ifdef LIBC_TYPES_HAS_INT128
   if constexpr (TOTAL_BITS == 128) {
     using type = cpp::conditional_t<is_signed, __int128_t, __uint128_t>;
@@ -272,6 +268,8 @@ LIBC_INLINE constexpr cpp::array<word, N> shift(cpp::array<word, N> array,
     return cpp::bit_cast<cpp::array<word, N>>(tmp);
   }
 #endif
+  if (LIBC_UNLIKELY(offset == 0))
+    return array;
   const bool is_neg = is_signed && is_negative(array);
   constexpr auto at = [](size_t index) -> int {
     // reverse iteration when direction == LEFT.
diff --git a/libc/test/src/__support/big_int_test.cpp b/libc/test/src/__support/big_int_test.cpp
index fadec0cc313b16..1c4f0ac29171fa 100644
--- a/libc/test/src/__support/big_int_test.cpp
+++ b/libc/test/src/__support/big_int_test.cpp
@@ -192,7 +192,7 @@ TYPED_TEST(LlvmLibcUIntClassTest, Masks, Types) {
 
 TYPED_TEST(LlvmLibcUIntClassTest, CountBits, Types) {
   if constexpr (!T::SIGNED) {
-    for (size_t i = 0; i <= T::BITS; ++i) {
+    for (size_t i = 0; i < T::BITS; ++i) {
       const auto l_one = T::all_ones() << i; // 0b111...000
       const auto r_one = T::all_ones() >> i; // 0b000...111
       const int zeros = i;
@@ -559,10 +559,6 @@ TEST(LlvmLibcUIntClassTest, ShiftLeftTests) {
   LL_UInt128 result5({0, 0x2468ace000000000});
   EXPECT_EQ((val2 << 100), result5);
 
-  LL_UInt128 result6({0, 0});
-  EXPECT_EQ((val2 << 128), result6);
-  EXPECT_EQ((val2 << 256), result6);
-
   LL_UInt192 val3({1, 0, 0});
   LL_UInt192 result7({0, 1, 0});
   EXPECT_EQ((val3 << 64), result7);
@@ -589,10 +585,6 @@ TEST(LlvmLibcUIntClassTest, ShiftRightTests) {
   LL_UInt128 result5({0x0000000001234567, 0});
   EXPECT_EQ((val2 >> 100), result5);
 
-  LL_UInt128 result6({0, 0});
-  EXPECT_EQ((val2 >> 128), result6);
-  EXPECT_EQ((val2 >> 256), result6);
-
   LL_UInt128 v1({0x1111222233334444, 0xaaaabbbbccccdddd});
   LL_UInt128 r1({0xaaaabbbbccccdddd, 0});
   EXPECT_EQ((v1 >> 64), r1);