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[libc] Make BigInt bitwise shift consistent with regular integral semantics. #87874

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Merged
merged 2 commits into from
Apr 6, 2024
Merged

[libc] Make BigInt bitwise shift consistent with regular integral semantics. #87874

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22c269c
[libc] Make `BigInt` bitwise shift consistent with regular integral s…
gchatelet Apr 5, 2024
39c0256
Fix test that was still shifting by the wrong amount
gchatelet Apr 6, 2024
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6 changes: 4 additions & 2 deletions libc/src/__support/FPUtil/dyadic_float.h
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Original file line number Diff line number Diff line change
Expand Up @@ -122,15 +122,17 @@ 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,
(static_cast<output_bits_t>(m_hi) & FPBits<T>::SIG_MASK) |
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();
Expand Down
10 changes: 4 additions & 6 deletions libc/src/__support/big_int.h
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Expand Up @@ -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>;
Expand All @@ -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.
Expand Down
10 changes: 1 addition & 9 deletions libc/test/src/__support/big_int_test.cpp
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Expand Up @@ -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;
Expand Down Expand Up @@ -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);
Expand All @@ -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);
Expand Down