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[ADT] Add llvm::countr_zero_constexpr #157391

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[ADT] Add llvm::countr_zero_constexpr #157391

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36 changes: 23 additions & 13 deletions llvm/include/llvm/ADT/bit.h
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Original file line number Diff line number Diff line change
Expand Up @@ -154,6 +154,27 @@ template <typename T, typename = std::enable_if_t<std::is_unsigned_v<T>>>
/// Only unsigned integral types are allowed.
///
/// Returns std::numeric_limits<T>::digits on an input of 0.
template <typename T> [[nodiscard]] constexpr int countr_zero_constexpr(T Val) {
static_assert(std::is_unsigned_v<T>,
"Only unsigned integral types are allowed.");
if (!Val)
return std::numeric_limits<T>::digits;

// Use the bisection method.
unsigned ZeroBits = 0;
T Shift = std::numeric_limits<T>::digits >> 1;
T Mask = std::numeric_limits<T>::max() >> Shift;
while (Shift) {
if ((Val & Mask) == 0) {
Val >>= Shift;
ZeroBits |= Shift;
}
Shift >>= 1;
Mask >>= Shift;
}
return ZeroBits;
}

template <typename T> [[nodiscard]] int countr_zero(T Val) {
static_assert(std::is_unsigned_v<T>,
"Only unsigned integral types are allowed.");
Expand All @@ -179,19 +200,8 @@ template <typename T> [[nodiscard]] int countr_zero(T Val) {
#endif
}

// Fall back to the bisection method.
unsigned ZeroBits = 0;
T Shift = std::numeric_limits<T>::digits >> 1;
T Mask = std::numeric_limits<T>::max() >> Shift;
while (Shift) {
if ((Val & Mask) == 0) {
Val >>= Shift;
ZeroBits |= Shift;
}
Shift >>= 1;
Mask >>= Shift;
}
return ZeroBits;
// Fall back to the constexpr implementation.
return countr_zero_constexpr(Val);
}

/// Count number of 0's from the most significant bit to the least
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20 changes: 20 additions & 0 deletions llvm/unittests/ADT/BitTest.cpp
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Expand Up @@ -279,6 +279,26 @@ TEST(BitTest, CountlZero) {
}
}

TEST(BitTest, CountrZeroConstexpr) {
constexpr uint8_t Z8 = 0;
constexpr uint16_t Z16 = 0;
constexpr uint32_t Z32 = 0;
constexpr uint64_t Z64 = 0;
static_assert(llvm::countr_zero_constexpr(Z8) == 8, "");
static_assert(llvm::countr_zero_constexpr(Z16) == 16, "");
static_assert(llvm::countr_zero_constexpr(Z32) == 32, "");
static_assert(llvm::countr_zero_constexpr(Z64) == 64, "");

constexpr uint8_t NZ8 = 42;
constexpr uint16_t NZ16 = 42;
constexpr uint32_t NZ32 = 42;
constexpr uint64_t NZ64 = 42;
static_assert(llvm::countr_zero_constexpr(NZ8) == 1, "");
static_assert(llvm::countr_zero_constexpr(NZ16) == 1, "");
static_assert(llvm::countr_zero_constexpr(NZ32) == 1, "");
static_assert(llvm::countr_zero_constexpr(NZ64) == 1, "");
}

TEST(BitTest, CountrZero) {
uint8_t Z8 = 0;
uint16_t Z16 = 0;
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