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[ADT] Define countr_zero in terms of popcount (NFC) #158519

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Merged
merged 3 commits into from
Sep 15, 2025
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[ADT] Define countr_zero in terms of popcount (NFC) #158519

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1c885b0
[ADT] Define countr_zero in terms of popcount (NFC)
kazutakahirata Sep 14, 2025
6ca2e58
Address comments.
kazutakahirata Sep 15, 2025
2ce4e6c
Fix formatting.
kazutakahirata Sep 15, 2025
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74 changes: 32 additions & 42 deletions llvm/include/llvm/ADT/bit.h
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Original file line number Diff line number Diff line change
Expand Up @@ -148,6 +148,35 @@ template <typename T, typename = std::enable_if_t<std::is_unsigned_v<T>>>
return (Value != 0) && ((Value & (Value - 1)) == 0);
}

/// Count the number of set bits in a value.
/// Ex. popcount(0xF000F000) = 8
/// Returns 0 if Value is zero.
template <typename T> [[nodiscard]] inline int popcount(T Value) noexcept {
static_assert(std::is_unsigned_v<T>, "T must be an unsigned integer type");
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My suggestion was that we allow unsigned types and call make_unsigned as a part of the implementation. Would there be any issues with that?

static_assert(sizeof(T) <= 8, "T must be 8 bytes or less");

if constexpr (sizeof(T) <= 4) {
#if defined(__GNUC__)
return (int)__builtin_popcount(Value);
#else
uint32_t V = Value;
V = V - ((V >> 1) & 0x55555555);
V = (V & 0x33333333) + ((V >> 2) & 0x33333333);
return int(((V + (V >> 4) & 0xF0F0F0F) * 0x1010101) >> 24);
#endif
} else {
#if defined(__GNUC__)
return (int)__builtin_popcountll(Value);
#else
uint64_t V = Value;
V = V - ((V >> 1) & 0x5555555555555555ULL);
V = (V & 0x3333333333333333ULL) + ((V >> 2) & 0x3333333333333333ULL);
V = (V + (V >> 4)) & 0x0F0F0F0F0F0F0F0FULL;
return int((uint64_t)(V * 0x0101010101010101ULL) >> 56);
#endif
}
}

/// Count number of 0's from the least significant bit to the most
/// stopping at the first 1.
///
Expand Down Expand Up @@ -179,19 +208,9 @@ 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;
// Fallback to popcount. "(Val & -Val) - 1" is a bitmask with all bits below
// the least significant 1 set.
return llvm::popcount(static_cast<std::make_unsigned_t<T>>((Val & -Val) - 1));
}

/// Count number of 0's from the most significant bit to the least
Expand Down Expand Up @@ -300,35 +319,6 @@ template <typename T> [[nodiscard]] T bit_ceil(T Value) {
return T(1) << llvm::bit_width<T>(Value - 1u);
}

/// Count the number of set bits in a value.
/// Ex. popcount(0xF000F000) = 8
/// Returns 0 if the word is zero.
template <typename T, typename = std::enable_if_t<std::is_unsigned_v<T>>>
[[nodiscard]] inline int popcount(T Value) noexcept {
if constexpr (sizeof(T) <= 4) {
#if defined(__GNUC__)
return (int)__builtin_popcount(Value);
#else
uint32_t v = Value;
v = v - ((v >> 1) & 0x55555555);
v = (v & 0x33333333) + ((v >> 2) & 0x33333333);
return int(((v + (v >> 4) & 0xF0F0F0F) * 0x1010101) >> 24);
#endif
} else if constexpr (sizeof(T) <= 8) {
#if defined(__GNUC__)
return (int)__builtin_popcountll(Value);
#else
uint64_t v = Value;
v = v - ((v >> 1) & 0x5555555555555555ULL);
v = (v & 0x3333333333333333ULL) + ((v >> 2) & 0x3333333333333333ULL);
v = (v + (v >> 4)) & 0x0F0F0F0F0F0F0F0FULL;
return int((uint64_t)(v * 0x0101010101010101ULL) >> 56);
#endif
} else {
static_assert(sizeof(T) == 0, "T must be 8 bytes or less");
}
}

// Forward-declare rotr so that rotl can use it.
template <typename T, typename = std::enable_if_t<std::is_unsigned_v<T>>>
[[nodiscard]] constexpr T rotr(T V, int R);
Expand Down
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