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[ADT] Simplify popcount with constexpr if (NFC) #143339

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Merged
merged 1 commit into from
Jun 9, 2025
Merged

[ADT] Simplify popcount with constexpr if (NFC) #143339

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28 changes: 9 additions & 19 deletions llvm/include/llvm/ADT/bit.h
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Original file line number Diff line number Diff line change
Expand Up @@ -300,11 +300,12 @@ template <typename T> [[nodiscard]] T bit_ceil(T Value) {
return T(1) << llvm::bit_width<T>(Value - 1u);
}

namespace detail {
template <typename T, std::size_t SizeOfT> struct PopulationCounter {
static int count(T Value) {
// Generic version, forward to 32 bits.
static_assert(SizeOfT <= 4, "Not implemented!");
/// 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
Expand All @@ -313,11 +314,7 @@ template <typename T, std::size_t SizeOfT> struct PopulationCounter {
v = (v & 0x33333333) + ((v >> 2) & 0x33333333);
return int(((v + (v >> 4) & 0xF0F0F0F) * 0x1010101) >> 24);
#endif
}
};

template <typename T> struct PopulationCounter<T, 8> {
static int count(T Value) {
} else if constexpr (sizeof(T) <= 8) {
#if defined(__GNUC__)
return (int)__builtin_popcountll(Value);
#else
Expand All @@ -327,16 +324,9 @@ template <typename T> struct PopulationCounter<T, 8> {
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");
}
};
} // namespace detail

/// 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 {
return detail::PopulationCounter<T, sizeof(T)>::count(Value);
}

// Forward-declare rotr so that rotl can use it.
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