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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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29 changes: 15 additions & 14 deletions llvm/include/llvm/ADT/bit.h
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Original file line number Diff line number Diff line change
Expand Up @@ -150,30 +150,31 @@ template <typename T, typename = std::enable_if_t<std::is_unsigned_v<T>>>

/// 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>>>
/// 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);
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) {
} 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);
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");
}
}

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