[llvm] [ADT] Define countr_zero in terms of popcount (NFC) (PR #158519)
Kazu Hirata via llvm-commits
llvm-commits at lists.llvm.org
Sun Sep 14 21:27:15 PDT 2025
https://github.com/kazutakahirata updated https://github.com/llvm/llvm-project/pull/158519
>From 1c885b0df6a94f7e927b443cdb9e46fe8560bd63 Mon Sep 17 00:00:00 2001
From: Kazu Hirata <kazu at google.com>
Date: Sun, 14 Sep 2025 00:11:15 -0700
Subject: [PATCH 1/3] [ADT] Define countr_zero in terms of popcount (NFC)
We can express the fallback mechanism of llvm::countr_zero a lot more
concisely with llvm::popcount. Since llvm::countr_zero now requires
llvm::popcount, this patch moves llvm::popcount earlier.
---
llvm/include/llvm/ADT/bit.h | 74 ++++++++++++++++---------------------
1 file changed, 32 insertions(+), 42 deletions(-)
diff --git a/llvm/include/llvm/ADT/bit.h b/llvm/include/llvm/ADT/bit.h
index d6e33c3e6133a..73aa84504d6e9 100644
--- a/llvm/include/llvm/ADT/bit.h
+++ b/llvm/include/llvm/ADT/bit.h
@@ -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 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");
+ }
+}
+
/// Count number of 0's from the least significant bit to the most
/// stopping at the first 1.
///
@@ -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
@@ -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);
>From 6ca2e58d3d43ce5d31d1ee150da459644d462073 Mon Sep 17 00:00:00 2001
From: Kazu Hirata <kazu at google.com>
Date: Sun, 14 Sep 2025 20:10:41 -0700
Subject: [PATCH 2/3] Address comments.
---
llvm/include/llvm/ADT/bit.h | 29 +++++++++++++++--------------
1 file changed, 15 insertions(+), 14 deletions(-)
diff --git a/llvm/include/llvm/ADT/bit.h b/llvm/include/llvm/ADT/bit.h
index 73aa84504d6e9..fec8340f9689a 100644
--- a/llvm/include/llvm/ADT/bit.h
+++ b/llvm/include/llvm/ADT/bit.h
@@ -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");
+ 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");
}
}
>From 2ce4e6c7dc195eb6c3ec83b748efb9f50aa72c33 Mon Sep 17 00:00:00 2001
From: Kazu Hirata <kazu at google.com>
Date: Sun, 14 Sep 2025 21:26:56 -0700
Subject: [PATCH 3/3] Fix formatting.
---
llvm/include/llvm/ADT/bit.h | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)
diff --git a/llvm/include/llvm/ADT/bit.h b/llvm/include/llvm/ADT/bit.h
index fec8340f9689a..671d9bfa78bb2 100644
--- a/llvm/include/llvm/ADT/bit.h
+++ b/llvm/include/llvm/ADT/bit.h
@@ -151,8 +151,7 @@ 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 Value is zero.
-template <typename T>
-[[nodiscard]] inline int popcount(T Value) noexcept {
+template <typename T> [[nodiscard]] inline int popcount(T Value) noexcept {
static_assert(std::is_unsigned_v<T>, "T must be an unsigned integer type");
static_assert(sizeof(T) <= 8, "T must be 8 bytes or less");
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