[ADT] Simplify countr_zero and countl_zero with constexpr if (NFC) #141517
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The high-level idea in countr_zero and countl_zero is to use the
intrinsic if available and fall back otherwise, but the actual
implementation is unnecessarily complicated.
Specifically, without this patch, we are going through a long chain of
macros to check the availability of _BitScanForward64:
#if defined(__GNUC__) || defined(_MSC_VER)
#if !defined(_MSC_VER) || defined(_M_X64)
#if __has_builtin(__builtin_ctzll) || defined(__GNUC__)
...
#elif defined(_MSC_VER)
_BitScanForward64(...);
#endif
#endif
#endif
This basically says we can use _BitScanForward64 if:
defined(_MSC_VER) && defined(_M_X64)
This patch simplifies all this with "constexpr if" and consolidates
the implementation into the body of countr_zero and countl_zero.
|
@llvm/pr-subscribers-llvm-adt Author: Kazu Hirata (kazutakahirata) ChangesThe high-level idea in countr_zero and countl_zero is to use the Specifically, without this patch, we are going through a long chain of #if defined(GNUC) || defined(_MSC_VER) This basically says we can use _BitScanForward64 if: defined(_MSC_VER) && defined(_M_X64) This patch simplifies all this with "constexpr if" and consolidates Full diff: https://github.com/llvm/llvm-project/pull/141517.diff 1 Files Affected:
diff --git a/llvm/include/llvm/ADT/bit.h b/llvm/include/llvm/ADT/bit.h
index 8544efb5c45d4..b952825e574bc 100644
--- a/llvm/include/llvm/ADT/bit.h
+++ b/llvm/include/llvm/ADT/bit.h
@@ -148,36 +148,20 @@ template <typename T, typename = std::enable_if_t<std::is_unsigned_v<T>>>
return (Value != 0) && ((Value & (Value - 1)) == 0);
}
-namespace detail {
-template <typename T, std::size_t SizeOfT> struct TrailingZerosCounter {
- static unsigned count(T Val) {
- if (!Val)
- return std::numeric_limits<T>::digits;
- if (Val & 0x1)
- return 0;
-
- // 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;
- }
-};
-
-#if defined(__GNUC__) || defined(_MSC_VER)
-template <typename T> struct TrailingZerosCounter<T, 4> {
- static unsigned count(T Val) {
- if (Val == 0)
- return 32;
+/// Count number of 0's from the least significant bit to the most
+/// stopping at the first 1.
+///
+/// Only unsigned integral types are allowed.
+///
+/// Returns std::numeric_limits<T>::digits on an input of 0.
+template <typename T> [[nodiscard]] int countr_zero(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 intrinsic if available.
+ if constexpr (sizeof(T) == 4) {
#if __has_builtin(__builtin_ctz) || defined(__GNUC__)
return __builtin_ctz(Val);
#elif defined(_MSC_VER)
@@ -185,65 +169,45 @@ template <typename T> struct TrailingZerosCounter<T, 4> {
_BitScanForward(&Index, Val);
return Index;
#endif
- }
-};
-
-#if !defined(_MSC_VER) || defined(_M_X64)
-template <typename T> struct TrailingZerosCounter<T, 8> {
- static unsigned count(T Val) {
- if (Val == 0)
- return 64;
-
+ } else if constexpr (sizeof(T) == 8) {
#if __has_builtin(__builtin_ctzll) || defined(__GNUC__)
return __builtin_ctzll(Val);
-#elif defined(_MSC_VER)
+#elif defined(_MSC_VER) && defined(_M_X64)
unsigned long Index;
_BitScanForward64(&Index, Val);
return Index;
#endif
}
-};
-#endif
-#endif
-} // namespace detail
-/// Count number of 0's from the least significant bit to the most
+ // 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;
+}
+
+/// Count number of 0's from the most significant bit to the least
/// stopping at the first 1.
///
/// Only unsigned integral types are allowed.
///
/// Returns std::numeric_limits<T>::digits on an input of 0.
-template <typename T> [[nodiscard]] int countr_zero(T Val) {
+template <typename T> [[nodiscard]] int countl_zero(T Val) {
static_assert(std::is_unsigned_v<T>,
"Only unsigned integral types are allowed.");
- return llvm::detail::TrailingZerosCounter<T, sizeof(T)>::count(Val);
-}
-
-namespace detail {
-template <typename T, std::size_t SizeOfT> struct LeadingZerosCounter {
- static unsigned count(T Val) {
- if (!Val)
- return std::numeric_limits<T>::digits;
-
- // Bisection method.
- unsigned ZeroBits = 0;
- for (T Shift = std::numeric_limits<T>::digits >> 1; Shift; Shift >>= 1) {
- T Tmp = Val >> Shift;
- if (Tmp)
- Val = Tmp;
- else
- ZeroBits |= Shift;
- }
- return ZeroBits;
- }
-};
-
-#if defined(__GNUC__) || defined(_MSC_VER)
-template <typename T> struct LeadingZerosCounter<T, 4> {
- static unsigned count(T Val) {
- if (Val == 0)
- return 32;
+ if (!Val)
+ return std::numeric_limits<T>::digits;
+ // Use the intrinsic if available.
+ if constexpr (sizeof(T) == 4) {
#if __has_builtin(__builtin_clz) || defined(__GNUC__)
return __builtin_clz(Val);
#elif defined(_MSC_VER)
@@ -251,38 +215,26 @@ template <typename T> struct LeadingZerosCounter<T, 4> {
_BitScanReverse(&Index, Val);
return Index ^ 31;
#endif
- }
-};
-
-#if !defined(_MSC_VER) || defined(_M_X64)
-template <typename T> struct LeadingZerosCounter<T, 8> {
- static unsigned count(T Val) {
- if (Val == 0)
- return 64;
-
+ } else if constexpr (sizeof(T) == 8) {
#if __has_builtin(__builtin_clzll) || defined(__GNUC__)
return __builtin_clzll(Val);
-#elif defined(_MSC_VER)
+#elif defined(_MSC_VER) && defined(_M_X64)
unsigned long Index;
_BitScanReverse64(&Index, Val);
return Index ^ 63;
#endif
}
-};
-#endif
-#endif
-} // namespace detail
-/// Count number of 0's from the most significant bit to the least
-/// stopping at the first 1.
-///
-/// Only unsigned integral types are allowed.
-///
-/// Returns std::numeric_limits<T>::digits on an input of 0.
-template <typename T> [[nodiscard]] int countl_zero(T Val) {
- static_assert(std::is_unsigned_v<T>,
- "Only unsigned integral types are allowed.");
- return llvm::detail::LeadingZerosCounter<T, sizeof(T)>::count(Val);
+ // Fall back to the bisection method.
+ unsigned ZeroBits = 0;
+ for (T Shift = std::numeric_limits<T>::digits >> 1; Shift; Shift >>= 1) {
+ T Tmp = Val >> Shift;
+ if (Tmp)
+ Val = Tmp;
+ else
+ ZeroBits |= Shift;
+ }
+ return ZeroBits;
}
/// Count the number of ones from the most significant bit to the first
|
kazutakahirata
changed the title
nikic
approved these changes
Jun 1, 2025
|
LLVM Buildbot has detected a new failure on builder Full details are available at: https://lab.llvm.org/buildbot/#/builders/87/builds/1632 Here is the relevant piece of the build log for the reference |
|
LLVM Buildbot has detected a new failure on builder Full details are available at: https://lab.llvm.org/buildbot/#/builders/52/builds/8779 Here is the relevant piece of the build log for the reference |
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The high-level idea in countr_zero and countl_zero is to use the
intrinsic if available and fall back otherwise, but the actual
implementation is unnecessarily complicated.
Specifically, without this patch, we are going through a long chain of
macros to check the availability of _BitScanForward64:
#if defined(GNUC) || defined(_MSC_VER)
#if !defined(_MSC_VER) || defined(_M_X64)
#if __has_builtin(__builtin_ctzll) || defined(GNUC)
...
#elif defined(_MSC_VER)
_BitScanForward64(...);
#endif
#endif
#endif
This basically says we can use _BitScanForward64 if:
defined(_MSC_VER) && defined(_M_X64)
This patch simplifies all this with "constexpr if" and consolidates
the implementation into the body of countr_zero and countl_zero.