[ADT] Simplify countr_zero and countl_zero with constexpr if (NFC) (#141517)

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.

Author: kazutakahirata

llvm/include/llvm/ADT/bit.h

@@ -148,141 +148,93 @@ 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)
     unsigned long Index;
     _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)
     unsigned long Index;
     _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