[libc][support][bit] use new type generic builtins

[nickdesaulniers]

These are new in clang-19+, gcc-14+.

[nickdesaulniers]

cc @overmighty

[llvmbot]

@llvm/pr-subscribers-libc

Author: Nick Desaulniers (nickdesaulniers)

Changes

These are new in clang-19+, gcc-14+.

---

Full diff: https://github.com/llvm/llvm-project/pull/86746.diff

1 Files Affected:

• (modified) libc/src/__support/CPP/bit.h (+18)

diff --git a/libc/src/__support/CPP/bit.h b/libc/src/__support/CPP/bit.h
index 3f2fbec944054c..a578e256e8364e 100644
--- a/libc/src/__support/CPP/bit.h
+++ b/libc/src/__support/CPP/bit.h
@@ -73,6 +73,14 @@ has_single_bit(T value) {
 /// Only unsigned integral types are allowed.
 ///
 /// Returns cpp::numeric_limits<T>::digits on an input of 0.
+// clang-19+, gcc-14+
+#if __has_builtin(__builtin_ctzg)
+template <typename T>
+[[nodiscard]] LIBC_INLINE constexpr cpp::enable_if_t<cpp::is_unsigned_v<T>, int>
+countr_zero(T value) {
+  return __builtin_ctzg(value, cpp::numeric_limits<T>::digits);
+}
+#else
 template <typename T>
 [[nodiscard]] LIBC_INLINE constexpr cpp::enable_if_t<cpp::is_unsigned_v<T>, int>
 countr_zero(T value) {
@@ -100,6 +108,7 @@ ADD_SPECIALIZATION(countr_zero, unsigned short, __builtin_ctzs)
 ADD_SPECIALIZATION(countr_zero, unsigned int, __builtin_ctz)
 ADD_SPECIALIZATION(countr_zero, unsigned long, __builtin_ctzl)
 ADD_SPECIALIZATION(countr_zero, unsigned long long, __builtin_ctzll)
+#endif // __has_builtin(__builtin_ctzg)
 
 /// Count number of 0's from the most significant bit to the least
 ///   stopping at the first 1.
@@ -107,9 +116,17 @@ ADD_SPECIALIZATION(countr_zero, unsigned long long, __builtin_ctzll)
 /// Only unsigned integral types are allowed.
 ///
 /// Returns cpp::numeric_limits<T>::digits on an input of 0.
+// clang-19+, gcc-14+
+#if __has_builtin(__builtin_clzg)
 template <typename T>
 [[nodiscard]] LIBC_INLINE constexpr cpp::enable_if_t<cpp::is_unsigned_v<T>, int>
 countl_zero(T value) {
+  return __builtin_clzg(value, cpp::numeric_limits<T>::digits);
+}
+#else
+template <typename T [[nodiscard]] LIBC_INLINE constexpr cpp::enable_if_t<
+    cpp::is_unsigned_v<T>, int>
+              countl_zero(T value) {
   if (!value)
     return cpp::numeric_limits<T>::digits;
   // Bisection method.
@@ -130,6 +147,7 @@ ADD_SPECIALIZATION(countl_zero, unsigned short, __builtin_clzs)
 ADD_SPECIALIZATION(countl_zero, unsigned int, __builtin_clz)
 ADD_SPECIALIZATION(countl_zero, unsigned long, __builtin_clzl)
 ADD_SPECIALIZATION(countl_zero, unsigned long long, __builtin_clzll)
+#endif // __has_builtin(__builtin_clzg)
 
 #undef ADD_SPECIALIZATION
 

[nickdesaulniers]

Note: already did this for popcount in #86531.

[nickdesaulniers]

not sure what's going wrong in presubmit; will look more tomorrow.

[gchatelet]

@nickdesaulniers for my information, are you adding these generic builtins for documentation purposes ? As-is they don't provide anything that we don't already have with the specialization (and we have to keep them until clang-19+ / gcc-14+ becomes our minimum compiler version). I'm not opposing this patch, I'm just trying to get the rationale behind it.

[nickdesaulniers]

I'd like to simplify the implementations such that one day (when our minimum supported compiler versions advance past when these builtins were added), we can clean up/delete a bunch of code.

We might even get there sooner if we start reusing code from libc++. #86563 is where we're trying to utilize these builtins there, too.

[gchatelet]

LGTM (Sorry for dropping the ball!)