Merge #80: Add c++20 version of CountBits

82b6488 Add c++20 version of CountBits (Cory Fields)

Pull request description:

  This corresponds with an upcoming pr to Core to do the same.

  It's unclear why the optimized versions ignore the max param. I'm guessing it's because the output of the builtins is already capped? I wasn't sure, so I copied the behavior of the other builtins.

ACKs for top commit:
  sipa:
    ACK 82b6488

Tree-SHA512: 009f84bc67401533f57fd3a2765003d3baebfa87d7b3076ece16bda7e5af3756d082e8af5caf81098b4334a8c1f8c1a3eaa2a9bd21596801377aa58af91d08c9

Author: sipa

src/int_utils.h

@@ -14,7 +14,9 @@
 #include <algorithm>
 #include <type_traits>
 
-#ifdef _MSC_VER
+#if defined(__cpp_lib_int_pow2) && __cpp_lib_int_pow2 >= 202002L
+#  include <bit>
+#elif defined(_MSC_VER)
 #  include <intrin.h>
 #endif
 
@@ -142,7 +144,11 @@ constexpr inline I Mask() { return ((I((I(-1)) << (std::numeric_limits<I>::digit
 /** Compute the smallest power of two that is larger than val. */
 template<typename I>
 static inline int CountBits(I val, int max) {
-#ifdef _MSC_VER
+#if defined(__cpp_lib_int_pow2) && __cpp_lib_int_pow2 >= 202002L
+    // c++20 impl
+    (void)max;
+    return std::bit_width(val);
+#elif defined(_MSC_VER)
     (void)max;
     unsigned long index;
     unsigned char ret;