[Base] Add faster `bit_count` variants

- Use C++20 `popcount()` when available
- Else use compiler specific built-ins

src/xenia/base/math.h

@@ -17,6 +17,16 @@
 #include <limits>
 #include <numeric>
 #include <type_traits>
+
+#if defined __has_include
+#if __has_include(<version>)
+#include <version>
+#endif
+#endif
+#if __cpp_lib_bitops
+#include <bit>
+#endif
+
 #include "xenia/base/platform.h"
 
 #if XE_ARCH_AMD64
@@ -101,6 +111,23 @@ constexpr uint32_t select_bits(uint32_t value, uint32_t a, uint32_t b) {
   return (value & make_bitmask(a, b)) >> a;
 }
 
+#if __cpp_lib_bitops
+template <class T>
+constexpr inline uint32_t bit_count(T v) {
+  return static_cast<uint32_t>(std::popcount(v));
+}
+#else
+#if XE_COMPILER_MSVC || XE_COMPILER_INTEL
+inline uint32_t bit_count(uint32_t v) { return __popcnt(v); }
+inline uint32_t bit_count(uint64_t v) {
+  return static_cast<uint32_t>(__popcnt64(v));
+}
+#elif XE_COMPILER_GCC || XE_COMPILER_CLANG
+static_assert(sizeof(unsigned int) == sizeof(uint32_t));
+static_assert(sizeof(unsigned long long) == sizeof(uint64_t));
+inline uint32_t bit_count(uint32_t v) { return __builtin_popcount(v); }
+inline uint32_t bit_count(uint64_t v) { return __builtin_popcountll(v); }
+#else
 inline uint32_t bit_count(uint32_t v) {
   v = v - ((v >> 1) & 0x55555555);
   v = (v & 0x33333333) + ((v >> 2) & 0x33333333);
@@ -116,6 +143,8 @@ inline uint32_t bit_count(uint64_t v) {
   v = v + (v >> 32) & 0x0000007F;
   return static_cast<uint32_t>(v);
 }
+#endif
+#endif
 
 // lzcnt instruction, typed for integers of all sizes.
 // The number of leading zero bits in the value parameter. If value is zero, the