This is faster than using the software implementation, and the insn is
available on all half-way recent hardware. Use the respective compiler
builtins when available.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Jason Andryuk <jandryuk@gmail.com>

--- a/xen/arch/x86/include/asm/bitops.h
+++ b/xen/arch/x86/include/asm/bitops.h
@@ -475,9 +475,16 @@ static inline int fls(unsigned int x)
  *
  * The Hamming Weight of a number is the total number of bits set in it.
  */
+#ifdef __POPCNT__
+#define hweight64(x) __builtin_popcountll(x)
+#define hweight32(x) __builtin_popcount(x)
+#define hweight16(x) __builtin_popcount((uint16_t)(x))
+#define hweight8(x)  __builtin_popcount((uint8_t)(x))
+#else
 #define hweight64(x) generic_hweight64(x)
 #define hweight32(x) generic_hweight32(x)
 #define hweight16(x) generic_hweight16(x)
 #define hweight8(x) generic_hweight8(x)
+#endif
 
 #endif /* _X86_BITOPS_H */