use a better popcnt fallback algorithm

This one loops once for every set bit, rather than once for each bit
position to the right of the highest set bit.

https://en.wikipedia.org/wiki/Hamming_weight#Efficient_implementation

1 files changed, 2 insertions, 2 deletions

diff --git a/c/blake3_impl.h b/c/blake3_impl.h
index 576ccf4..d345246 100644
--- a/c/blake3_impl.h
+++ b/c/blake3_impl.h
@@ -50,8 +50,8 @@ INLINE uint8_t popcnt(uint64_t x) {
 #else
   uint8_t count = 0;
   while (x > 0) {
-    count += ((uint8_t)x) & 1;
-    x >>= 1;
+    count += 1;
+    x &= x - 1;
   }
   return count;
 #endif