diff --git a/lib/nghttp3_ringbuf.c b/lib/nghttp3_ringbuf.c
index 4888039..7d3ab39 100644
--- a/lib/nghttp3_ringbuf.c
+++ b/lib/nghttp3_ringbuf.c
@@ -33,26 +33,23 @@
 
 #include "nghttp3_macro.h"
 
+static int ispow2(size_t n) {
 #if defined(_MSC_VER) && !defined(__clang__) &&                                \
   (defined(_M_ARM) || (defined(_M_ARM64) && _MSC_VER < 1941))
-unsigned int __popcnt(unsigned int x) {
-  unsigned int c = 0;
-  for (; x; ++c) {
-    x &= x - 1;
-  }
-  return c;
+  return n && !(n & (n - 1));
+#elif defined(WIN32)
+  return 1 == __popcnt((unsigned int)n);
+#else  /* !((defined(_MSC_VER) && !defined(__clang__) && (defined(_M_ARM) ||   \
+          (defined(_M_ARM64) && _MSC_VER < 1941))) || defined(WIN32)) */
+  return 1 == __builtin_popcount((unsigned int)n);
+#endif /* !((defined(_MSC_VER) && !defined(__clang__) && (defined(_M_ARM) ||   \
+          (defined(_M_ARM64) && _MSC_VER < 1941))) || defined(WIN32)) */
 }
-#endif /* defined(_MSC_VER) && !defined(__clang__) && (defined(_M_ARM) ||      \
-          (defined(_M_ARM64) && _MSC_VER < 1941)) */
 
 int nghttp3_ringbuf_init(nghttp3_ringbuf *rb, size_t nmemb, size_t size,
                          const nghttp3_mem *mem) {
   if (nmemb) {
-#ifdef WIN32
-    assert(1 == __popcnt((unsigned int)nmemb));
-#else  /* !defined(WIN32) */
-    assert(1 == __builtin_popcount((unsigned int)nmemb));
-#endif /* !defined(WIN32) */
+    assert(ispow2(nmemb));
 
     rb->buf = nghttp3_mem_malloc(mem, nmemb * size);
     if (rb->buf == NULL) {
@@ -128,11 +125,7 @@ int nghttp3_ringbuf_reserve(nghttp3_ringbuf *rb, size_t nmemb) {
     return 0;
   }
 
-#ifdef WIN32
-  assert(1 == __popcnt((unsigned int)nmemb));
-#else  /* !defined(WIN32) */
-  assert(1 == __builtin_popcount((unsigned int)nmemb));
-#endif /* !defined(WIN32) */
+  assert(ispow2(nmemb));
 
   buf = nghttp3_mem_malloc(rb->mem, nmemb * rb->size);
   if (buf == NULL) {