diff --git a/include/jemalloc/internal/arena_inlines_b.h b/include/jemalloc/internal/arena_inlines_b.h
index b1cd84b4..1c98ffa0 100644
--- a/include/jemalloc/internal/arena_inlines_b.h
+++ b/include/jemalloc/internal/arena_inlines_b.h
@@ -378,7 +378,7 @@ arena_dalloc(tsdn_t *tsdn, void *ptr, tcache_t *tcache,
 	if (caller_alloc_ctx != NULL) {
 		alloc_ctx = *caller_alloc_ctx;
 	} else {
-		util_assume(!tsdn_null(tsdn));
+		util_assume(tsdn != NULL);
 		emap_alloc_ctx_lookup(tsdn, &arena_emap_global, ptr,
 		    &alloc_ctx);
 	}
diff --git a/include/jemalloc/internal/util.h b/include/jemalloc/internal/util.h
index 536c0970..2c35ef76 100644
--- a/include/jemalloc/internal/util.h
+++ b/include/jemalloc/internal/util.h
@@ -65,12 +65,19 @@ get_errno(void) {
 #endif
 }
 
-JEMALLOC_ALWAYS_INLINE void
-util_assume(bool b) {
-	if (!b) {
-		unreachable();
-	}
-}
+#ifdef _MSC_VER
+#define util_assume __assume
+#elif defined(__clang__) && (__clang_major__ > 3 || \
+    (__clang_major__ == 3 && __clang_minor__ >= 6))
+#define util_assume __builtin_assume
+#else
+#define util_assume(expr)		\
+	do {				\
+		if (!(expr)) {		\
+			unreachable();	\
+		}			\
+	} while(0)
+#endif
 
 /* ptr should be valid. */
 JEMALLOC_ALWAYS_INLINE void