diff --git a/libc/private/CFIShadow.h b/libc/private/CFIShadow.h
index cbdf0f7..b40c063 100644
--- a/libc/private/CFIShadow.h
+++ b/libc/private/CFIShadow.h
@@ -40,7 +40,7 @@
 // below) are interpreted as follows.
 //
 // For an address P and corresponding shadow value V, the address of __cfi_check is calculated as
-//   align_up(P, 2**kShadowGranularity) - (V - 2) * (2 ** kCfiCheckGranularity)
+//   __builtin_align_up(P, 2**kShadowGranularity) - (V - 2) * (2 ** kCfiCheckGranularity)
 //
 // Special shadow values:
 //        0 = kInvalidShadow, this memory range has no valid CFI targets.