diff --git a/libdl/libdl_cfi.cpp b/libdl/libdl_cfi.cpp
index 8adc342..e096f9a 100644
--- a/libdl/libdl_cfi.cpp
+++ b/libdl/libdl_cfi.cpp
@@ -55,8 +55,8 @@
   uintptr_t addr = reinterpret_cast<uintptr_t>(Ptr);
   // The aligned range of [0, kShadowAlign) uses a single shadow element, therefore all pointers in
   // this range must get the same aligned_addr below. This matches CFIShadowWriter::Add; not the
-  // same as align_up().
-  uintptr_t aligned_addr = align_down(addr, CFIShadow::kShadowAlign) + CFIShadow::kShadowAlign;
+  // same as just __builtin_align_up().
+  uintptr_t aligned_addr = __builtin_align_down(addr, CFIShadow::kShadowAlign) + CFIShadow::kShadowAlign;
   uintptr_t p = aligned_addr - (static_cast<uintptr_t>(v - CFIShadow::kRegularShadowMin)
                                 << CFIShadow::kCfiCheckGranularity);
 #ifdef __arm__