diff --git a/lib/compiler-rt/builtins/ctzdi2.c b/lib/compiler-rt/builtins/ctzdi2.c
index db3c6fd..26c908d 100644
--- a/lib/compiler-rt/builtins/ctzdi2.c
+++ b/lib/compiler-rt/builtins/ctzdi2.c
@@ -1,29 +1,35 @@
-/* ===-- ctzdi2.c - Implement __ctzdi2 -------------------------------------===
- *
- *                     The LLVM Compiler Infrastructure
- *
- * This file is dual licensed under the MIT and the University of Illinois Open
- * Source Licenses. See LICENSE.TXT for details.
- *
- * ===----------------------------------------------------------------------===
- *
- * This file implements __ctzdi2 for the compiler_rt library.
- *
- * ===----------------------------------------------------------------------===
- */
+//===-- ctzdi2.c - Implement __ctzdi2 -------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements __ctzdi2 for the compiler_rt library.
+//
+//===----------------------------------------------------------------------===//
 
 #include "int_lib.h"
 
-/* Returns: the number of trailing 0-bits  */
+// Returns: the number of trailing 0-bits
 
-/* Precondition: a != 0 */
+#if !defined(__clang__) &&                                                     \
+    ((defined(__sparc__) && defined(__arch64__)) || defined(__mips64) ||       \
+     (defined(__riscv) && __SIZEOF_POINTER__ >= 8))
+// On 64-bit architectures with neither a native clz instruction nor a native
+// ctz instruction, gcc resolves __builtin_ctz to __ctzdi2 rather than
+// __ctzsi2, leading to infinite recursion.
+#define __builtin_ctz(a) __ctzsi2(a)
+extern int __ctzsi2(si_int);
+#endif
 
-COMPILER_RT_ABI si_int
-__ctzdi2(di_int a)
-{
-    dwords x;
-    x.all = a;
-    const si_int f = -(x.s.low == 0);
-    return __builtin_ctz((x.s.high & f) | (x.s.low & ~f)) +
-              (f & ((si_int)(sizeof(si_int) * CHAR_BIT)));
+// Precondition: a != 0
+
+COMPILER_RT_ABI int __ctzdi2(di_int a) {
+  dwords x;
+  x.all = a;
+  const si_int f = -(x.s.low == 0);
+  return ctzsi((x.s.high & f) | (x.s.low & ~f)) +
+         (f & ((si_int)(sizeof(si_int) * CHAR_BIT)));
 }