llvm-project/compiler-rt/lib/builtins/ctzdi2.c

/* ===-- 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  */

#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 si_int __ctzsi2(si_int);
#endif

/* Precondition: a != 0 */

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)));
}