llvm-project/compiler-rt/lib/builtins/clzdi2.c

/* ===-- clzdi2.c - Implement __clzdi2 -------------------------------------===
 *
 *               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 __clzdi2 for the compiler_rt library.
 *
 * ===----------------------------------------------------------------------===
 */

#include "int_lib.h"

/* Returns: the number of leading 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_clz to __clzdi2 rather than
 * __clzsi2, leading to infinite recursion. */
#define __builtin_clz(a) __clzsi2(a)
extern si_int __clzsi2(si_int);
#endif

/* Precondition: a != 0 */

COMPILER_RT_ABI si_int
__clzdi2(di_int a)
{
    dwords x;
    x.all = a;
    const si_int f = -(x.s.high == 0);
    return __builtin_clz((x.s.high & ~f) | (x.s.low & f)) +
           (f & ((si_int)(sizeof(si_int) * CHAR_BIT)));
}