1 /* ===-- clzsi2.c - Implement __clzsi2 -------------------------------------=== 2 * 3 * The LLVM Compiler Infrastructure 4 * 5 * This file is dual licensed under the MIT and the University of Illinois Open 6 * Source Licenses. See LICENSE.TXT for details. 7 * 8 * ===----------------------------------------------------------------------=== 9 * 10 * This file implements __clzsi2 for the compiler_rt library. 11 * 12 * ===----------------------------------------------------------------------=== 13 */ 14 15 #include "int_lib.h" 16 17 /* Returns: the number of leading 0-bits */ 18 19 /* Precondition: a != 0 */ 20 21 COMPILER_RT_ABI si_int 22 __clzsi2(si_int a) 23 { 24 su_int x = (su_int)a; 25 si_int t = ((x & 0xFFFF0000) == 0) << 4; /* if (x is small) t = 16 else 0 */ 26 x >>= 16 - t; /* x = [0 - 0xFFFF] */ 27 su_int r = t; /* r = [0, 16] */ 28 /* return r + clz(x) */ 29 t = ((x & 0xFF00) == 0) << 3; 30 x >>= 8 - t; /* x = [0 - 0xFF] */ 31 r += t; /* r = [0, 8, 16, 24] */ 32 /* return r + clz(x) */ 33 t = ((x & 0xF0) == 0) << 2; 34 x >>= 4 - t; /* x = [0 - 0xF] */ 35 r += t; /* r = [0, 4, 8, 12, 16, 20, 24, 28] */ 36 /* return r + clz(x) */ 37 t = ((x & 0xC) == 0) << 1; 38 x >>= 2 - t; /* x = [0 - 3] */ 39 r += t; /* r = [0 - 30] and is even */ 40 /* return r + clz(x) */ 41 /* switch (x) 42 * { 43 * case 0: 44 * return r + 2; 45 * case 1: 46 * return r + 1; 47 * case 2: 48 * case 3: 49 * return r; 50 * } 51 */ 52 return r + ((2 - x) & -((x & 2) == 0)); 53 } 54
