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 count leading zeros for 32bit arguments. 11 * 12 * ===----------------------------------------------------------------------=== 13 */ 14#include "../assembly.h" 15 16 .syntax unified 17 18 .text 19 .p2align 2 20DEFINE_COMPILERRT_FUNCTION(__clzsi2) 21#ifdef __ARM_FEATURE_CLZ 22 clz r0, r0 23 JMP(lr) 24#else 25 /* Assumption: n != 0 */ 26 27 /* 28 * r0: n 29 * r1: count of leading zeros in n + 1 30 * r2: scratch register for shifted r0 31 */ 32 mov r1, 1 33 34 /* 35 * Basic block: 36 * if ((r0 >> SHIFT) == 0) 37 * r1 += SHIFT; 38 * else 39 * r0 >>= SHIFT; 40 * for descending powers of two as SHIFT. 41 */ 42 43#define BLOCK(shift) \ 44 lsrs r2, r0, shift; \ 45 movne r0, r2; \ 46 addeq r1, shift \ 47 48 BLOCK(16) 49 BLOCK(8) 50 BLOCK(4) 51 BLOCK(2) 52 53 /* 54 * The basic block invariants at this point are (r0 >> 2) == 0 and 55 * r0 != 0. This means 1 <= r0 <= 3 and 0 <= (r0 >> 1) <= 1. 56 * 57 * r0 | (r0 >> 1) == 0 | (r0 >> 1) == 1 | -(r0 >> 1) | 1 - (r0 >> 1) 58 * ---+----------------+----------------+------------+-------------- 59 * 1 | 1 | 0 | 0 | 1 60 * 2 | 0 | 1 | -1 | 0 61 * 3 | 0 | 1 | -1 | 0 62 * 63 * The r1's initial value of 1 compensates for the 1 here. 64 */ 65 sub r0, r1, r0, lsr #1 66 67 JMP(lr) 68#endif // __ARM_FEATURE_CLZ 69END_COMPILERRT_FUNCTION(__clzsi2) 70