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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	.text
18#if __ARM_ARCH_ISA_THUMB == 2
19	.thumb
20#endif
21
22	.p2align	2
23#if __ARM_ARCH_ISA_THUMB == 2
24DEFINE_COMPILERRT_THUMB_FUNCTION(__clzsi2)
25#else
26DEFINE_COMPILERRT_FUNCTION(__clzsi2)
27#endif
28#ifdef __ARM_FEATURE_CLZ
29	clz	r0, r0
30	JMP(lr)
31#else
32	/* Assumption: n != 0 */
33
34	/*
35	 * r0: n
36	 * r1: count of leading zeros in n + 1
37	 * r2: scratch register for shifted r0
38	 */
39	mov	r1, 1
40
41	/*
42	 * Basic block:
43	 * if ((r0 >> SHIFT) == 0)
44	 *   r1 += SHIFT;
45	 * else
46	 *   r0 >>= SHIFT;
47	 * for descending powers of two as SHIFT.
48	 */
49
50#define BLOCK(shift) \
51	lsrs	r2, r0, shift; \
52	movne	r0, r2; \
53	addeq	r1, shift \
54
55	BLOCK(16)
56	BLOCK(8)
57	BLOCK(4)
58	BLOCK(2)
59
60	/*
61	 * The basic block invariants at this point are (r0 >> 2) == 0 and
62	 * r0 != 0. This means 1 <= r0 <= 3 and 0 <= (r0 >> 1) <= 1.
63	 *
64	 * r0 | (r0 >> 1) == 0 | (r0 >> 1) == 1 | -(r0 >> 1) | 1 - (r0 >> 1)
65	 * ---+----------------+----------------+------------+--------------
66	 * 1  | 1              | 0              | 0          | 1
67	 * 2  | 0              | 1              | -1         | 0
68	 * 3  | 0              | 1              | -1         | 0
69	 *
70	 * The r1's initial value of 1 compensates for the 1 here.
71	 */
72	sub	r0, r1, r0, lsr #1
73
74	JMP(lr)
75#endif // __ARM_FEATURE_CLZ
76END_COMPILERRT_FUNCTION(__clzsi2)
77