1//===-- clzsi2.c - Implement __clzsi2 -------------------------------------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8//
9// This file implements count leading zeros for 32bit arguments.
10//
11//===----------------------------------------------------------------------===//
12
13#include "../assembly.h"
14
15	.syntax unified
16	.text
17	DEFINE_CODE_STATE
18
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	// r0: n
28	// r1: count of leading zeros in n + 1
29	// r2: scratch register for shifted r0
30	mov	r1, 1
31
32	// Basic block:
33	// if ((r0 >> SHIFT) == 0)
34	//   r1 += SHIFT;
35	// else
36	//   r0 >>= SHIFT;
37	// for descending powers of two as SHIFT.
38
39#define BLOCK(shift) \
40	lsrs	r2, r0, shift; \
41	movne	r0, r2; \
42	addeq	r1, shift \
43
44	BLOCK(16)
45	BLOCK(8)
46	BLOCK(4)
47	BLOCK(2)
48
49	// The basic block invariants at this point are (r0 >> 2) == 0 and
50	// r0 != 0. This means 1 <= r0 <= 3 and 0 <= (r0 >> 1) <= 1.
51	//
52	// r0 | (r0 >> 1) == 0 | (r0 >> 1) == 1 | -(r0 >> 1) | 1 - (r0 >> 1)
53	// ---+----------------+----------------+------------+--------------
54	// 1  | 1              | 0              | 0          | 1
55	// 2  | 0              | 1              | -1         | 0
56	// 3  | 0              | 1              | -1         | 0
57	//
58	// The r1's initial value of 1 compensates for the 1 here.
59	sub	r0, r1, r0, lsr #1
60
61	JMP(lr)
62#endif // __ARM_FEATURE_CLZ
63END_COMPILERRT_FUNCTION(__clzsi2)
64
65NO_EXEC_STACK_DIRECTIVE
66
67