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1a4d82fc JJ |
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 | |
1a4d82fc | 17 | .text |
92a42be0 SL |
18 | #if __ARM_ARCH_ISA_THUMB == 2 |
19 | .thumb | |
20 | #endif | |
21 | ||
22 | .p2align 2 | |
23 | #if __ARM_ARCH_ISA_THUMB == 2 | |
24 | DEFINE_COMPILERRT_THUMB_FUNCTION(__clzsi2) | |
25 | #else | |
1a4d82fc | 26 | DEFINE_COMPILERRT_FUNCTION(__clzsi2) |
92a42be0 | 27 | #endif |
1a4d82fc JJ |
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 | |
76 | END_COMPILERRT_FUNCTION(__clzsi2) |