1 /* SPDX-License-Identifier: GPL-2.0-only */
2 /*
3 * linux/arch/arm/lib/div64.S
4 *
5 * Optimized computation of 64-bit dividend / 32-bit divisor
6 *
7 * Author: Nicolas Pitre
8 * Created: Oct 5, 2003
9 * Copyright: Monta Vista Software, Inc.
10 */
11
12 #include <linux/linkage.h>
13 #include <asm/assembler.h>
14 #include <asm/unwind.h>
15
16 #ifdef __ARMEB__
17 #define xh r0
18 #define xl r1
19 #define yh r2
20 #define yl r3
21 #else
22 #define xl r0
23 #define xh r1
24 #define yl r2
25 #define yh r3
26 #endif
27
28 /*
29 * __do_div64: perform a division with 64-bit dividend and 32-bit divisor.
30 *
31 * Note: Calling convention is totally non standard for optimal code.
32 * This is meant to be used by do_div() from include/asm/div64.h only.
33 *
34 * Input parameters:
35 * xh-xl = dividend (clobbered)
36 * r4 = divisor (preserved)
37 *
38 * Output values:
39 * yh-yl = result
40 * xh = remainder
41 *
42 * Clobbered regs: xl, ip
43 */
44
45 ENTRY(__do_div64)
46 UNWIND(.fnstart)
47
48 @ Test for easy paths first.
49 subs ip, r4, #1
50 bls 9f @ divisor is 0 or 1
51 tst ip, r4
52 beq 8f @ divisor is power of 2
53
54 @ See if we need to handle upper 32-bit result.
55 cmp xh, r4
56 mov yh, #0
57 blo 3f
58
59 @ Align divisor with upper part of dividend.
60 @ The aligned divisor is stored in yl preserving the original.
61 @ The bit position is stored in ip.
62
63 #if __LINUX_ARM_ARCH__ >= 5
64
65 clz yl, r4
66 clz ip, xh
67 sub yl, yl, ip
68 mov ip, #1
69 mov ip, ip, lsl yl
70 mov yl, r4, lsl yl
71
72 #else
73
74 mov yl, r4
75 mov ip, #1
76 1: cmp yl, #0x80000000
77 cmpcc yl, xh
78 movcc yl, yl, lsl #1
79 movcc ip, ip, lsl #1
80 bcc 1b
81
82 #endif
83
84 @ The division loop for needed upper bit positions.
85 @ Break out early if dividend reaches 0.
86 2: cmp xh, yl
87 orrcs yh, yh, ip
88 subscs xh, xh, yl
89 movsne ip, ip, lsr #1
90 mov yl, yl, lsr #1
91 bne 2b
92
93 @ See if we need to handle lower 32-bit result.
94 3: cmp xh, #0
95 mov yl, #0
96 cmpeq xl, r4
97 movlo xh, xl
98 retlo lr
99
100 @ The division loop for lower bit positions.
101 @ Here we shift remainer bits leftwards rather than moving the
102 @ divisor for comparisons, considering the carry-out bit as well.
103 mov ip, #0x80000000
104 4: movs xl, xl, lsl #1
105 adcs xh, xh, xh
106 beq 6f
107 cmpcc xh, r4
108 5: orrcs yl, yl, ip
109 subcs xh, xh, r4
110 movs ip, ip, lsr #1
111 bne 4b
112 ret lr
113
114 @ The top part of remainder became zero. If carry is set
115 @ (the 33th bit) this is a false positive so resume the loop.
116 @ Otherwise, if lower part is also null then we are done.
117 6: bcs 5b
118 cmp xl, #0
119 reteq lr
120
121 @ We still have remainer bits in the low part. Bring them up.
122
123 #if __LINUX_ARM_ARCH__ >= 5
124
125 clz xh, xl @ we know xh is zero here so...
126 add xh, xh, #1
127 mov xl, xl, lsl xh
128 mov ip, ip, lsr xh
129
130 #else
131
132 7: movs xl, xl, lsl #1
133 mov ip, ip, lsr #1
134 bcc 7b
135
136 #endif
137
138 @ Current remainder is now 1. It is worthless to compare with
139 @ divisor at this point since divisor can not be smaller than 3 here.
140 @ If possible, branch for another shift in the division loop.
141 @ If no bit position left then we are done.
142 movs ip, ip, lsr #1
143 mov xh, #1
144 bne 4b
145 ret lr
146
147 8: @ Division by a power of 2: determine what that divisor order is
148 @ then simply shift values around
149
150 #if __LINUX_ARM_ARCH__ >= 5
151
152 clz ip, r4
153 rsb ip, ip, #31
154
155 #else
156
157 mov yl, r4
158 cmp r4, #(1 << 16)
159 mov ip, #0
160 movhs yl, yl, lsr #16
161 movhs ip, #16
162
163 cmp yl, #(1 << 8)
164 movhs yl, yl, lsr #8
165 addhs ip, ip, #8
166
167 cmp yl, #(1 << 4)
168 movhs yl, yl, lsr #4
169 addhs ip, ip, #4
170
171 cmp yl, #(1 << 2)
172 addhi ip, ip, #3
173 addls ip, ip, yl, lsr #1
174
175 #endif
176
177 mov yh, xh, lsr ip
178 mov yl, xl, lsr ip
179 rsb ip, ip, #32
180 ARM( orr yl, yl, xh, lsl ip )
181 THUMB( lsl xh, xh, ip )
182 THUMB( orr yl, yl, xh )
183 mov xh, xl, lsl ip
184 mov xh, xh, lsr ip
185 ret lr
186
187 @ eq -> division by 1: obvious enough...
188 9: moveq yl, xl
189 moveq yh, xh
190 moveq xh, #0
191 reteq lr
192 UNWIND(.fnend)
193
194 UNWIND(.fnstart)
195 UNWIND(.pad #4)
196 UNWIND(.save {lr})
197 Ldiv0_64:
198 @ Division by 0:
199 str lr, [sp, #-8]!
200 bl __div0
201
202 @ as wrong as it could be...
203 mov yl, #0
204 mov yh, #0
205 mov xh, #0
206 ldr pc, [sp], #8
207
208 UNWIND(.fnend)
209 ENDPROC(__do_div64)
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