1// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
2// See https://llvm.org/LICENSE.txt for license information.
3// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
4
5#include "../assembly.h"
6
7// di_int __moddi3(di_int a, di_int b);
8
9// result = remainder of a / b.
10// both inputs and the output are 64-bit signed integers.
11// This will do whatever the underlying hardware is set to do on division by zero.
12// No other exceptions are generated, as the divide cannot overflow.
13//
14// This is targeted at 32-bit x86 *only*, as this can be done directly in hardware
15// on x86_64. The performance goal is ~40 cycles per divide, which is faster than
16// currently possible via simulation of integer divides on the x87 unit.
17//
18
19// Stephen Canon, December 2008
20
21#ifdef __i386__
22
23.text
24.balign 4
25DEFINE_COMPILERRT_FUNCTION(__moddi3)
26
27// This is currently implemented by wrapping the unsigned modulus up in an absolute
28// value. This could certainly be improved upon.
29
30 pushl %esi
31 movl 20(%esp), %edx // high word of b
32 movl 16(%esp), %eax // low word of b
33 movl %edx, %ecx
34 sarl $31, %ecx // (b < 0) ? -1 : 0
35 xorl %ecx, %eax
36 xorl %ecx, %edx // EDX:EAX = (b < 0) ? not(b) : b
37 subl %ecx, %eax
38 sbbl %ecx, %edx // EDX:EAX = abs(b)
39 movl %edx, 20(%esp)
40 movl %eax, 16(%esp) // store abs(b) back to stack
41
42 movl 12(%esp), %edx // high word of b
43 movl 8(%esp), %eax // low word of b
44 movl %edx, %ecx
45 sarl $31, %ecx // (a < 0) ? -1 : 0
46 xorl %ecx, %eax
47 xorl %ecx, %edx // EDX:EAX = (a < 0) ? not(a) : a
48 subl %ecx, %eax
49 sbbl %ecx, %edx // EDX:EAX = abs(a)
50 movl %edx, 12(%esp)
51 movl %eax, 8(%esp) // store abs(a) back to stack
52 movl %ecx, %esi // set aside sign of a
53
54 pushl %ebx
55 movl 24(%esp), %ebx // Find the index i of the leading bit in b.
56 bsrl %ebx, %ecx // If the high word of b is zero, jump to
57 jz 9f // the code to handle that special case [9].
58
59 // High word of b is known to be non-zero on this branch
60
61 movl 20(%esp), %eax // Construct bhi, containing bits [1+i:32+i] of b
62
63 shrl %cl, %eax // Practically, this means that bhi is given by:
64 shrl %eax //
65 notl %ecx // bhi = (high word of b) << (31 - i) |
66 shll %cl, %ebx // (low word of b) >> (1 + i)
67 orl %eax, %ebx //
68 movl 16(%esp), %edx // Load the high and low words of a, and jump
69 movl 12(%esp), %eax // to [2] if the high word is larger than bhi
70 cmpl %ebx, %edx // to avoid overflowing the upcoming divide.
71 jae 2f
72
73 // High word of a is greater than or equal to (b >> (1 + i)) on this branch
74
75 divl %ebx // eax <-- qs, edx <-- r such that ahi:alo = bs*qs + r
76
77 pushl %edi
78 notl %ecx
79 shrl %eax
80 shrl %cl, %eax // q = qs >> (1 + i)
81 movl %eax, %edi
82 mull 24(%esp) // q*blo
83 movl 16(%esp), %ebx
84 movl 20(%esp), %ecx // ECX:EBX = a
85 subl %eax, %ebx
86 sbbl %edx, %ecx // ECX:EBX = a - q*blo
87 movl 28(%esp), %eax
88 imull %edi, %eax // q*bhi
89 subl %eax, %ecx // ECX:EBX = a - q*b
90
91 jnc 1f // if positive, this is the result.
92 addl 24(%esp), %ebx // otherwise
93 adcl 28(%esp), %ecx // ECX:EBX = a - (q-1)*b = result
941: movl %ebx, %eax
95 movl %ecx, %edx
96
97 addl %esi, %eax // Restore correct sign to result
98 adcl %esi, %edx
99 xorl %esi, %eax
100 xorl %esi, %edx
101 popl %edi // Restore callee-save registers
102 popl %ebx
103 popl %esi
104 retl // Return
105
1062: // High word of a is greater than or equal to (b >> (1 + i)) on this branch
107
108 subl %ebx, %edx // subtract bhi from ahi so that divide will not
109 divl %ebx // overflow, and find q and r such that
110 //
111 // ahi:alo = (1:q)*bhi + r
112 //
113 // Note that q is a number in (31-i).(1+i)
114 // fix point.
115
116 pushl %edi
117 notl %ecx
118 shrl %eax
119 orl $0x80000000, %eax
120 shrl %cl, %eax // q = (1:qs) >> (1 + i)
121 movl %eax, %edi
122 mull 24(%esp) // q*blo
123 movl 16(%esp), %ebx
124 movl 20(%esp), %ecx // ECX:EBX = a
125 subl %eax, %ebx
126 sbbl %edx, %ecx // ECX:EBX = a - q*blo
127 movl 28(%esp), %eax
128 imull %edi, %eax // q*bhi
129 subl %eax, %ecx // ECX:EBX = a - q*b
130
131 jnc 3f // if positive, this is the result.
132 addl 24(%esp), %ebx // otherwise
133 adcl 28(%esp), %ecx // ECX:EBX = a - (q-1)*b = result
1343: movl %ebx, %eax
135 movl %ecx, %edx
136
137 addl %esi, %eax // Restore correct sign to result
138 adcl %esi, %edx
139 xorl %esi, %eax
140 xorl %esi, %edx
141 popl %edi // Restore callee-save registers
142 popl %ebx
143 popl %esi
144 retl // Return
145
1469: // High word of b is zero on this branch
147
148 movl 16(%esp), %eax // Find qhi and rhi such that
149 movl 20(%esp), %ecx //
150 xorl %edx, %edx // ahi = qhi*b + rhi with 0 ≤ rhi < b
151 divl %ecx //
152 movl %eax, %ebx //
153 movl 12(%esp), %eax // Find rlo such that
154 divl %ecx //
155 movl %edx, %eax // rhi:alo = qlo*b + rlo with 0 ≤ rlo < b
156 popl %ebx //
157 xorl %edx, %edx // and return 0:rlo
158
159 addl %esi, %eax // Restore correct sign to result
160 adcl %esi, %edx
161 xorl %esi, %eax
162 xorl %esi, %edx
163 popl %esi
164 retl // Return
165END_COMPILERRT_FUNCTION(__moddi3)
166
167#endif // __i386__
168
169NO_EXEC_STACK_DIRECTIVE
170
171

source code of compiler-rt/lib/builtins/i386/moddi3.S