1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
|
// This file is dual licensed under the MIT and the University of Illinois Open
// Source Licenses. See LICENSE.TXT for details.
#include "../assembly.h"
// du_int __udivdi3(du_int a, du_int b);
// result = a / b.
// both inputs and the output are 64-bit unsigned integers.
// This will do whatever the underlying hardware is set to do on division by zero.
// No other exceptions are generated, as the divide cannot overflow.
//
// This is targeted at 32-bit x86 *only*, as this can be done directly in hardware
// on x86_64. The performance goal is ~40 cycles per divide, which is faster than
// currently possible via simulation of integer divides on the x87 unit.
//
// Stephen Canon, December 2008
#ifdef __i386__
.text
.balign 4
DEFINE_COMPILERRT_FUNCTION(__udivdi3)
pushl %ebx
movl 20(%esp), %ebx // Find the index i of the leading bit in b.
bsrl %ebx, %ecx // If the high word of b is zero, jump to
jz 9f // the code to handle that special case [9].
/* High word of b is known to be non-zero on this branch */
movl 16(%esp), %eax // Construct bhi, containing bits [1+i:32+i] of b
shrl %cl, %eax // Practically, this means that bhi is given by:
shrl %eax //
notl %ecx // bhi = (high word of b) << (31 - i) |
shll %cl, %ebx // (low word of b) >> (1 + i)
orl %eax, %ebx //
movl 12(%esp), %edx // Load the high and low words of a, and jump
movl 8(%esp), %eax // to [1] if the high word is larger than bhi
cmpl %ebx, %edx // to avoid overflowing the upcoming divide.
jae 1f
/* High word of a is greater than or equal to (b >> (1 + i)) on this branch */
divl %ebx // eax <-- qs, edx <-- r such that ahi:alo = bs*qs + r
pushl %edi
notl %ecx
shrl %eax
shrl %cl, %eax // q = qs >> (1 + i)
movl %eax, %edi
mull 20(%esp) // q*blo
movl 12(%esp), %ebx
movl 16(%esp), %ecx // ECX:EBX = a
subl %eax, %ebx
sbbl %edx, %ecx // ECX:EBX = a - q*blo
movl 24(%esp), %eax
imull %edi, %eax // q*bhi
subl %eax, %ecx // ECX:EBX = a - q*b
sbbl $0, %edi // decrement q if remainder is negative
xorl %edx, %edx
movl %edi, %eax
popl %edi
popl %ebx
retl
1: /* High word of a is greater than or equal to (b >> (1 + i)) on this branch */
subl %ebx, %edx // subtract bhi from ahi so that divide will not
divl %ebx // overflow, and find q and r such that
//
// ahi:alo = (1:q)*bhi + r
//
// Note that q is a number in (31-i).(1+i)
// fix point.
pushl %edi
notl %ecx
shrl %eax
orl $0x80000000, %eax
shrl %cl, %eax // q = (1:qs) >> (1 + i)
movl %eax, %edi
mull 20(%esp) // q*blo
movl 12(%esp), %ebx
movl 16(%esp), %ecx // ECX:EBX = a
subl %eax, %ebx
sbbl %edx, %ecx // ECX:EBX = a - q*blo
movl 24(%esp), %eax
imull %edi, %eax // q*bhi
subl %eax, %ecx // ECX:EBX = a - q*b
sbbl $0, %edi // decrement q if remainder is negative
xorl %edx, %edx
movl %edi, %eax
popl %edi
popl %ebx
retl
9: /* High word of b is zero on this branch */
movl 12(%esp), %eax // Find qhi and rhi such that
movl 16(%esp), %ecx //
xorl %edx, %edx // ahi = qhi*b + rhi with 0 ≤ rhi < b
divl %ecx //
movl %eax, %ebx //
movl 8(%esp), %eax // Find qlo such that
divl %ecx //
movl %ebx, %edx // rhi:alo = qlo*b + rlo with 0 ≤ rlo < b
popl %ebx //
retl // and return qhi:qlo
END_COMPILERRT_FUNCTION(__udivdi3)
#endif // __i386__
|
