udivdi3.S revision 2d1fdb26e458c4ddc04155c1d421bced3ba90cd0 
1// This file is dual licensed under the MIT and the University of Illinois Open
2// Source Licenses. See LICENSE.TXT for details.
3
4#include "../assembly.h"
5
6// du_int __udivdi3(du_int a, du_int b);
7
8// result = a / b.
9// both inputs and the output are 64-bit unsigned integers.
10// This will do whatever the underlying hardware is set to do on division by zero.
11// No other exceptions are generated, as the divide cannot overflow.
12//
13// This is targeted at 32-bit x86 *only*, as this can be done directly in hardware
14// on x86_64.  The performance goal is ~40 cycles per divide, which is faster than
15// currently possible via simulation of integer divides on the x87 unit.
16//
17// Stephen Canon, December 2008
18
19#ifdef __i386__
20
21.text
22.balign 4
23DEFINE_COMPILERRT_FUNCTION(__udivdi3)
24
25	pushl		%ebx
26	movl	 20(%esp),			%ebx	// Find the index i of the leading bit in b.
27	bsrl		%ebx,			%ecx	// If the high word of b is zero, jump to
28	jz			9f						// the code to handle that special case [9].
29
30	/* High word of b is known to be non-zero on this branch */
31
32	movl	 16(%esp),			%eax	// Construct bhi, containing bits [1+i:32+i] of b
33
34	shrl		%cl,			%eax	// Practically, this means that bhi is given by:
35	shrl		%eax					//
36	notl		%ecx					//		bhi = (high word of b) << (31 - i) |
37	shll		%cl,			%ebx	//			  (low word of b) >> (1 + i)
38	orl			%eax,			%ebx	//
39	movl	 12(%esp),			%edx	// Load the high and low words of a, and jump
40	movl	  8(%esp),			%eax	// to [1] if the high word is larger than bhi
41	cmpl		%ebx,			%edx	// to avoid overflowing the upcoming divide.
42	jae			1f
43
44	/* High word of a is greater than or equal to (b >> (1 + i)) on this branch */
45
46	divl		%ebx					// eax <-- qs, edx <-- r such that ahi:alo = bs*qs + r
47
48	pushl		%edi
49	notl		%ecx
50	shrl		%eax
51	shrl		%cl,			%eax	// q = qs >> (1 + i)
52	movl		%eax,			%edi
53	mull	 20(%esp)					// q*blo
54	movl	 12(%esp),			%ebx
55	movl	 16(%esp),			%ecx	// ECX:EBX = a
56	subl		%eax,			%ebx
57	sbbl		%edx,			%ecx	// ECX:EBX = a - q*blo
58	movl	 24(%esp),			%eax
59	imull		%edi,			%eax	// q*bhi
60	subl		%eax,			%ecx	// ECX:EBX = a - q*b
61	sbbl		$0,				%edi	// decrement q if remainder is negative
62	xorl		%edx,			%edx
63	movl		%edi,			%eax
64	popl		%edi
65	popl		%ebx
66	retl
67
68
691:	/* High word of a is greater than or equal to (b >> (1 + i)) on this branch */
70
71	subl		%ebx,			%edx	// subtract bhi from ahi so that divide will not
72	divl		%ebx					// overflow, and find q and r such that
73										//
74										//		ahi:alo = (1:q)*bhi + r
75										//
76										// Note that q is a number in (31-i).(1+i)
77										// fix point.
78
79	pushl		%edi
80	notl		%ecx
81	shrl		%eax
82	orl			$0x80000000,	%eax
83	shrl		%cl,			%eax	// q = (1:qs) >> (1 + i)
84	movl		%eax,			%edi
85	mull	 20(%esp)					// q*blo
86	movl	 12(%esp),			%ebx
87	movl	 16(%esp),			%ecx	// ECX:EBX = a
88	subl		%eax,			%ebx
89	sbbl		%edx,			%ecx	// ECX:EBX = a - q*blo
90	movl	 24(%esp),			%eax
91	imull		%edi,			%eax	// q*bhi
92	subl		%eax,			%ecx	// ECX:EBX = a - q*b
93	sbbl		$0,				%edi	// decrement q if remainder is negative
94	xorl		%edx,			%edx
95	movl		%edi,			%eax
96	popl		%edi
97	popl		%ebx
98	retl
99
100
1019:	/* High word of b is zero on this branch */
102
103	movl	 12(%esp),			%eax	// Find qhi and rhi such that
104	movl	 16(%esp),			%ecx	//
105	xorl		%edx,			%edx	//		ahi = qhi*b + rhi	with	0 ≤ rhi < b
106	divl		%ecx					//
107	movl		%eax,			%ebx	//
108	movl	  8(%esp),			%eax	// Find qlo such that
109	divl		%ecx					//
110	movl		%ebx,			%edx	//		rhi:alo = qlo*b + rlo  with 0 ≤ rlo < b
111	popl		%ebx					//
112	retl								// and return qhi:qlo
113END_COMPILERRT_FUNCTION(__udivdi3)
114
115#endif // __i386__
116