xref: /openbmc/linux/arch/alpha/lib/ev6-divide.S (revision 6fa24b41) 
1/* SPDX-License-Identifier: GPL-2.0 */
2/*
3 * arch/alpha/lib/ev6-divide.S
4 *
5 * 21264 version contributed by Rick Gorton <rick.gorton@alpha-processor.com>
6 *
7 * Alpha division..
8 */
9
10/*
11 * The alpha chip doesn't provide hardware division, so we have to do it
12 * by hand.  The compiler expects the functions
13 *
14 *	__divqu: 64-bit unsigned long divide
15 *	__remqu: 64-bit unsigned long remainder
16 *	__divqs/__remqs: signed 64-bit
17 *	__divlu/__remlu: unsigned 32-bit
18 *	__divls/__remls: signed 32-bit
19 *
20 * These are not normal C functions: instead of the normal
21 * calling sequence, these expect their arguments in registers
22 * $24 and $25, and return the result in $27. Register $28 may
23 * be clobbered (assembly temporary), anything else must be saved.
24 *
25 * In short: painful.
26 *
27 * This is a rather simple bit-at-a-time algorithm: it's very good
28 * at dividing random 64-bit numbers, but the more usual case where
29 * the divisor is small is handled better by the DEC algorithm
30 * using lookup tables. This uses much less memory, though, and is
31 * nicer on the cache.. Besides, I don't know the copyright status
32 * of the DEC code.
33 */
34
35/*
36 * My temporaries:
37 *	$0 - current bit
38 *	$1 - shifted divisor
39 *	$2 - modulus/quotient
40 *
41 *	$23 - return address
42 *	$24 - dividend
43 *	$25 - divisor
44 *
45 *	$27 - quotient/modulus
46 *	$28 - compare status
47 *
48 * Much of the information about 21264 scheduling/coding comes from:
49 *	Compiler Writer's Guide for the Alpha 21264
50 *	abbreviated as 'CWG' in other comments here
51 *	ftp.digital.com/pub/Digital/info/semiconductor/literature/dsc-library.html
52 * Scheduling notation:
53 *	E	- either cluster
54 *	U	- upper subcluster; U0 - subcluster U0; U1 - subcluster U1
55 *	L	- lower subcluster; L0 - subcluster L0; L1 - subcluster L1
56 * Try not to change the actual algorithm if possible for consistency.
57 */
58
59#include <linux/export.h>
60#define halt .long 0
61
62/*
63 * Select function type and registers
64 */
65#define mask	$0
66#define divisor	$1
67#define compare $28
68#define tmp1	$3
69#define tmp2	$4
70
71#ifdef DIV
72#define DIV_ONLY(x,y...) x,##y
73#define MOD_ONLY(x,y...)
74#define func(x) __div##x
75#define modulus $2
76#define quotient $27
77#define GETSIGN(x) xor $24,$25,x
78#define STACK 48
79#else
80#define DIV_ONLY(x,y...)
81#define MOD_ONLY(x,y...) x,##y
82#define func(x) __rem##x
83#define modulus $27
84#define quotient $2
85#define GETSIGN(x) bis $24,$24,x
86#define STACK 32
87#endif
88
89/*
90 * For 32-bit operations, we need to extend to 64-bit
91 */
92#ifdef INTSIZE
93#define ufunction func(lu)
94#define sfunction func(l)
95#define LONGIFY(x) zapnot x,15,x
96#define SLONGIFY(x) addl x,0,x
97#else
98#define ufunction func(qu)
99#define sfunction func(q)
100#define LONGIFY(x)
101#define SLONGIFY(x)
102#endif
103
104.set noat
105.align	4
106.globl	ufunction
107.ent	ufunction
108ufunction:
109	subq	$30,STACK,$30		# E :
110	.frame	$30,STACK,$23
111	.prologue 0
112
1137:	stq	$1, 0($30)		# L :
114	bis	$25,$25,divisor		# E :
115	stq	$2, 8($30)		# L : L U L U
116
117	bis	$24,$24,modulus		# E :
118	stq	$0,16($30)		# L :
119	bis	$31,$31,quotient	# E :
120	LONGIFY(divisor)		# E : U L L U
121
122	stq	tmp1,24($30)		# L :
123	LONGIFY(modulus)		# E :
124	bis	$31,1,mask		# E :
125	DIV_ONLY(stq tmp2,32($30))	# L : L U U L
126
127	beq	divisor, 9f			/* div by zero */
128	/*
129	 * In spite of the DIV_ONLY being either a non-instruction
130	 * or an actual stq, the addition of the .align directive
131	 * below ensures that label 1 is going to be nicely aligned
132	 */
133
134	.align	4
135#ifdef INTSIZE
136	/*
137	 * shift divisor left, using 3-bit shifts for
138	 * 32-bit divides as we can't overflow. Three-bit
139	 * shifts will result in looping three times less
140	 * here, but can result in two loops more later.
141	 * Thus using a large shift isn't worth it (and
142	 * s8add pairs better than a sll..)
143	 */
1441:	cmpult	divisor,modulus,compare	# E :
145	s8addq	divisor,$31,divisor	# E :
146	s8addq	mask,$31,mask		# E :
147	bne	compare,1b		# U : U L U L
148#else
1491:	cmpult	divisor,modulus,compare	# E :
150	nop				# E :
151	nop				# E :
152	blt     divisor, 2f		# U : U L U L
153
154	addq	divisor,divisor,divisor	# E :
155	addq	mask,mask,mask		# E :
156	unop				# E :
157	bne	compare,1b		# U : U L U L
158#endif
159
160	/* ok, start to go right again.. */
1612:
162	/*
163	 * Keep things nicely bundled... use a nop instead of not
164	 * having an instruction for DIV_ONLY
165	 */
166#ifdef DIV
167	DIV_ONLY(addq quotient,mask,tmp2) # E :
168#else
169	nop				# E :
170#endif
171	srl	mask,1,mask		# U :
172	cmpule	divisor,modulus,compare	# E :
173	subq	modulus,divisor,tmp1	# E :
174
175#ifdef DIV
176	DIV_ONLY(cmovne compare,tmp2,quotient)	# E : Latency 2, extra map slot
177	nop				# E : as part of the cmovne
178	srl	divisor,1,divisor	# U :
179	nop				# E : L U L U
180
181	nop				# E :
182	cmovne	compare,tmp1,modulus	# E : Latency 2, extra map slot
183	nop				# E : as part of the cmovne
184	bne	mask,2b			# U : U L U L
185#else
186	srl	divisor,1,divisor	# U :
187	cmovne	compare,tmp1,modulus	# E : Latency 2, extra map slot
188	nop				# E : as part of the cmovne
189	bne	mask,2b			# U : U L L U
190#endif
191
1929:	ldq	$1, 0($30)		# L :
193	ldq	$2, 8($30)		# L :
194	nop				# E :
195	nop				# E : U U L L
196
197	ldq	$0,16($30)		# L :
198	ldq	tmp1,24($30)		# L :
199	nop				# E :
200	nop				# E :
201
202#ifdef DIV
203	DIV_ONLY(ldq tmp2,32($30))	# L :
204#else
205	nop				# E :
206#endif
207	addq	$30,STACK,$30		# E :
208	ret	$31,($23),1		# L0 : L U U L
209	.end	ufunction
210EXPORT_SYMBOL(ufunction)
211
212/*
213 * Uhh.. Ugly signed division. I'd rather not have it at all, but
214 * it's needed in some circumstances. There are different ways to
215 * handle this, really. This does:
216 * 	-a / b = a / -b = -(a / b)
217 *	-a % b = -(a % b)
218 *	a % -b = a % b
219 * which is probably not the best solution, but at least should
220 * have the property that (x/y)*y + (x%y) = x.
221 */
222.align 4
223.globl	sfunction
224.ent	sfunction
225sfunction:
226	subq	$30,STACK,$30		# E :
227	.frame	$30,STACK,$23
228	.prologue 0
229	bis	$24,$25,$28		# E :
230	SLONGIFY($28)			# E :
231	bge	$28,7b			# U :
232
233	stq	$24,0($30)		# L :
234	subq	$31,$24,$28		# E :
235	stq	$25,8($30)		# L :
236	nop				# E : U L U L
237
238	cmovlt	$24,$28,$24	/* abs($24) */ # E : Latency 2, extra map slot
239	nop				# E : as part of the cmov
240	stq	$23,16($30)		# L :
241	subq	$31,$25,$28		# E : U L U L
242
243	stq	tmp1,24($30)		# L :
244	cmovlt	$25,$28,$25	/* abs($25) */ # E : Latency 2, extra map slot
245	nop				# E :
246	bsr	$23,ufunction		# L0: L U L U
247
248	ldq	$24,0($30)		# L :
249	ldq	$25,8($30)		# L :
250	GETSIGN($28)			# E :
251	subq	$31,$27,tmp1		# E : U U L L
252
253	SLONGIFY($28)			# E :
254	ldq	$23,16($30)		# L :
255	cmovlt	$28,tmp1,$27		# E : Latency 2, extra map slot
256	nop				# E : U L L U : as part of the cmov
257
258	ldq	tmp1,24($30)		# L :
259	nop				# E : as part of the cmov
260	addq	$30,STACK,$30		# E :
261	ret	$31,($23),1		# L0 : L U U L
262	.end	sfunction
263EXPORT_SYMBOL(sfunction)
264