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/arch/alpha/lib/divide.S

http://github.com/mirrors/linux
Assembly | 199 lines | 198 code | 1 blank | 0 comment | 0 complexity | 373b16bdbad87095dfd9ec3079516cf5 MD5 | raw file
  1/* SPDX-License-Identifier: GPL-2.0 */
  2/*
  3 * arch/alpha/lib/divide.S
  4 *
  5 * (C) 1995 Linus Torvalds
  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
 49#include <asm/export.h>
 50#define halt .long 0
 51
 52/*
 53 * Select function type and registers
 54 */
 55#define mask	$0
 56#define divisor	$1
 57#define compare $28
 58#define tmp1	$3
 59#define tmp2	$4
 60
 61#ifdef DIV
 62#define DIV_ONLY(x,y...) x,##y
 63#define MOD_ONLY(x,y...)
 64#define func(x) __div##x
 65#define modulus $2
 66#define quotient $27
 67#define GETSIGN(x) xor $24,$25,x
 68#define STACK 48
 69#else
 70#define DIV_ONLY(x,y...)
 71#define MOD_ONLY(x,y...) x,##y
 72#define func(x) __rem##x
 73#define modulus $27
 74#define quotient $2
 75#define GETSIGN(x) bis $24,$24,x
 76#define STACK 32
 77#endif
 78
 79/*
 80 * For 32-bit operations, we need to extend to 64-bit
 81 */
 82#ifdef INTSIZE
 83#define ufunction func(lu)
 84#define sfunction func(l)
 85#define LONGIFY(x) zapnot x,15,x
 86#define SLONGIFY(x) addl x,0,x
 87#else
 88#define ufunction func(qu)
 89#define sfunction func(q)
 90#define LONGIFY(x)
 91#define SLONGIFY(x)
 92#endif
 93
 94.set noat
 95.align	3
 96.globl	ufunction
 97.ent	ufunction
 98ufunction:
 99	subq	$30,STACK,$30
100	.frame	$30,STACK,$23
101	.prologue 0
102
1037:	stq	$1, 0($30)
104	bis	$25,$25,divisor
105	stq	$2, 8($30)
106	bis	$24,$24,modulus
107	stq	$0,16($30)
108	bis	$31,$31,quotient
109	LONGIFY(divisor)
110	stq	tmp1,24($30)
111	LONGIFY(modulus)
112	bis	$31,1,mask
113	DIV_ONLY(stq tmp2,32($30))
114	beq	divisor, 9f			/* div by zero */
115
116#ifdef INTSIZE
117	/*
118	 * shift divisor left, using 3-bit shifts for
119	 * 32-bit divides as we can't overflow. Three-bit
120	 * shifts will result in looping three times less
121	 * here, but can result in two loops more later.
122	 * Thus using a large shift isn't worth it (and
123	 * s8add pairs better than a sll..)
124	 */
1251:	cmpult	divisor,modulus,compare
126	s8addq	divisor,$31,divisor
127	s8addq	mask,$31,mask
128	bne	compare,1b
129#else
1301:	cmpult	divisor,modulus,compare
131	blt     divisor, 2f
132	addq	divisor,divisor,divisor
133	addq	mask,mask,mask
134	bne	compare,1b
135	unop
136#endif
137
138	/* ok, start to go right again.. */
1392:	DIV_ONLY(addq quotient,mask,tmp2)
140	srl	mask,1,mask
141	cmpule	divisor,modulus,compare
142	subq	modulus,divisor,tmp1
143	DIV_ONLY(cmovne compare,tmp2,quotient)
144	srl	divisor,1,divisor
145	cmovne	compare,tmp1,modulus
146	bne	mask,2b
147
1489:	ldq	$1, 0($30)
149	ldq	$2, 8($30)
150	ldq	$0,16($30)
151	ldq	tmp1,24($30)
152	DIV_ONLY(ldq tmp2,32($30))
153	addq	$30,STACK,$30
154	ret	$31,($23),1
155	.end	ufunction
156EXPORT_SYMBOL(ufunction)
157
158/*
159 * Uhh.. Ugly signed division. I'd rather not have it at all, but
160 * it's needed in some circumstances. There are different ways to
161 * handle this, really. This does:
162 * 	-a / b = a / -b = -(a / b)
163 *	-a % b = -(a % b)
164 *	a % -b = a % b
165 * which is probably not the best solution, but at least should
166 * have the property that (x/y)*y + (x%y) = x.
167 */
168.align 3
169.globl	sfunction
170.ent	sfunction
171sfunction:
172	subq	$30,STACK,$30
173	.frame	$30,STACK,$23
174	.prologue 0
175	bis	$24,$25,$28
176	SLONGIFY($28)
177	bge	$28,7b
178	stq	$24,0($30)
179	subq	$31,$24,$28
180	stq	$25,8($30)
181	cmovlt	$24,$28,$24	/* abs($24) */
182	stq	$23,16($30)
183	subq	$31,$25,$28
184	stq	tmp1,24($30)
185	cmovlt	$25,$28,$25	/* abs($25) */
186	unop
187	bsr	$23,ufunction
188	ldq	$24,0($30)
189	ldq	$25,8($30)
190	GETSIGN($28)
191	subq	$31,$27,tmp1
192	SLONGIFY($28)
193	ldq	$23,16($30)
194	cmovlt	$28,tmp1,$27
195	ldq	tmp1,24($30)
196	addq	$30,STACK,$30
197	ret	$31,($23),1
198	.end	sfunction
199EXPORT_SYMBOL(sfunction)