LXR linux/arch/alpha/lib/divide.S

   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)
 200