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

   1/*
   2 * arch/alpha/lib/divide.S
   3 *
   4 * (C) 1995 Linus Torvalds
   5 *
   6 * Alpha division..
   7 */
   8
   9/*
  10 * The alpha chip doesn't provide hardware division, so we have to do it
  11 * by hand.  The compiler expects the functions
  12 *
  13 *      __divqu: 64-bit unsigned long divide
  14 *      __remqu: 64-bit unsigned long remainder
  15 *      __divqs/__remqs: signed 64-bit
  16 *      __divlu/__remlu: unsigned 32-bit
  17 *      __divls/__remls: signed 32-bit
  18 *
  19 * These are not normal C functions: instead of the normal
  20 * calling sequence, these expect their arguments in registers
  21 * $24 and $25, and return the result in $27. Register $28 may
  22 * be clobbered (assembly temporary), anything else must be saved. 
  23 *
  24 * In short: painful.
  25 *
  26 * This is a rather simple bit-at-a-time algorithm: it's very good
  27 * at dividing random 64-bit numbers, but the more usual case where
  28 * the divisor is small is handled better by the DEC algorithm
  29 * using lookup tables. This uses much less memory, though, and is
  30 * nicer on the cache.. Besides, I don't know the copyright status
  31 * of the DEC code.
  32 */
  33
  34/*
  35 * My temporaries:
  36 *      $0 - current bit
  37 *      $1 - shifted divisor
  38 *      $2 - modulus/quotient
  39 *
  40 *      $23 - return address
  41 *      $24 - dividend
  42 *      $25 - divisor
  43 *
  44 *      $27 - quotient/modulus
  45 *      $28 - compare status
  46 */
  47
  48#define halt .long 0
  49
  50/*
  51 * Select function type and registers
  52 */
  53#define mask    $0
  54#define divisor $1
  55#define compare $28
  56#define tmp1    $3
  57#define tmp2    $4
  58
  59#ifdef DIV
  60#define DIV_ONLY(x,y...) x,##y
  61#define MOD_ONLY(x,y...)
  62#define func(x) __div##x
  63#define modulus $2
  64#define quotient $27
  65#define GETSIGN(x) xor $24,$25,x
  66#define STACK 48
  67#else
  68#define DIV_ONLY(x,y...)
  69#define MOD_ONLY(x,y...) x,##y
  70#define func(x) __rem##x
  71#define modulus $27
  72#define quotient $2
  73#define GETSIGN(x) bis $24,$24,x
  74#define STACK 32
  75#endif
  76
  77/*
  78 * For 32-bit operations, we need to extend to 64-bit
  79 */
  80#ifdef INTSIZE
  81#define ufunction func(lu)
  82#define sfunction func(l)
  83#define LONGIFY(x) zapnot x,15,x
  84#define SLONGIFY(x) addl x,0,x
  85#else
  86#define ufunction func(qu)
  87#define sfunction func(q)
  88#define LONGIFY(x)
  89#define SLONGIFY(x)
  90#endif
  91
  92.set noat
  93.align  3
  94.globl  ufunction
  95.ent    ufunction
  96ufunction:
  97        subq    $30,STACK,$30
  98        .frame  $30,STACK,$23
  99        .prologue 0
 100
 1017:      stq     $1, 0($30)
 102        bis     $25,$25,divisor
 103        stq     $2, 8($30)
 104        bis     $24,$24,modulus
 105        stq     $0,16($30)
 106        bis     $31,$31,quotient
 107        LONGIFY(divisor)
 108        stq     tmp1,24($30)
 109        LONGIFY(modulus)
 110        bis     $31,1,mask
 111        DIV_ONLY(stq tmp2,32($30))
 112        beq     divisor, 9f                     /* div by zero */
 113
 114#ifdef INTSIZE
 115        /*
 116         * shift divisor left, using 3-bit shifts for
 117         * 32-bit divides as we can't overflow. Three-bit
 118         * shifts will result in looping three times less
 119         * here, but can result in two loops more later.
 120         * Thus using a large shift isn't worth it (and
 121         * s8add pairs better than a sll..)
 122         */
 1231:      cmpult  divisor,modulus,compare
 124        s8addq  divisor,$31,divisor
 125        s8addq  mask,$31,mask
 126        bne     compare,1b
 127#else
 1281:      cmpult  divisor,modulus,compare
 129        blt     divisor, 2f
 130        addq    divisor,divisor,divisor
 131        addq    mask,mask,mask
 132        bne     compare,1b
 133        unop
 134#endif
 135
 136        /* ok, start to go right again.. */
 1372:      DIV_ONLY(addq quotient,mask,tmp2)
 138        srl     mask,1,mask
 139        cmpule  divisor,modulus,compare
 140        subq    modulus,divisor,tmp1
 141        DIV_ONLY(cmovne compare,tmp2,quotient)
 142        srl     divisor,1,divisor
 143        cmovne  compare,tmp1,modulus
 144        bne     mask,2b
 145
 1469:      ldq     $1, 0($30)
 147        ldq     $2, 8($30)
 148        ldq     $0,16($30)
 149        ldq     tmp1,24($30)
 150        DIV_ONLY(ldq tmp2,32($30))
 151        addq    $30,STACK,$30
 152        ret     $31,($23),1
 153        .end    ufunction
 154
 155/*
 156 * Uhh.. Ugly signed division. I'd rather not have it at all, but
 157 * it's needed in some circumstances. There are different ways to
 158 * handle this, really. This does:
 159 *      -a / b = a / -b = -(a / b)
 160 *      -a % b = -(a % b)
 161 *      a % -b = a % b
 162 * which is probably not the best solution, but at least should
 163 * have the property that (x/y)*y + (x%y) = x.
 164 */
 165.align 3
 166.globl  sfunction
 167.ent    sfunction
 168sfunction:
 169        subq    $30,STACK,$30
 170        .frame  $30,STACK,$23
 171        .prologue 0
 172        bis     $24,$25,$28
 173        SLONGIFY($28)
 174        bge     $28,7b
 175        stq     $24,0($30)
 176        subq    $31,$24,$28
 177        stq     $25,8($30)
 178        cmovlt  $24,$28,$24     /* abs($24) */
 179        stq     $23,16($30)
 180        subq    $31,$25,$28
 181        stq     tmp1,24($30)
 182        cmovlt  $25,$28,$25     /* abs($25) */
 183        unop
 184        bsr     $23,ufunction
 185        ldq     $24,0($30)
 186        ldq     $25,8($30)
 187        GETSIGN($28)
 188        subq    $31,$27,tmp1
 189        SLONGIFY($28)
 190        ldq     $23,16($30)
 191        cmovlt  $28,tmp1,$27
 192        ldq     tmp1,24($30)
 193        addq    $30,STACK,$30
 194        ret     $31,($23),1
 195        .end    sfunction
 196