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
  96 ufunction:
  97         subq    $30,STACK,$30
  98         .frame  $30,STACK,$23
  99         .prologue 0
 100
 101 7:      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          */
 123 1:      cmpult  divisor,modulus,compare
 124         s8addq  divisor,$31,divisor
 125         s8addq  mask,$31,mask
 126         bne     compare,1b
 127 #else
 128 1:      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.. */
 137 2:      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
 146 9:      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
 168 sfunction:
 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