root/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
  98 ufunction:
  99         subq    $30,STACK,$30
 100         .frame  $30,STACK,$23
 101         .prologue 0
 102 
 103 7:      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          */
 125 1:      cmpult  divisor,modulus,compare
 126         s8addq  divisor,$31,divisor
 127         s8addq  mask,$31,mask
 128         bne     compare,1b
 129 #else
 130 1:      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.. */
 139 2:      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 
 148 9:      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
 156 EXPORT_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
 171 sfunction:
 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
 199 EXPORT_SYMBOL(sfunction)