arch/alpha/lib/ev6-divide.S

   1 /*
   2  * arch/alpha/lib/ev6-divide.S
   3  *
   4  * 21264 version contributed by Rick Gorton <rick.gorton@alpha-processor.com>
   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  * Much of the information about 21264 scheduling/coding comes from:
  48  *      Compiler Writer's Guide for the Alpha 21264
  49  *      abbreviated as 'CWG' in other comments here
  50  *      ftp.digital.com/pub/Digital/info/semiconductor/literature/dsc-library.html
  51  * Scheduling notation:
  52  *      E       - either cluster
  53  *      U       - upper subcluster; U0 - subcluster U0; U1 - subcluster U1
  54  *      L       - lower subcluster; L0 - subcluster L0; L1 - subcluster L1
  55  * Try not to change the actual algorithm if possible for consistency.
  56  */
  57
  58 #include <asm/export.h>
  59 #define halt .long 0
  60
  61 /*
  62  * Select function type and registers
  63  */
  64 #define mask    $0
  65 #define divisor $1
  66 #define compare $28
  67 #define tmp1    $3
  68 #define tmp2    $4
  69
  70 #ifdef DIV
  71 #define DIV_ONLY(x,y...) x,##y
  72 #define MOD_ONLY(x,y...)
  73 #define func(x) __div##x
  74 #define modulus $2
  75 #define quotient $27
  76 #define GETSIGN(x) xor $24,$25,x
  77 #define STACK 48
  78 #else
  79 #define DIV_ONLY(x,y...)
  80 #define MOD_ONLY(x,y...) x,##y
  81 #define func(x) __rem##x
  82 #define modulus $27
  83 #define quotient $2
  84 #define GETSIGN(x) bis $24,$24,x
  85 #define STACK 32
  86 #endif
  87
  88 /*
  89  * For 32-bit operations, we need to extend to 64-bit
  90  */
  91 #ifdef INTSIZE
  92 #define ufunction func(lu)
  93 #define sfunction func(l)
  94 #define LONGIFY(x) zapnot x,15,x
  95 #define SLONGIFY(x) addl x,0,x
  96 #else
  97 #define ufunction func(qu)
  98 #define sfunction func(q)
  99 #define LONGIFY(x)
 100 #define SLONGIFY(x)
 101 #endif
 102
 103 .set noat
 104 .align  4
 105 .globl  ufunction
 106 .ent    ufunction
 107 ufunction:
 108         subq    $30,STACK,$30           # E :
 109         .frame  $30,STACK,$23
 110         .prologue 0
 111
 112 7:      stq     $1, 0($30)              # L :
 113         bis     $25,$25,divisor         # E :
 114         stq     $2, 8($30)              # L : L U L U
 115
 116         bis     $24,$24,modulus         # E :
 117         stq     $0,16($30)              # L :
 118         bis     $31,$31,quotient        # E :
 119         LONGIFY(divisor)                # E : U L L U
 120
 121         stq     tmp1,24($30)            # L :
 122         LONGIFY(modulus)                # E :
 123         bis     $31,1,mask              # E :
 124         DIV_ONLY(stq tmp2,32($30))      # L : L U U L
 125
 126         beq     divisor, 9f                     /* div by zero */
 127         /*
 128          * In spite of the DIV_ONLY being either a non-instruction
 129          * or an actual stq, the addition of the .align directive
 130          * below ensures that label 1 is going to be nicely aligned
 131          */
 132
 133         .align  4
 134 #ifdef INTSIZE
 135         /*
 136          * shift divisor left, using 3-bit shifts for
 137          * 32-bit divides as we can't overflow. Three-bit
 138          * shifts will result in looping three times less
 139          * here, but can result in two loops more later.
 140          * Thus using a large shift isn't worth it (and
 141          * s8add pairs better than a sll..)
 142          */
 143 1:      cmpult  divisor,modulus,compare # E :
 144         s8addq  divisor,$31,divisor     # E :
 145         s8addq  mask,$31,mask           # E :
 146         bne     compare,1b              # U : U L U L
 147 #else
 148 1:      cmpult  divisor,modulus,compare # E :
 149         nop                             # E :
 150         nop                             # E :
 151         blt     divisor, 2f             # U : U L U L
 152
 153         addq    divisor,divisor,divisor # E :
 154         addq    mask,mask,mask          # E :
 155         unop                            # E :
 156         bne     compare,1b              # U : U L U L
 157 #endif
 158
 159         /* ok, start to go right again.. */
 160 2:
 161         /*
 162          * Keep things nicely bundled... use a nop instead of not
 163          * having an instruction for DIV_ONLY
 164          */
 165 #ifdef DIV
 166         DIV_ONLY(addq quotient,mask,tmp2) # E :
 167 #else
 168         nop                             # E :
 169 #endif
 170         srl     mask,1,mask             # U :
 171         cmpule  divisor,modulus,compare # E :
 172         subq    modulus,divisor,tmp1    # E :
 173
 174 #ifdef DIV
 175         DIV_ONLY(cmovne compare,tmp2,quotient)  # E : Latency 2, extra map slot
 176         nop                             # E : as part of the cmovne
 177         srl     divisor,1,divisor       # U :
 178         nop                             # E : L U L U
 179
 180         nop                             # E :
 181         cmovne  compare,tmp1,modulus    # E : Latency 2, extra map slot
 182         nop                             # E : as part of the cmovne
 183         bne     mask,2b                 # U : U L U L
 184 #else
 185         srl     divisor,1,divisor       # U :
 186         cmovne  compare,tmp1,modulus    # E : Latency 2, extra map slot
 187         nop                             # E : as part of the cmovne
 188         bne     mask,2b                 # U : U L L U
 189 #endif
 190
 191 9:      ldq     $1, 0($30)              # L :
 192         ldq     $2, 8($30)              # L :
 193         nop                             # E :
 194         nop                             # E : U U L L
 195
 196         ldq     $0,16($30)              # L :
 197         ldq     tmp1,24($30)            # L :
 198         nop                             # E :
 199         nop                             # E :
 200
 201 #ifdef DIV
 202         DIV_ONLY(ldq tmp2,32($30))      # L :
 203 #else
 204         nop                             # E :
 205 #endif
 206         addq    $30,STACK,$30           # E :
 207         ret     $31,($23),1             # L0 : L U U L
 208         .end    ufunction
 209 EXPORT_SYMBOL(ufunction)
 210
 211 /*
 212  * Uhh.. Ugly signed division. I'd rather not have it at all, but
 213  * it's needed in some circumstances. There are different ways to
 214  * handle this, really. This does:
 215  *      -a / b = a / -b = -(a / b)
 216  *      -a % b = -(a % b)
 217  *      a % -b = a % b
 218  * which is probably not the best solution, but at least should
 219  * have the property that (x/y)*y + (x%y) = x.
 220  */
 221 .align 4
 222 .globl  sfunction
 223 .ent    sfunction
 224 sfunction:
 225         subq    $30,STACK,$30           # E :
 226         .frame  $30,STACK,$23
 227         .prologue 0
 228         bis     $24,$25,$28             # E :
 229         SLONGIFY($28)                   # E :
 230         bge     $28,7b                  # U :
 231
 232         stq     $24,0($30)              # L :
 233         subq    $31,$24,$28             # E :
 234         stq     $25,8($30)              # L :
 235         nop                             # E : U L U L
 236
 237         cmovlt  $24,$28,$24     /* abs($24) */ # E : Latency 2, extra map slot
 238         nop                             # E : as part of the cmov
 239         stq     $23,16($30)             # L :
 240         subq    $31,$25,$28             # E : U L U L
 241
 242         stq     tmp1,24($30)            # L :
 243         cmovlt  $25,$28,$25     /* abs($25) */ # E : Latency 2, extra map slot
 244         nop                             # E :
 245         bsr     $23,ufunction           # L0: L U L U
 246
 247         ldq     $24,0($30)              # L :
 248         ldq     $25,8($30)              # L :
 249         GETSIGN($28)                    # E :
 250         subq    $31,$27,tmp1            # E : U U L L
 251
 252         SLONGIFY($28)                   # E :
 253         ldq     $23,16($30)             # L :
 254         cmovlt  $28,tmp1,$27            # E : Latency 2, extra map slot
 255         nop                             # E : U L L U : as part of the cmov
 256
 257         ldq     tmp1,24($30)            # L :
 258         nop                             # E : as part of the cmov
 259         addq    $30,STACK,$30           # E :
 260         ret     $31,($23),1             # L0 : L U U L
 261         .end    sfunction
 262 EXPORT_SYMBOL(sfunction)