divide.S

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/*
 * arch/alpha/lib/divide.S
 *
 * (C) 1995 Linus Torvalds
 *
 * Alpha division..
 */

/*
 * The alpha chip doesn't provide hardware division, so we have to do it
 * by hand.  The compiler expects the functions
 *
 *      __divqu: 64-bit unsigned long divide
 *      __remqu: 64-bit unsigned long remainder
 *      __divqs/__remqs: signed 64-bit
 *      __divlu/__remlu: unsigned 32-bit
 *      __divls/__remls: signed 32-bit
 *
 * These are not normal C functions: instead of the normal
 * calling sequence, these expect their arguments in registers
 * $24 and $25, and return the result in $27. Register $28 may
 * be clobbered (assembly temporary), anything else must be saved. 
 *
 * In short: painful.
 *
 * This is a rather simple bit-at-a-time algorithm: it's very good
 * at dividing random 64-bit numbers, but the more usual case where
 * the divisor is small is handled better by the DEC algorithm
 * using lookup tables. This uses much less memory, though, and is
 * nicer on the cache.. Besides, I don't know the copyright status
 * of the DEC code.
 */

/*
 * My temporaries:
 *      $0 - current bit
 *      $1 - shifted divisor
 *      $2 - modulus/quotient
 *
 *      $23 - return address
 *      $24 - dividend
 *      $25 - divisor
 *
 *      $27 - quotient/modulus
 *      $28 - compare status
 */

#define halt .long 0

/*
 * Select function type and registers
 */
#define mask    $0
#define divisor $1
#define compare $28
#define tmp1    $3
#define tmp2    $4

#ifdef DIV
#define DIV_ONLY(x,y...) x,##y
#define MOD_ONLY(x,y...)
#define func(x) __div##x
#define modulus $2
#define quotient $27
#define GETSIGN(x) xor $24,$25,x
#define STACK 48
#else
#define DIV_ONLY(x,y...)
#define MOD_ONLY(x,y...) x,##y
#define func(x) __rem##x
#define modulus $27
#define quotient $2
#define GETSIGN(x) bis $24,$24,x
#define STACK 32
#endif

/*
 * For 32-bit operations, we need to extend to 64-bit
 */
#ifdef INTSIZE
#define ufunction func(lu)
#define sfunction func(l)
#define LONGIFY(x) zapnot x,15,x
#define SLONGIFY(x) addl x,0,x
#else
#define ufunction func(qu)
#define sfunction func(q)
#define LONGIFY(x)
#define SLONGIFY(x)
#endif

.set noat
.align  3
.globl  ufunction
.ent    ufunction
ufunction:
        subq    $30,STACK,$30
        .frame  $30,STACK,$23
        .prologue 0

7:      stq     $1, 0($30)
        bis     $25,$25,divisor
        stq     $2, 8($30)
        bis     $24,$24,modulus
        stq     $0,16($30)
        bis     $31,$31,quotient
        LONGIFY(divisor)
        stq     tmp1,24($30)
        LONGIFY(modulus)
        bis     $31,1,mask
        DIV_ONLY(stq tmp2,32($30))
        beq     divisor, 9f                     /* div by zero */

#ifdef INTSIZE
        /*
         * shift divisor left, using 3-bit shifts for
         * 32-bit divides as we can't overflow. Three-bit
         * shifts will result in looping three times less
         * here, but can result in two loops more later.
         * Thus using a large shift isn't worth it (and
         * s8add pairs better than a sll..)
         */
1:      cmpult  divisor,modulus,compare
        s8addq  divisor,$31,divisor
        s8addq  mask,$31,mask
        bne     compare,1b
#else
1:      cmpult  divisor,modulus,compare
        blt     divisor, 2f
        addq    divisor,divisor,divisor
        addq    mask,mask,mask
        bne     compare,1b
        unop
#endif

        /* ok, start to go right again.. */
2:      DIV_ONLY(addq quotient,mask,tmp2)
        srl     mask,1,mask
        cmpule  divisor,modulus,compare
        subq    modulus,divisor,tmp1
        DIV_ONLY(cmovne compare,tmp2,quotient)
        srl     divisor,1,divisor
        cmovne  compare,tmp1,modulus
        bne     mask,2b

9:      ldq     $1, 0($30)
        ldq     $2, 8($30)
        ldq     $0,16($30)
        ldq     tmp1,24($30)
        DIV_ONLY(ldq tmp2,32($30))
        addq    $30,STACK,$30
        ret     $31,($23),1
        .end    ufunction

/*
 * Uhh.. Ugly signed division. I'd rather not have it at all, but
 * it's needed in some circumstances. There are different ways to
 * handle this, really. This does:
 *      -a / b = a / -b = -(a / b)
 *      -a % b = -(a % b)
 *      a % -b = a % b
 * which is probably not the best solution, but at least should
 * have the property that (x/y)*y + (x%y) = x.
 */
.align 3
.globl  sfunction
.ent    sfunction
sfunction:
        subq    $30,STACK,$30
        .frame  $30,STACK,$23
        .prologue 0
        bis     $24,$25,$28
        SLONGIFY($28)
        bge     $28,7b
        stq     $24,0($30)
        subq    $31,$24,$28
        stq     $25,8($30)
        cmovlt  $24,$28,$24     /* abs($24) */
        stq     $23,16($30)
        subq    $31,$25,$28
        stq     tmp1,24($30)
        cmovlt  $25,$28,$25     /* abs($25) */
        unop
        bsr     $23,ufunction
        ldq     $24,0($30)
        ldq     $25,8($30)
        GETSIGN($28)
        subq    $31,$27,tmp1
        SLONGIFY($28)
        ldq     $23,16($30)
        cmovlt  $28,tmp1,$27
        ldq     tmp1,24($30)
        addq    $30,STACK,$30
        ret     $31,($23),1
        .end    sfunction

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