/* SPDX-License-Identifier: GPL-2.0 */
/*
 * arch/alpha/lib/ev6-divide.S
 *
 * 21264 version contributed by Rick Gorton <rick.gorton@alpha-processor.com>
 *
 * 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
 *
 * Much of the information about 21264 scheduling/coding comes from:
 *      Compiler Writer's Guide for the Alpha 21264
 *      abbreviated as 'CWG' in other comments here
 *      ftp.digital.com/pub/Digital/info/semiconductor/literature/dsc-library.html
 * Scheduling notation:
 *      E       - either cluster
 *      U       - upper subcluster; U0 - subcluster U0; U1 - subcluster U1
 *      L       - lower subcluster; L0 - subcluster L0; L1 - subcluster L1
 * Try not to change the actual algorithm if possible for consistency.
 */

#include <linux/export.h>
#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  4
.globl  ufunction
.ent    ufunction
ufunction:
        subq    $30,STACK,$30           # E :
        .frame  $30,STACK,$23
        .prologue 0

7:      stq     $1, 0($30)              # L :
        bis     $25,$25,divisor         # E :
        stq     $2, 8($30)              # L : L U L U

        bis     $24,$24,modulus         # E :
        stq     $0,16($30)              # L :
        bis     $31,$31,quotient        # E :
        LONGIFY(divisor)                # E : U L L U

        stq     tmp1,24($30)            # L :
        LONGIFY(modulus)                # E :
        bis     $31,1,mask              # E :
        DIV_ONLY(stq tmp2,32($30))      # L : L U U L

        beq     divisor, 9f                     /* div by zero */
        /*
         * In spite of the DIV_ONLY being either a non-instruction
         * or an actual stq, the addition of the .align directive
         * below ensures that label 1 is going to be nicely aligned
         */

        .align  4
#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 # E :
        s8addq  divisor,$31,divisor     # E :
        s8addq  mask,$31,mask           # E :
        bne     compare,1b              # U : U L U L
#else
1:      cmpult  divisor,modulus,compare # E :
        nop                             # E :
        nop                             # E :
        blt     divisor, 2f             # U : U L U L

        addq    divisor,divisor,divisor # E :
        addq    mask,mask,mask          # E :
        unop                            # E :
        bne     compare,1b              # U : U L U L
#endif

        /* ok, start to go right again.. */
2:
        /*
         * Keep things nicely bundled... use a nop instead of not
         * having an instruction for DIV_ONLY
         */
#ifdef DIV
        DIV_ONLY(addq quotient,mask,tmp2) # E :
#else
        nop                             # E :
#endif
        srl     mask,1,mask             # U :
        cmpule  divisor,modulus,compare # E :
        subq    modulus,divisor,tmp1    # E :

#ifdef DIV
        DIV_ONLY(cmovne compare,tmp2,quotient)  # E : Latency 2, extra map slot
        nop                             # E : as part of the cmovne
        srl     divisor,1,divisor       # U :
        nop                             # E : L U L U

        nop                             # E :
        cmovne  compare,tmp1,modulus    # E : Latency 2, extra map slot
        nop                             # E : as part of the cmovne
        bne     mask,2b                 # U : U L U L
#else
        srl     divisor,1,divisor       # U :
        cmovne  compare,tmp1,modulus    # E : Latency 2, extra map slot
        nop                             # E : as part of the cmovne
        bne     mask,2b                 # U : U L L U
#endif

9:      ldq     $1, 0($30)              # L :
        ldq     $2, 8($30)              # L :
        nop                             # E :
        nop                             # E : U U L L

        ldq     $0,16($30)              # L :
        ldq     tmp1,24($30)            # L :
        nop                             # E :
        nop                             # E :

#ifdef DIV
        DIV_ONLY(ldq tmp2,32($30))      # L :
#else
        nop                             # E :
#endif
        addq    $30,STACK,$30           # E :
        ret     $31,($23),1             # L0 : L U U L
        .end    ufunction
EXPORT_SYMBOL(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 4
.globl  sfunction
.ent    sfunction
sfunction:
        subq    $30,STACK,$30           # E :
        .frame  $30,STACK,$23
        .prologue 0
        bis     $24,$25,$28             # E :
        SLONGIFY($28)                   # E :
        bge     $28,7b                  # U :

        stq     $24,0($30)              # L :
        subq    $31,$24,$28             # E :
        stq     $25,8($30)              # L :
        nop                             # E : U L U L

        cmovlt  $24,$28,$24     /* abs($24) */ # E : Latency 2, extra map slot
        nop                             # E : as part of the cmov
        stq     $23,16($30)             # L :
        subq    $31,$25,$28             # E : U L U L

        stq     tmp1,24($30)            # L :
        cmovlt  $25,$28,$25     /* abs($25) */ # E : Latency 2, extra map slot
        nop                             # E :
        bsr     $23,ufunction           # L0: L U L U

        ldq     $24,0($30)              # L :
        ldq     $25,8($30)              # L :
        GETSIGN($28)                    # E :
        subq    $31,$27,tmp1            # E : U U L L

        SLONGIFY($28)                   # E :
        ldq     $23,16($30)             # L :
        cmovlt  $28,tmp1,$27            # E : Latency 2, extra map slot
        nop                             # E : U L L U : as part of the cmov

        ldq     tmp1,24($30)            # L :
        nop                             # E : as part of the cmov
        addq    $30,STACK,$30           # E :
        ret     $31,($23),1             # L0 : L U U L
        .end    sfunction
EXPORT_SYMBOL(sfunction)