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/*
 * arch/alpha/lib/ev6-strncpy_from_user.S
 * 21264 version contributed by Rick Gorton <rick.gorton@alpha-processor.com>
 *
 * Just like strncpy except in the return value:
 *
 * -EFAULT       if an exception occurs before the terminator is copied.
 * N             if the buffer filled.
 *
 * Otherwise the length of the string is returned.
 *
 * 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
 * A bunch of instructions got moved and temp registers were changed
 * to aid in scheduling.  Control flow was also re-arranged to eliminate
 * branches, and to provide longer code sequences to enable better scheduling.
 * A total rewrite (using byte load/stores for start & tail sequences)
 * is desirable, but very difficult to do without a from-scratch rewrite.
 * Save that for the future.
 */


#include <asm/errno.h>
#include <asm/regdef.h>


/* Allow an exception for an insn; exit if we get one.  */
#define EX(x,y...)                      \
        99: x,##y;                      \
        .section __ex_table,"a";        \
        .gprel32 99b;                   \
        lda $31, $exception-99b($0);    \
        .previous


        .set noat
        .set noreorder
        .text

        .globl __strncpy_from_user
        .ent __strncpy_from_user
        .frame $30, 0, $26
        .prologue 1

        .align 4
__strncpy_from_user:
        ldgp    $29, 0($27)     # E E : becomes 2 instructions (for exceptions)
        and     a0, 7, t3       # E : find dest misalignment
        beq     a2, $zerolength # U :

        /* Are source and destination co-aligned?  */
        mov     a0, v0          # E : save the string start
        xor     a0, a1, t4      # E :
        EX( ldq_u t1, 0(a1) )   # L : Latency=3 load first quadword
        ldq_u   t0, 0(a0)       # L : load first (partial) aligned dest quadword

        addq    a2, t3, a2      # E : bias count by dest misalignment
        subq    a2, 1, a3       # E :
        addq    zero, 1, t10    # E :
        and     t4, 7, t4       # E : misalignment between the two

        and     a3, 7, t6       # E : number of tail bytes
        sll     t10, t6, t10    # E : t10 = bitmask of last count byte
        bne     t4, $unaligned  # U :
        lda     t2, -1          # E : build a mask against false zero

        /*
         * We are co-aligned; take care of a partial first word.
         * On entry to this basic block:
         * t0 == the first destination word for masking back in
         * t1 == the first source word.
         */

        srl     a3, 3, a2       # E : a2 = loop counter = (count - 1)/8
        addq    a1, 8, a1       # E :
        mskqh   t2, a1, t2      # U :   detection in the src word
        nop

        /* Create the 1st output word and detect 0's in the 1st input word.  */
        mskqh   t1, a1, t3      # U :
        mskql   t0, a1, t0      # U : assemble the first output word
        ornot   t1, t2, t2      # E :
        nop

        cmpbge  zero, t2, t8    # E : bits set iff null found
        or      t0, t3, t0      # E :
        beq     a2, $a_eoc      # U :
        bne     t8, $a_eos      # U : 2nd branch in a quad.  Bad.

        /* On entry to this basic block:
         * t0 == a source quad not containing a null.
         * a0 - current aligned destination address
         * a1 - current aligned source address
         * a2 - count of quadwords to move.
         * NOTE: Loop improvement - unrolling this is going to be
         *      a huge win, since we're going to stall otherwise.
         *      Fix this later.  For _really_ large copies, look
         *      at using wh64 on a look-ahead basis.  See the code
         *      in clear_user.S and copy_user.S.
         * Presumably, since (a0) and (a1) do not overlap (by C definition)
         * Lots of nops here:
         *      - Separate loads from stores
         *      - Keep it to 1 branch/quadpack so the branch predictor
         *        can train.
         */
$a_loop:
        stq_u   t0, 0(a0)       # L :
        addq    a0, 8, a0       # E :
        nop
        subq    a2, 1, a2       # E :

        EX( ldq_u t0, 0(a1) )   # L :
        addq    a1, 8, a1       # E :
        cmpbge  zero, t0, t8    # E : Stall 2 cycles on t0
        beq     a2, $a_eoc      # U :

        beq     t8, $a_loop     # U :
        nop
        nop
        nop

        /* Take care of the final (partial) word store.  At this point
         * the end-of-count bit is set in t8 iff it applies.
         *
         * On entry to this basic block we have:
         * t0 == the source word containing the null
         * t8 == the cmpbge mask that found it.
         */
$a_eos:
        negq    t8, t12         # E : find low bit set
        and     t8, t12, t12    # E : 

        /* We're doing a partial word store and so need to combine
           our source and original destination words.  */
        ldq_u   t1, 0(a0)       # L :
        subq    t12, 1, t6      # E :

        or      t12, t6, t8     # E :
        zapnot  t0, t8, t0      # U : clear src bytes > null
        zap     t1, t8, t1      # U : clear dst bytes <= null
        or      t0, t1, t0      # E :

        stq_u   t0, 0(a0)       # L :
        br      $finish_up      # L0 :
        nop
        nop

        /* Add the end-of-count bit to the eos detection bitmask.  */
        .align 4
$a_eoc:
        or      t10, t8, t8
        br      $a_eos
        nop
        nop


/* The source and destination are not co-aligned.  Align the destination
   and cope.  We have to be very careful about not reading too much and
   causing a SEGV.  */

        .align 4
$u_head:
        /* We know just enough now to be able to assemble the first
           full source word.  We can still find a zero at the end of it
           that prevents us from outputting the whole thing.

           On entry to this basic block:
           t0 == the first dest word, unmasked
           t1 == the shifted low bits of the first source word
           t6 == bytemask that is -1 in dest word bytes */

        EX( ldq_u t2, 8(a1) )   # L : load second src word
        addq    a1, 8, a1       # E :
        mskql   t0, a0, t0      # U : mask trailing garbage in dst
        extqh   t2, a1, t4      # U :

        or      t1, t4, t1      # E : first aligned src word complete
        mskqh   t1, a0, t1      # U : mask leading garbage in src
        or      t0, t1, t0      # E : first output word complete
        or      t0, t6, t6      # E : mask original data for zero test

        cmpbge  zero, t6, t8    # E :
        beq     a2, $u_eocfin   # U :
        bne     t8, $u_final    # U : bad news - 2nd branch in a quad
        lda     t6, -1          # E : mask out the bits we have

        mskql   t6, a1, t6      # U :   already seen
        stq_u   t0, 0(a0)       # L : store first output word
        or      t6, t2, t2      # E :
        cmpbge  zero, t2, t8    # E : find nulls in second partial

        addq    a0, 8, a0               # E :
        subq    a2, 1, a2               # E :
        bne     t8, $u_late_head_exit   # U :
        nop

        /* Finally, we've got all the stupid leading edge cases taken care
           of and we can set up to enter the main loop.  */

        extql   t2, a1, t1      # U : position hi-bits of lo word
        EX( ldq_u t2, 8(a1) )   # L : read next high-order source word
        addq    a1, 8, a1       # E :
        cmpbge  zero, t2, t8    # E :

        beq     a2, $u_eoc      # U :
        bne     t8, $u_eos      # U :
        nop
        nop

        /* Unaligned copy main loop.  In order to avoid reading too much,
           the loop is structured to detect zeros in aligned source words.
           This has, unfortunately, effectively pulled half of a loop
           iteration out into the head and half into the tail, but it does
           prevent nastiness from accumulating in the very thing we want
           to run as fast as possible.

           On entry to this basic block:
           t1 == the shifted high-order bits from the previous source word
           t2 == the unshifted current source word

           We further know that t2 does not contain a null terminator.  */

        /*
         * Extra nops here:
         *      separate load quads from store quads
         *      only one branch/quad to permit predictor training
         */

        .align 4
$u_loop:
        extqh   t2, a1, t0      # U : extract high bits for current word
        addq    a1, 8, a1       # E :
        extql   t2, a1, t3      # U : extract low bits for next time
        addq    a0, 8, a0       # E :

        or      t0, t1, t0      # E : current dst word now complete
        EX( ldq_u t2, 0(a1) )   # L : load high word for next time
        subq    a2, 1, a2       # E :
        nop

        stq_u   t0, -8(a0)      # L : save the current word
        mov     t3, t1          # E :
        cmpbge  zero, t2, t8    # E : test new word for eos
        beq     a2, $u_eoc      # U :

        beq     t8, $u_loop     # U :
        nop
        nop
        nop

        /* We've found a zero somewhere in the source word we just read.
           If it resides in the lower half, we have one (probably partial)
           word to write out, and if it resides in the upper half, we
           have one full and one partial word left to write out.

           On entry to this basic block:
           t1 == the shifted high-order bits from the previous source word
           t2 == the unshifted current source word.  */
        .align 4
$u_eos:
        extqh   t2, a1, t0      # U :
        or      t0, t1, t0      # E : first (partial) source word complete
        cmpbge  zero, t0, t8    # E : is the null in this first bit?
        nop

        bne     t8, $u_final    # U :
        stq_u   t0, 0(a0)       # L : the null was in the high-order bits
        addq    a0, 8, a0       # E :
        subq    a2, 1, a2       # E :

        .align 4
$u_late_head_exit:
        extql   t2, a1, t0      # U :
        cmpbge  zero, t0, t8    # E :
        or      t8, t10, t6     # E :
        cmoveq  a2, t6, t8      # E :

        /* Take care of a final (probably partial) result word.
           On entry to this basic block:
           t0 == assembled source word
           t8 == cmpbge mask that found the null.  */
        .align 4
$u_final:
        negq    t8, t6          # E : isolate low bit set
        and     t6, t8, t12     # E :
        ldq_u   t1, 0(a0)       # L :
        subq    t12, 1, t6      # E :

        or      t6, t12, t8     # E :
        zapnot  t0, t8, t0      # U : kill source bytes > null
        zap     t1, t8, t1      # U : kill dest bytes <= null
        or      t0, t1, t0      # E :

        stq_u   t0, 0(a0)       # E :
        br      $finish_up      # U :
        nop
        nop

        .align 4
$u_eoc:                         # end-of-count
        extqh   t2, a1, t0      # U :
        or      t0, t1, t0      # E :
        cmpbge  zero, t0, t8    # E :
        nop

        .align 4
$u_eocfin:                      # end-of-count, final word
        or      t10, t8, t8     # E :
        br      $u_final        # U :
        nop
        nop

        /* Unaligned copy entry point.  */
        .align 4
$unaligned:

        srl     a3, 3, a2       # U : a2 = loop counter = (count - 1)/8
        and     a0, 7, t4       # E : find dest misalignment
        and     a1, 7, t5       # E : find src misalignment
        mov     zero, t0        # E :

        /* Conditionally load the first destination word and a bytemask
           with 0xff indicating that the destination byte is sacrosanct.  */

        mov     zero, t6        # E :
        beq     t4, 1f          # U :
        ldq_u   t0, 0(a0)       # L :
        lda     t6, -1          # E :

        mskql   t6, a0, t6      # E :
        nop
        nop
        nop

        .align 4
1:
        subq    a1, t4, a1      # E : sub dest misalignment from src addr
        /* If source misalignment is larger than dest misalignment, we need
           extra startup checks to avoid SEGV.  */
        cmplt   t4, t5, t12     # E :
        extql   t1, a1, t1      # U : shift src into place
        lda     t2, -1          # E : for creating masks later

        beq     t12, $u_head    # U :
        mskqh   t2, t5, t2      # U : begin src byte validity mask
        cmpbge  zero, t1, t8    # E : is there a zero?
        nop

        extql   t2, a1, t2      # U :
        or      t8, t10, t5     # E : test for end-of-count too
        cmpbge  zero, t2, t3    # E :
        cmoveq  a2, t5, t8      # E : Latency=2, extra map slot

        nop                     # E : goes with cmov
        andnot  t8, t3, t8      # E :
        beq     t8, $u_head     # U :
        nop

        /* At this point we've found a zero in the first partial word of
           the source.  We need to isolate the valid source data and mask
           it into the original destination data.  (Incidentally, we know
           that we'll need at least one byte of that original dest word.) */

        ldq_u   t0, 0(a0)       # L :
        negq    t8, t6          # E : build bitmask of bytes <= zero
        mskqh   t1, t4, t1      # U :
        and     t6, t8, t12     # E :

        subq    t12, 1, t6      # E :
        or      t6, t12, t8     # E :
        zapnot  t2, t8, t2      # U : prepare source word; mirror changes
        zapnot  t1, t8, t1      # U : to source validity mask

        andnot  t0, t2, t0      # E : zero place for source to reside
        or      t0, t1, t0      # E : and put it there
        stq_u   t0, 0(a0)       # L :
        nop

        .align 4
$finish_up:
        zapnot  t0, t12, t4     # U : was last byte written null?
        and     t12, 0xf0, t3   # E : binary search for the address of the
        cmovne  t4, 1, t4       # E : Latency=2, extra map slot
        nop                     # E : with cmovne

        and     t12, 0xcc, t2   # E : last byte written
        and     t12, 0xaa, t1   # E :
        cmovne  t3, 4, t3       # E : Latency=2, extra map slot
        nop                     # E : with cmovne

        bic     a0, 7, t0
        cmovne  t2, 2, t2       # E : Latency=2, extra map slot
        nop                     # E : with cmovne
        nop

        cmovne  t1, 1, t1       # E : Latency=2, extra map slot
        nop                     # E : with cmovne
        addq    t0, t3, t0      # E :
        addq    t1, t2, t1      # E :

        addq    t0, t1, t0      # E :
        addq    t0, t4, t0      # add one if we filled the buffer
        subq    t0, v0, v0      # find string length
        ret                     # L0 :

        .align 4
$zerolength:
        nop
        nop
        nop
        clr     v0

$exception:
        nop
        nop
        nop
        ret

        .end __strncpy_from_user