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/* match.s -- Pentium-optimized version of longest_match()
 * Written for zlib 1.1.2
 * Copyright (C) 1998 Brian Raiter <breadbox@muppetlabs.com>
 *
 * This is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License.
 */

#ifndef NO_UNDERLINE
#define match_init      _match_init
#define longest_match   _longest_match
#endif

#define MAX_MATCH       (258)
#define MIN_MATCH       (3)
#define MIN_LOOKAHEAD   (MAX_MATCH + MIN_MATCH + 1)
#define MAX_MATCH_8     ((MAX_MATCH + 7) & ~7)

/* stack frame offsets */

#define wmask                   0        /* local copy of s->wmask       */
#define window                  4       /* local copy of s->window      */
#define windowbestlen           8       /* s->window + bestlen          */
#define chainlenscanend         12      /* high word: current chain len */
                                        /* low word: last bytes sought  */
#define scanstart               16      /* first two bytes of string    */
#define scanalign               20      /* dword-misalignment of string */
#define nicematch               24      /* a good enough match size     */
#define bestlen                 28      /* size of best match so far    */
#define scan                    32      /* ptr to string wanting match  */

#define LocalVarsSize           (36)
/*      saved ebx               36 */
/*      saved edi               40 */
/*      saved esi               44 */
/*      saved ebp               48 */
/*      return address          52 */
#define deflatestate            56      /* the function arguments       */
#define curmatch                60

/* Offsets for fields in the deflate_state structure. These numbers
 * are calculated from the definition of deflate_state, with the
 * assumption that the compiler will dword-align the fields. (Thus,
 * changing the definition of deflate_state could easily cause this
 * program to crash horribly, without so much as a warning at
 * compile time. Sigh.)
 */

/* All the +zlib1222add offsets are due to the addition of fields
 *  in zlib in the deflate_state structure since the asm code was first written
 * (if you compile with zlib 1.0.4 or older, use "zlib1222add equ (-4)").
 * (if you compile with zlib between 1.0.5 and 1.2.2.1, use "zlib1222add equ 0").
 * if you compile with zlib 1.2.2.2 or later , use "zlib1222add equ 8").
 */

#define zlib1222add             (8)

#define dsWSize                 (36+zlib1222add)
#define dsWMask                 (44+zlib1222add)
#define dsWindow                (48+zlib1222add)
#define dsPrev                  (56+zlib1222add)
#define dsMatchLen              (88+zlib1222add)
#define dsPrevMatch             (92+zlib1222add)
#define dsStrStart              (100+zlib1222add)
#define dsMatchStart            (104+zlib1222add)
#define dsLookahead             (108+zlib1222add)
#define dsPrevLen               (112+zlib1222add)
#define dsMaxChainLen           (116+zlib1222add)
#define dsGoodMatch             (132+zlib1222add)
#define dsNiceMatch             (136+zlib1222add)


.file "match.S"

.globl  match_init, longest_match

.text

/* uInt longest_match(deflate_state *deflatestate, IPos curmatch) */

longest_match:

/* Save registers that the compiler may be using, and adjust %esp to    */
/* make room for our stack frame.                                       */

                pushl   %ebp
                pushl   %edi
                pushl   %esi
                pushl   %ebx
                subl    $LocalVarsSize, %esp

/* Retrieve the function arguments. %ecx will hold cur_match            */
/* throughout the entire function. %edx will hold the pointer to the    */
/* deflate_state structure during the function's setup (before          */
/* entering the main loop).                                             */

                movl    deflatestate(%esp), %edx
                movl    curmatch(%esp), %ecx

/* if ((uInt)nice_match > s->lookahead) nice_match = s->lookahead;      */

                movl    dsNiceMatch(%edx), %eax
                movl    dsLookahead(%edx), %ebx
                cmpl    %eax, %ebx
                jl      LookaheadLess
                movl    %eax, %ebx
LookaheadLess:  movl    %ebx, nicematch(%esp)

/* register Bytef *scan = s->window + s->strstart;                      */

                movl    dsWindow(%edx), %esi
                movl    %esi, window(%esp)
                movl    dsStrStart(%edx), %ebp
                lea     (%esi,%ebp), %edi
                movl    %edi, scan(%esp)

/* Determine how many bytes the scan ptr is off from being              */
/* dword-aligned.                                                       */

                movl    %edi, %eax
                negl    %eax
                andl    $3, %eax
                movl    %eax, scanalign(%esp)

/* IPos limit = s->strstart > (IPos)MAX_DIST(s) ?                       */
/*     s->strstart - (IPos)MAX_DIST(s) : NIL;                           */

                movl    dsWSize(%edx), %eax
                subl    $MIN_LOOKAHEAD, %eax
                subl    %eax, %ebp
                jg      LimitPositive
                xorl    %ebp, %ebp
LimitPositive:

/* unsigned chain_length = s->max_chain_length;                         */
/* if (s->prev_length >= s->good_match) {                               */
/*     chain_length >>= 2;                                              */
/* }                                                                    */

                movl    dsPrevLen(%edx), %eax
                movl    dsGoodMatch(%edx), %ebx
                cmpl    %ebx, %eax
                movl    dsMaxChainLen(%edx), %ebx
                jl      LastMatchGood
                shrl    $2, %ebx
LastMatchGood:

/* chainlen is decremented once beforehand so that the function can     */
/* use the sign flag instead of the zero flag for the exit test.        */
/* It is then shifted into the high word, to make room for the scanend  */
/* scanend value, which it will always accompany.                       */

                decl    %ebx
                shll    $16, %ebx

/* int best_len = s->prev_length;                                       */

                movl    dsPrevLen(%edx), %eax
                movl    %eax, bestlen(%esp)

/* Store the sum of s->window + best_len in %esi locally, and in %esi.  */

                addl    %eax, %esi
                movl    %esi, windowbestlen(%esp)

/* register ush scan_start = *(ushf*)scan;                              */
/* register ush scan_end   = *(ushf*)(scan+best_len-1);                 */

                movw    (%edi), %bx
                movw    %bx, scanstart(%esp)
                movw    -1(%edi,%eax), %bx
                movl    %ebx, chainlenscanend(%esp)

/* Posf *prev = s->prev;                                                */
/* uInt wmask = s->w_mask;                                              */

                movl    dsPrev(%edx), %edi
                movl    dsWMask(%edx), %edx
                mov     %edx, wmask(%esp)

/* Jump into the main loop.                                             */

                jmp     LoopEntry

.balign 16

/* do {
 *     match = s->window + cur_match;
 *     if (*(ushf*)(match+best_len-1) != scan_end ||
 *         *(ushf*)match != scan_start) continue;
 *     [...]
 * } while ((cur_match = prev[cur_match & wmask]) > limit
 *          && --chain_length != 0);
 *
 * Here is the inner loop of the function. The function will spend the
 * majority of its time in this loop, and majority of that time will
 * be spent in the first ten instructions.
 *
 * Within this loop:
 * %ebx = chainlenscanend - i.e., ((chainlen << 16) | scanend)
 * %ecx = curmatch
 * %edx = curmatch & wmask
 * %esi = windowbestlen - i.e., (window + bestlen)
 * %edi = prev
 * %ebp = limit
 *
 * Two optimization notes on the choice of instructions:
 *
 * The first instruction uses a 16-bit address, which costs an extra,
 * unpairable cycle. This is cheaper than doing a 32-bit access and
 * zeroing the high word, due to the 3-cycle misalignment penalty which
 * would occur half the time. This also turns out to be cheaper than
 * doing two separate 8-bit accesses, as the memory is so rarely in the
 * L1 cache.
 *
 * The window buffer, however, apparently spends a lot of time in the
 * cache, and so it is faster to retrieve the word at the end of the
 * match string with two 8-bit loads. The instructions that test the
 * word at the beginning of the match string, however, are executed
 * much less frequently, and there it was cheaper to use 16-bit
 * instructions, which avoided the necessity of saving off and
 * subsequently reloading one of the other registers.
 */
LookupLoop:
                                                        /* 1 U & V  */
                movw    (%edi,%edx,2), %cx              /* 2 U pipe */
                movl    wmask(%esp), %edx               /* 2 V pipe */
                cmpl    %ebp, %ecx                      /* 3 U pipe */
                jbe     LeaveNow                        /* 3 V pipe */
                subl    $0x00010000, %ebx               /* 4 U pipe */
                js      LeaveNow                        /* 4 V pipe */
LoopEntry:      movb    -1(%esi,%ecx), %al              /* 5 U pipe */
                andl    %ecx, %edx                      /* 5 V pipe */
                cmpb    %bl, %al                        /* 6 U pipe */
                jnz     LookupLoop                      /* 6 V pipe */
                movb    (%esi,%ecx), %ah
                cmpb    %bh, %ah
                jnz     LookupLoop
                movl    window(%esp), %eax
                movw    (%eax,%ecx), %ax
                cmpw    scanstart(%esp), %ax
                jnz     LookupLoop

/* Store the current value of chainlen.                                 */

                movl    %ebx, chainlenscanend(%esp)

/* Point %edi to the string under scrutiny, and %esi to the string we   */
/* are hoping to match it up with. In actuality, %esi and %edi are      */
/* both pointed (MAX_MATCH_8 - scanalign) bytes ahead, and %edx is      */
/* initialized to -(MAX_MATCH_8 - scanalign).                           */

                movl    window(%esp), %esi
                movl    scan(%esp), %edi
                addl    %ecx, %esi
                movl    scanalign(%esp), %eax
                movl    $(-MAX_MATCH_8), %edx
                lea     MAX_MATCH_8(%edi,%eax), %edi
                lea     MAX_MATCH_8(%esi,%eax), %esi

/* Test the strings for equality, 8 bytes at a time. At the end,
 * adjust %edx so that it is offset to the exact byte that mismatched.
 *
 * We already know at this point that the first three bytes of the
 * strings match each other, and they can be safely passed over before
 * starting the compare loop. So what this code does is skip over 0-3
 * bytes, as much as necessary in order to dword-align the %edi
 * pointer. (%esi will still be misaligned three times out of four.)
 *
 * It should be confessed that this loop usually does not represent
 * much of the total running time. Replacing it with a more
 * straightforward "rep cmpsb" would not drastically degrade
 * performance.
 */
LoopCmps:
                movl    (%esi,%edx), %eax
                movl    (%edi,%edx), %ebx
                xorl    %ebx, %eax
                jnz     LeaveLoopCmps
                movl    4(%esi,%edx), %eax
                movl    4(%edi,%edx), %ebx
                xorl    %ebx, %eax
                jnz     LeaveLoopCmps4
                addl    $8, %edx
                jnz     LoopCmps
                jmp     LenMaximum
LeaveLoopCmps4: addl    $4, %edx
LeaveLoopCmps:  testl   $0x0000FFFF, %eax
                jnz     LenLower
                addl    $2, %edx
                shrl    $16, %eax
LenLower:       subb    $1, %al
                adcl    $0, %edx

/* Calculate the length of the match. If it is longer than MAX_MATCH,   */
/* then automatically accept it as the best possible match and leave.   */

                lea     (%edi,%edx), %eax
                movl    scan(%esp), %edi
                subl    %edi, %eax
                cmpl    $MAX_MATCH, %eax
                jge     LenMaximum

/* If the length of the match is not longer than the best match we      */
/* have so far, then forget it and return to the lookup loop.           */

                movl    deflatestate(%esp), %edx
                movl    bestlen(%esp), %ebx
                cmpl    %ebx, %eax
                jg      LongerMatch
                movl    chainlenscanend(%esp), %ebx
                movl    windowbestlen(%esp), %esi
                movl    dsPrev(%edx), %edi
                movl    wmask(%esp), %edx
                andl    %ecx, %edx
                jmp     LookupLoop

/*         s->match_start = cur_match;                                  */
/*         best_len = len;                                              */
/*         if (len >= nice_match) break;                                */
/*         scan_end = *(ushf*)(scan+best_len-1);                        */

LongerMatch:    movl    nicematch(%esp), %ebx
                movl    %eax, bestlen(%esp)
                movl    %ecx, dsMatchStart(%edx)
                cmpl    %ebx, %eax
                jge     LeaveNow
                movl    window(%esp), %esi
                addl    %eax, %esi
                movl    %esi, windowbestlen(%esp)
                movl    chainlenscanend(%esp), %ebx
                movw    -1(%edi,%eax), %bx
                movl    dsPrev(%edx), %edi
                movl    %ebx, chainlenscanend(%esp)
                movl    wmask(%esp), %edx
                andl    %ecx, %edx
                jmp     LookupLoop

/* Accept the current string, with the maximum possible length.         */

LenMaximum:     movl    deflatestate(%esp), %edx
                movl    $MAX_MATCH, bestlen(%esp)
                movl    %ecx, dsMatchStart(%edx)

/* if ((uInt)best_len <= s->lookahead) return (uInt)best_len;           */
/* return s->lookahead;                                                 */

LeaveNow:
                movl    deflatestate(%esp), %edx
                movl    bestlen(%esp), %ebx
                movl    dsLookahead(%edx), %eax
                cmpl    %eax, %ebx
                jg      LookaheadRet
                movl    %ebx, %eax
LookaheadRet:

/* Restore the stack and return from whence we came.                    */

                addl    $LocalVarsSize, %esp
                popl    %ebx
                popl    %esi
                popl    %edi
                popl    %ebp
match_init:     ret