/* 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
Submit an article or tip