oc_cksum.s revision 1.1
1 | $NetBSD: oc_cksum.s,v 1.1 1995/07/25 23:12:31 chuck Exp $ 2 3 | Copyright (c) 1988 Regents of the University of California. 4 | All rights reserved. 5 | 6 | Redistribution and use in source and binary forms, with or without 7 | modification, are permitted provided that the following conditions 8 | are met: 9 | 1. Redistributions of source code must retain the above copyright 10 | notice, this list of conditions and the following disclaimer. 11 | 2. Redistributions in binary form must reproduce the above copyright 12 | notice, this list of conditions and the following disclaimer in the 13 | documentation and/or other materials provided with the distribution. 14 | 3. All advertising materials mentioning features or use of this software 15 | must display the following acknowledgement: 16 | This product includes software developed by the University of 17 | California, Berkeley and its contributors. 18 | 4. Neither the name of the University nor the names of its contributors 19 | may be used to endorse or promote products derived from this software 20 | without specific prior written permission. 21 | 22 | THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 23 | ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 24 | IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 25 | ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 26 | FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 27 | DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 28 | OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 29 | HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 30 | LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 31 | OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 32 | SUCH DAMAGE. 33 | 34 | @(#)oc_cksum.s 7.2 (Berkeley) 11/3/90 35 | 36 | 37 | oc_cksum: ones complement 16 bit checksum for MC68020. 38 | 39 | oc_cksum (buffer, count, strtval) 40 | 41 | Do a 16 bit one's complement sum of 'count' bytes from 'buffer'. 42 | 'strtval' is the starting value of the sum (usually zero). 43 | 44 | It simplifies life in in_cksum if strtval can be >= 2^16. 45 | This routine will work as long as strtval is < 2^31. 46 | 47 | Performance 48 | ----------- 49 | This routine is intended for MC 68020s but should also work 50 | for 68030s. It (deliberately) doesn't worry about the alignment 51 | of the buffer so will only work on a 68010 if the buffer is 52 | aligned on an even address. (Also, a routine written to use 53 | 68010 "loop mode" would almost certainly be faster than this 54 | code on a 68010). 55 | 56 | We don't worry about alignment because this routine is frequently 57 | called with small counts: 20 bytes for IP header checksums and 40 58 | bytes for TCP ack checksums. For these small counts, testing for 59 | bad alignment adds ~10% to the per-call cost. Since, by the nature 60 | of the kernel's allocator, the data we're called with is almost 61 | always longword aligned, there is no benefit to this added cost 62 | and we're better off letting the loop take a big performance hit 63 | in the rare cases where we're handed an unaligned buffer. 64 | 65 | Loop unrolling constants of 2, 4, 8, 16, 32 and 64 times were 66 | tested on random data on four different types of processors (see 67 | list below -- 64 was the largest unrolling because anything more 68 | overflows the 68020 Icache). On all the processors, the 69 | throughput asymptote was located between 8 and 16 (closer to 8). 70 | However, 16 was substantially better than 8 for small counts. 71 | (It's clear why this happens for a count of 40: unroll-8 pays a 72 | loop branch cost and unroll-16 doesn't. But the tests also showed 73 | that 16 was better than 8 for a count of 20. It's not obvious to 74 | me why.) So, since 16 was good for both large and small counts, 75 | the loop below is unrolled 16 times. 76 | 77 | The processors tested and their average time to checksum 1024 bytes 78 | of random data were: 79 | Sun 3/50 (15MHz) 190 us/KB 80 | Sun 3/180 (16.6MHz) 175 us/KB 81 | Sun 3/60 (20MHz) 134 us/KB 82 | Sun 3/280 (25MHz) 95 us/KB 83 | 84 | The cost of calling this routine was typically 10% of the per- 85 | kilobyte cost. E.g., checksumming zero bytes on a 3/60 cost 9us 86 | and each additional byte cost 125ns. With the high fixed cost, 87 | it would clearly be a gain to "inline" this routine -- the 88 | subroutine call adds 400% overhead to an IP header checksum. 89 | However, in absolute terms, inlining would only gain 10us per 90 | packet -- a 1% effect for a 1ms ethernet packet. This is not 91 | enough gain to be worth the effort. 92 93 #include <m68k/asm.h> 94 95 .text 96 97 .text; .even; .globl _oc_cksum; _oc_cksum: 98 movl sp@(4),a0 | get buffer ptr 99 movl sp@(8),d1 | get byte count 100 movl sp@(12),d0 | get starting value 101 movl d2,sp@- | free a reg 102 103 | test for possible 1, 2 or 3 bytes of excess at end 104 | of buffer. The usual case is no excess (the usual 105 | case is header checksums) so we give that the faster 106 | 'not taken' leg of the compare. (We do the excess 107 | first because we're about the trash the low order 108 | bits of the count in d1.) 109 110 btst #0,d1 111 jne L5 | if one or three bytes excess 112 btst #1,d1 113 jne L7 | if two bytes excess 114 L1: 115 movl d1,d2 116 lsrl #6,d1 | make cnt into # of 64 byte chunks 117 andl #0x3c,d2 | then find fractions of a chunk 118 negl d2 119 andb #0xf,cc | clear X 120 jmp pc@(L3-.-2:b,d2) 121 L2: 122 movl a0@+,d2 123 addxl d2,d0 124 movl a0@+,d2 125 addxl d2,d0 126 movl a0@+,d2 127 addxl d2,d0 128 movl a0@+,d2 129 addxl d2,d0 130 movl a0@+,d2 131 addxl d2,d0 132 movl a0@+,d2 133 addxl d2,d0 134 movl a0@+,d2 135 addxl d2,d0 136 movl a0@+,d2 137 addxl d2,d0 138 movl a0@+,d2 139 addxl d2,d0 140 movl a0@+,d2 141 addxl d2,d0 142 movl a0@+,d2 143 addxl d2,d0 144 movl a0@+,d2 145 addxl d2,d0 146 movl a0@+,d2 147 addxl d2,d0 148 movl a0@+,d2 149 addxl d2,d0 150 movl a0@+,d2 151 addxl d2,d0 152 movl a0@+,d2 153 addxl d2,d0 154 L3: 155 dbra d1,L2 | (NB- dbra doesn't affect X) 156 157 movl d0,d1 | fold 32 bit sum to 16 bits 158 swap d1 | (NB- swap doesn't affect X) 159 addxw d1,d0 160 jcc L4 161 addw #1,d0 162 L4: 163 andl #0xffff,d0 164 movl sp@+,d2 165 rts 166 167 L5: | deal with 1 or 3 excess bytes at the end of the buffer. 168 btst #1,d1 169 jeq L6 | if 1 excess 170 171 | 3 bytes excess 172 clrl d2 173 movw a0@(-3,d1:l),d2 | add in last full word then drop 174 addl d2,d0 | through to pick up last byte 175 176 L6: | 1 byte excess 177 clrl d2 178 movb a0@(-1,d1:l),d2 179 lsll #8,d2 180 addl d2,d0 181 jra L1 182 183 L7: | 2 bytes excess 184 clrl d2 185 movw a0@(-2,d1:l),d2 186 addl d2,d0 187 jra L1 188
Indexes created Sat Oct 11 14:09:46 GMT 2025