Annotation of sys/arch/mvme88k/stand/sboot/oc_cksum.S, Revision 1.1.1.1
1.1 nbrk 1: | $OpenBSD: oc_cksum.S,v 1.3 2006/05/16 22:52:26 miod 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 ones 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) does not 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 do not 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 allocator, the data we are called with is almost
61: | always longword aligned, there is no benefit to this added cost
62: | and we are better off letting the loop take a big performance hit
63: | in the rare cases where we are 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 is clear why this happens for a count of 40: unroll-8 pays a
72: | loop branch cost and unroll-16 does not. But the tests also showed
73: | that 16 was better than 8 for a count of 20. It is 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 <machine/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 are 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 does not affect X)
156:
157: movl d0,d1 | fold 32 bit sum to 16 bits
158: swap d1 | (NB- swap does not 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
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