ia64/xen-unstable

view xen/arch/ia64/linux/strlen.S @ 10888:5379548bfc79

[NET] Enable TCPv4 segmentation offload in front/back drivers.
Signed-off-by: Keir Fraser <keir@xensource.com>
author kfraser@localhost.localdomain
date Tue Aug 01 11:54:45 2006 +0100 (2006-08-01)
parents 3ca4ca7a9cc2
children
line source
1 /*
2 *
3 * Optimized version of the standard strlen() function
4 *
5 *
6 * Inputs:
7 * in0 address of string
8 *
9 * Outputs:
10 * ret0 the number of characters in the string (0 if empty string)
11 * does not count the \0
12 *
13 * Copyright (C) 1999, 2001 Hewlett-Packard Co
14 * Stephane Eranian <eranian@hpl.hp.com>
15 *
16 * 09/24/99 S.Eranian add speculation recovery code
17 */
19 #include <asm/asmmacro.h>
21 //
22 //
23 // This is an enhanced version of the basic strlen. it includes a combination
24 // of compute zero index (czx), parallel comparisons, speculative loads and
25 // loop unroll using rotating registers.
26 //
27 // General Ideas about the algorithm:
28 // The goal is to look at the string in chunks of 8 bytes.
29 // so we need to do a few extra checks at the beginning because the
30 // string may not be 8-byte aligned. In this case we load the 8byte
31 // quantity which includes the start of the string and mask the unused
32 // bytes with 0xff to avoid confusing czx.
33 // We use speculative loads and software pipelining to hide memory
34 // latency and do read ahead safely. This way we defer any exception.
35 //
36 // Because we don't want the kernel to be relying on particular
37 // settings of the DCR register, we provide recovery code in case
38 // speculation fails. The recovery code is going to "redo" the work using
39 // only normal loads. If we still get a fault then we generate a
40 // kernel panic. Otherwise we return the strlen as usual.
41 //
42 // The fact that speculation may fail can be caused, for instance, by
43 // the DCR.dm bit being set. In this case TLB misses are deferred, i.e.,
44 // a NaT bit will be set if the translation is not present. The normal
45 // load, on the other hand, will cause the translation to be inserted
46 // if the mapping exists.
47 //
48 // It should be noted that we execute recovery code only when we need
49 // to use the data that has been speculatively loaded: we don't execute
50 // recovery code on pure read ahead data.
51 //
52 // Remarks:
53 // - the cmp r0,r0 is used as a fast way to initialize a predicate
54 // register to 1. This is required to make sure that we get the parallel
55 // compare correct.
56 //
57 // - we don't use the epilogue counter to exit the loop but we need to set
58 // it to zero beforehand.
59 //
60 // - after the loop we must test for Nat values because neither the
61 // czx nor cmp instruction raise a NaT consumption fault. We must be
62 // careful not to look too far for a Nat for which we don't care.
63 // For instance we don't need to look at a NaT in val2 if the zero byte
64 // was in val1.
65 //
66 // - Clearly performance tuning is required.
67 //
68 //
69 //
70 #define saved_pfs r11
71 #define tmp r10
72 #define base r16
73 #define orig r17
74 #define saved_pr r18
75 #define src r19
76 #define mask r20
77 #define val r21
78 #define val1 r22
79 #define val2 r23
81 GLOBAL_ENTRY(strlen)
82 .prologue
83 .save ar.pfs, saved_pfs
84 alloc saved_pfs=ar.pfs,11,0,0,8 // rotating must be multiple of 8
86 .rotr v[2], w[2] // declares our 4 aliases
88 extr.u tmp=in0,0,3 // tmp=least significant 3 bits
89 mov orig=in0 // keep trackof initial byte address
90 dep src=0,in0,0,3 // src=8byte-aligned in0 address
91 .save pr, saved_pr
92 mov saved_pr=pr // preserve predicates (rotation)
93 ;;
95 .body
97 ld8 v[1]=[src],8 // must not speculate: can fail here
98 shl tmp=tmp,3 // multiply by 8bits/byte
99 mov mask=-1 // our mask
100 ;;
101 ld8.s w[1]=[src],8 // speculatively load next
102 cmp.eq p6,p0=r0,r0 // sets p6 to true for cmp.and
103 sub tmp=64,tmp // how many bits to shift our mask on the right
104 ;;
105 shr.u mask=mask,tmp // zero enough bits to hold v[1] valuable part
106 mov ar.ec=r0 // clear epilogue counter (saved in ar.pfs)
107 ;;
108 add base=-16,src // keep track of aligned base
109 or v[1]=v[1],mask // now we have a safe initial byte pattern
110 ;;
111 1:
112 ld8.s v[0]=[src],8 // speculatively load next
113 czx1.r val1=v[1] // search 0 byte from right
114 czx1.r val2=w[1] // search 0 byte from right following 8bytes
115 ;;
116 ld8.s w[0]=[src],8 // speculatively load next to next
117 cmp.eq.and p6,p0=8,val1 // p6 = p6 and val1==8
118 cmp.eq.and p6,p0=8,val2 // p6 = p6 and mask==8
119 (p6) br.wtop.dptk 1b // loop until p6 == 0
120 ;;
121 //
122 // We must return try the recovery code iff
123 // val1_is_nat || (val1==8 && val2_is_nat)
124 //
125 // XXX Fixme
126 // - there must be a better way of doing the test
127 //
128 cmp.eq p8,p9=8,val1 // p6 = val1 had zero (disambiguate)
129 tnat.nz p6,p7=val1 // test NaT on val1
130 (p6) br.cond.spnt .recover // jump to recovery if val1 is NaT
131 ;;
132 //
133 // if we come here p7 is true, i.e., initialized for // cmp
134 //
135 cmp.eq.and p7,p0=8,val1// val1==8?
136 tnat.nz.and p7,p0=val2 // test NaT if val2
137 (p7) br.cond.spnt .recover // jump to recovery if val2 is NaT
138 ;;
139 (p8) mov val1=val2 // the other test got us out of the loop
140 (p8) adds src=-16,src // correct position when 3 ahead
141 (p9) adds src=-24,src // correct position when 4 ahead
142 ;;
143 sub ret0=src,orig // distance from base
144 sub tmp=8,val1 // which byte in word
145 mov pr=saved_pr,0xffffffffffff0000
146 ;;
147 sub ret0=ret0,tmp // adjust
148 mov ar.pfs=saved_pfs // because of ar.ec, restore no matter what
149 br.ret.sptk.many rp // end of normal execution
151 //
152 // Outlined recovery code when speculation failed
153 //
154 // This time we don't use speculation and rely on the normal exception
155 // mechanism. that's why the loop is not as good as the previous one
156 // because read ahead is not possible
157 //
158 // IMPORTANT:
159 // Please note that in the case of strlen() as opposed to strlen_user()
160 // we don't use the exception mechanism, as this function is not
161 // supposed to fail. If that happens it means we have a bug and the
162 // code will cause of kernel fault.
163 //
164 // XXX Fixme
165 // - today we restart from the beginning of the string instead
166 // of trying to continue where we left off.
167 //
168 .recover:
169 ld8 val=[base],8 // will fail if unrecoverable fault
170 ;;
171 or val=val,mask // remask first bytes
172 cmp.eq p0,p6=r0,r0 // nullify first ld8 in loop
173 ;;
174 //
175 // ar.ec is still zero here
176 //
177 2:
178 (p6) ld8 val=[base],8 // will fail if unrecoverable fault
179 ;;
180 czx1.r val1=val // search 0 byte from right
181 ;;
182 cmp.eq p6,p0=8,val1 // val1==8 ?
183 (p6) br.wtop.dptk 2b // loop until p6 == 0
184 ;; // (avoid WAW on p63)
185 sub ret0=base,orig // distance from base
186 sub tmp=8,val1
187 mov pr=saved_pr,0xffffffffffff0000
188 ;;
189 sub ret0=ret0,tmp // length=now - back -1
190 mov ar.pfs=saved_pfs // because of ar.ec, restore no matter what
191 br.ret.sptk.many rp // end of successful recovery code
192 END(strlen)