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1da177e4 LT |
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 | */ | |
18 | ||
19 | #include <asm/asmmacro.h> | |
e007c533 | 20 | #include <asm/export.h> |
1da177e4 LT |
21 | |
22 | // | |
23 | // | |
24 | // This is an enhanced version of the basic strlen. it includes a combination | |
25 | // of compute zero index (czx), parallel comparisons, speculative loads and | |
26 | // loop unroll using rotating registers. | |
27 | // | |
28 | // General Ideas about the algorithm: | |
29 | // The goal is to look at the string in chunks of 8 bytes. | |
30 | // so we need to do a few extra checks at the beginning because the | |
31 | // string may not be 8-byte aligned. In this case we load the 8byte | |
32 | // quantity which includes the start of the string and mask the unused | |
33 | // bytes with 0xff to avoid confusing czx. | |
34 | // We use speculative loads and software pipelining to hide memory | |
35 | // latency and do read ahead safely. This way we defer any exception. | |
36 | // | |
37 | // Because we don't want the kernel to be relying on particular | |
38 | // settings of the DCR register, we provide recovery code in case | |
39 | // speculation fails. The recovery code is going to "redo" the work using | |
40 | // only normal loads. If we still get a fault then we generate a | |
41 | // kernel panic. Otherwise we return the strlen as usual. | |
42 | // | |
43 | // The fact that speculation may fail can be caused, for instance, by | |
44 | // the DCR.dm bit being set. In this case TLB misses are deferred, i.e., | |
45 | // a NaT bit will be set if the translation is not present. The normal | |
46 | // load, on the other hand, will cause the translation to be inserted | |
47 | // if the mapping exists. | |
48 | // | |
49 | // It should be noted that we execute recovery code only when we need | |
50 | // to use the data that has been speculatively loaded: we don't execute | |
51 | // recovery code on pure read ahead data. | |
52 | // | |
53 | // Remarks: | |
54 | // - the cmp r0,r0 is used as a fast way to initialize a predicate | |
55 | // register to 1. This is required to make sure that we get the parallel | |
56 | // compare correct. | |
57 | // | |
58 | // - we don't use the epilogue counter to exit the loop but we need to set | |
59 | // it to zero beforehand. | |
60 | // | |
61 | // - after the loop we must test for Nat values because neither the | |
62 | // czx nor cmp instruction raise a NaT consumption fault. We must be | |
63 | // careful not to look too far for a Nat for which we don't care. | |
64 | // For instance we don't need to look at a NaT in val2 if the zero byte | |
65 | // was in val1. | |
66 | // | |
67 | // - Clearly performance tuning is required. | |
68 | // | |
69 | // | |
70 | // | |
71 | #define saved_pfs r11 | |
72 | #define tmp r10 | |
73 | #define base r16 | |
74 | #define orig r17 | |
75 | #define saved_pr r18 | |
76 | #define src r19 | |
77 | #define mask r20 | |
78 | #define val r21 | |
79 | #define val1 r22 | |
80 | #define val2 r23 | |
81 | ||
82 | GLOBAL_ENTRY(strlen) | |
83 | .prologue | |
84 | .save ar.pfs, saved_pfs | |
85 | alloc saved_pfs=ar.pfs,11,0,0,8 // rotating must be multiple of 8 | |
86 | ||
87 | .rotr v[2], w[2] // declares our 4 aliases | |
88 | ||
89 | extr.u tmp=in0,0,3 // tmp=least significant 3 bits | |
90 | mov orig=in0 // keep trackof initial byte address | |
91 | dep src=0,in0,0,3 // src=8byte-aligned in0 address | |
92 | .save pr, saved_pr | |
93 | mov saved_pr=pr // preserve predicates (rotation) | |
94 | ;; | |
95 | ||
96 | .body | |
97 | ||
98 | ld8 v[1]=[src],8 // must not speculate: can fail here | |
99 | shl tmp=tmp,3 // multiply by 8bits/byte | |
100 | mov mask=-1 // our mask | |
101 | ;; | |
102 | ld8.s w[1]=[src],8 // speculatively load next | |
103 | cmp.eq p6,p0=r0,r0 // sets p6 to true for cmp.and | |
104 | sub tmp=64,tmp // how many bits to shift our mask on the right | |
105 | ;; | |
106 | shr.u mask=mask,tmp // zero enough bits to hold v[1] valuable part | |
107 | mov ar.ec=r0 // clear epilogue counter (saved in ar.pfs) | |
108 | ;; | |
109 | add base=-16,src // keep track of aligned base | |
110 | or v[1]=v[1],mask // now we have a safe initial byte pattern | |
111 | ;; | |
112 | 1: | |
113 | ld8.s v[0]=[src],8 // speculatively load next | |
114 | czx1.r val1=v[1] // search 0 byte from right | |
115 | czx1.r val2=w[1] // search 0 byte from right following 8bytes | |
116 | ;; | |
117 | ld8.s w[0]=[src],8 // speculatively load next to next | |
118 | cmp.eq.and p6,p0=8,val1 // p6 = p6 and val1==8 | |
119 | cmp.eq.and p6,p0=8,val2 // p6 = p6 and mask==8 | |
120 | (p6) br.wtop.dptk 1b // loop until p6 == 0 | |
121 | ;; | |
122 | // | |
123 | // We must return try the recovery code iff | |
124 | // val1_is_nat || (val1==8 && val2_is_nat) | |
125 | // | |
126 | // XXX Fixme | |
127 | // - there must be a better way of doing the test | |
128 | // | |
129 | cmp.eq p8,p9=8,val1 // p6 = val1 had zero (disambiguate) | |
130 | tnat.nz p6,p7=val1 // test NaT on val1 | |
131 | (p6) br.cond.spnt .recover // jump to recovery if val1 is NaT | |
132 | ;; | |
133 | // | |
134 | // if we come here p7 is true, i.e., initialized for // cmp | |
135 | // | |
136 | cmp.eq.and p7,p0=8,val1// val1==8? | |
137 | tnat.nz.and p7,p0=val2 // test NaT if val2 | |
138 | (p7) br.cond.spnt .recover // jump to recovery if val2 is NaT | |
139 | ;; | |
140 | (p8) mov val1=val2 // the other test got us out of the loop | |
141 | (p8) adds src=-16,src // correct position when 3 ahead | |
142 | (p9) adds src=-24,src // correct position when 4 ahead | |
143 | ;; | |
144 | sub ret0=src,orig // distance from base | |
145 | sub tmp=8,val1 // which byte in word | |
146 | mov pr=saved_pr,0xffffffffffff0000 | |
147 | ;; | |
148 | sub ret0=ret0,tmp // adjust | |
149 | mov ar.pfs=saved_pfs // because of ar.ec, restore no matter what | |
150 | br.ret.sptk.many rp // end of normal execution | |
151 | ||
152 | // | |
153 | // Outlined recovery code when speculation failed | |
154 | // | |
155 | // This time we don't use speculation and rely on the normal exception | |
156 | // mechanism. that's why the loop is not as good as the previous one | |
157 | // because read ahead is not possible | |
158 | // | |
159 | // IMPORTANT: | |
160 | // Please note that in the case of strlen() as opposed to strlen_user() | |
161 | // we don't use the exception mechanism, as this function is not | |
162 | // supposed to fail. If that happens it means we have a bug and the | |
163 | // code will cause of kernel fault. | |
164 | // | |
165 | // XXX Fixme | |
166 | // - today we restart from the beginning of the string instead | |
167 | // of trying to continue where we left off. | |
168 | // | |
169 | .recover: | |
170 | ld8 val=[base],8 // will fail if unrecoverable fault | |
171 | ;; | |
172 | or val=val,mask // remask first bytes | |
173 | cmp.eq p0,p6=r0,r0 // nullify first ld8 in loop | |
174 | ;; | |
175 | // | |
176 | // ar.ec is still zero here | |
177 | // | |
178 | 2: | |
179 | (p6) ld8 val=[base],8 // will fail if unrecoverable fault | |
180 | ;; | |
181 | czx1.r val1=val // search 0 byte from right | |
182 | ;; | |
183 | cmp.eq p6,p0=8,val1 // val1==8 ? | |
184 | (p6) br.wtop.dptk 2b // loop until p6 == 0 | |
185 | ;; // (avoid WAW on p63) | |
186 | sub ret0=base,orig // distance from base | |
187 | sub tmp=8,val1 | |
188 | mov pr=saved_pr,0xffffffffffff0000 | |
189 | ;; | |
190 | sub ret0=ret0,tmp // length=now - back -1 | |
191 | mov ar.pfs=saved_pfs // because of ar.ec, restore no matter what | |
192 | br.ret.sptk.many rp // end of successful recovery code | |
193 | END(strlen) | |
e007c533 | 194 | EXPORT_SYMBOL(strlen) |