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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> |
20 | |
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 |
80 | |
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 |
85 | |
86 | .rotr v[2], w[2] // declares our 4 aliases |
87 | |
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 | ;; |
94 | |
95 | .body |
96 | |
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 |
150 | |
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) |
193 |
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