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1 | /* |
2 | * Optimized version of the strlen_user() function |
3 | * |
4 | * Inputs: |
5 | * in0 address of buffer |
6 | * |
7 | * Outputs: |
8 | * ret0 0 in case of fault, strlen(buffer)+1 otherwise |
9 | * |
10 | * Copyright (C) 1998, 1999, 2001 Hewlett-Packard Co |
11 | * David Mosberger-Tang <davidm@hpl.hp.com> |
12 | * Stephane Eranian <eranian@hpl.hp.com> |
13 | * |
14 | * 01/19/99 S.Eranian heavily enhanced version (see details below) |
15 | * 09/24/99 S.Eranian added speculation recovery code |
16 | */ |
17 | |
18 | #include <asm/asmmacro.h> |
19 | |
20 | // |
21 | // int strlen_user(char *) |
22 | // ------------------------ |
23 | // Returns: |
24 | // - length of string + 1 |
25 | // - 0 in case an exception is raised |
26 | // |
27 | // This is an enhanced version of the basic strlen_user. it includes a |
28 | // combination of compute zero index (czx), parallel comparisons, speculative |
29 | // loads and loop unroll using rotating registers. |
30 | // |
31 | // General Ideas about the algorithm: |
32 | // The goal is to look at the string in chunks of 8 bytes. |
33 | // so we need to do a few extra checks at the beginning because the |
34 | // string may not be 8-byte aligned. In this case we load the 8byte |
35 | // quantity which includes the start of the string and mask the unused |
36 | // bytes with 0xff to avoid confusing czx. |
37 | // We use speculative loads and software pipelining to hide memory |
38 | // latency and do read ahead safely. This way we defer any exception. |
39 | // |
40 | // Because we don't want the kernel to be relying on particular |
41 | // settings of the DCR register, we provide recovery code in case |
42 | // speculation fails. The recovery code is going to "redo" the work using |
43 | // only normal loads. If we still get a fault then we return an |
44 | // error (ret0=0). Otherwise we return the strlen+1 as usual. |
45 | // The fact that speculation may fail can be caused, for instance, by |
46 | // the DCR.dm bit being set. In this case TLB misses are deferred, i.e., |
47 | // a NaT bit will be set if the translation is not present. The normal |
48 | // load, on the other hand, will cause the translation to be inserted |
49 | // if the mapping exists. |
50 | // |
51 | // It should be noted that we execute recovery code only when we need |
52 | // to use the data that has been speculatively loaded: we don't execute |
53 | // recovery code on pure read ahead data. |
54 | // |
55 | // Remarks: |
56 | // - the cmp r0,r0 is used as a fast way to initialize a predicate |
57 | // register to 1. This is required to make sure that we get the parallel |
58 | // compare correct. |
59 | // |
60 | // - we don't use the epilogue counter to exit the loop but we need to set |
61 | // it to zero beforehand. |
62 | // |
63 | // - after the loop we must test for Nat values because neither the |
64 | // czx nor cmp instruction raise a NaT consumption fault. We must be |
65 | // careful not to look too far for a Nat for which we don't care. |
66 | // For instance we don't need to look at a NaT in val2 if the zero byte |
67 | // was in val1. |
68 | // |
69 | // - Clearly performance tuning is required. |
70 | // |
71 | |
72 | #define saved_pfs r11 |
73 | #define tmp r10 |
74 | #define base r16 |
75 | #define orig r17 |
76 | #define saved_pr r18 |
77 | #define src r19 |
78 | #define mask r20 |
79 | #define val r21 |
80 | #define val1 r22 |
81 | #define val2 r23 |
82 | |
83 | GLOBAL_ENTRY(__strlen_user) |
84 | .prologue |
85 | .save ar.pfs, saved_pfs |
86 | alloc saved_pfs=ar.pfs,11,0,0,8 |
87 | |
88 | .rotr v[2], w[2] // declares our 4 aliases |
89 | |
90 | extr.u tmp=in0,0,3 // tmp=least significant 3 bits |
91 | mov orig=in0 // keep trackof initial byte address |
92 | dep src=0,in0,0,3 // src=8byte-aligned in0 address |
93 | .save pr, saved_pr |
94 | mov saved_pr=pr // preserve predicates (rotation) |
95 | ;; |
96 | |
97 | .body |
98 | |
99 | ld8.s v[1]=[src],8 // load the initial 8bytes (must speculate) |
100 | shl tmp=tmp,3 // multiply by 8bits/byte |
101 | mov mask=-1 // our mask |
102 | ;; |
103 | ld8.s w[1]=[src],8 // load next 8 bytes in 2nd pipeline |
104 | cmp.eq p6,p0=r0,r0 // sets p6 (required because of // cmp.and) |
105 | sub tmp=64,tmp // how many bits to shift our mask on the right |
106 | ;; |
107 | shr.u mask=mask,tmp // zero enough bits to hold v[1] valuable part |
108 | mov ar.ec=r0 // clear epilogue counter (saved in ar.pfs) |
109 | ;; |
110 | add base=-16,src // keep track of aligned base |
111 | chk.s v[1], .recover // if already NaT, then directly skip to recover |
112 | or v[1]=v[1],mask // now we have a safe initial byte pattern |
113 | ;; |
114 | 1: |
115 | ld8.s v[0]=[src],8 // speculatively load next |
116 | czx1.r val1=v[1] // search 0 byte from right |
117 | czx1.r val2=w[1] // search 0 byte from right following 8bytes |
118 | ;; |
119 | ld8.s w[0]=[src],8 // speculatively load next to next |
120 | cmp.eq.and p6,p0=8,val1 // p6 = p6 and val1==8 |
121 | cmp.eq.and p6,p0=8,val2 // p6 = p6 and mask==8 |
122 | (p6) br.wtop.dptk.few 1b // loop until p6 == 0 |
123 | ;; |
124 | // |
125 | // We must return try the recovery code iff |
126 | // val1_is_nat || (val1==8 && val2_is_nat) |
127 | // |
128 | // XXX Fixme |
129 | // - there must be a better way of doing the test |
130 | // |
131 | cmp.eq p8,p9=8,val1 // p6 = val1 had zero (disambiguate) |
132 | tnat.nz p6,p7=val1 // test NaT on val1 |
133 | (p6) br.cond.spnt .recover // jump to recovery if val1 is NaT |
134 | ;; |
135 | // |
136 | // if we come here p7 is true, i.e., initialized for // cmp |
137 | // |
138 | cmp.eq.and p7,p0=8,val1// val1==8? |
139 | tnat.nz.and p7,p0=val2 // test NaT if val2 |
140 | (p7) br.cond.spnt .recover // jump to recovery if val2 is NaT |
141 | ;; |
142 | (p8) mov val1=val2 // val2 contains the value |
143 | (p8) adds src=-16,src // correct position when 3 ahead |
144 | (p9) adds src=-24,src // correct position when 4 ahead |
145 | ;; |
146 | sub ret0=src,orig // distance from origin |
147 | sub tmp=7,val1 // 7=8-1 because this strlen returns strlen+1 |
148 | mov pr=saved_pr,0xffffffffffff0000 |
149 | ;; |
150 | sub ret0=ret0,tmp // length=now - back -1 |
151 | mov ar.pfs=saved_pfs // because of ar.ec, restore no matter what |
152 | br.ret.sptk.many rp // end of normal execution |
153 | |
154 | // |
155 | // Outlined recovery code when speculation failed |
156 | // |
157 | // This time we don't use speculation and rely on the normal exception |
158 | // mechanism. that's why the loop is not as good as the previous one |
159 | // because read ahead is not possible |
160 | // |
161 | // XXX Fixme |
162 | // - today we restart from the beginning of the string instead |
163 | // of trying to continue where we left off. |
164 | // |
165 | .recover: |
166 | EX(.Lexit1, ld8 val=[base],8) // load the initial bytes |
167 | ;; |
168 | or val=val,mask // remask first bytes |
169 | cmp.eq p0,p6=r0,r0 // nullify first ld8 in loop |
170 | ;; |
171 | // |
172 | // ar.ec is still zero here |
173 | // |
174 | 2: |
175 | EX(.Lexit1, (p6) ld8 val=[base],8) |
176 | ;; |
177 | czx1.r val1=val // search 0 byte from right |
178 | ;; |
179 | cmp.eq p6,p0=8,val1 // val1==8 ? |
180 | (p6) br.wtop.dptk.few 2b // loop until p6 == 0 |
181 | ;; |
182 | sub ret0=base,orig // distance from base |
183 | sub tmp=7,val1 // 7=8-1 because this strlen returns strlen+1 |
184 | mov pr=saved_pr,0xffffffffffff0000 |
185 | ;; |
186 | sub ret0=ret0,tmp // length=now - back -1 |
187 | mov ar.pfs=saved_pfs // because of ar.ec, restore no matter what |
188 | br.ret.sptk.many rp // end of successful recovery code |
189 | |
190 | // |
191 | // We failed even on the normal load (called from exception handler) |
192 | // |
193 | .Lexit1: |
194 | mov ret0=0 |
195 | mov pr=saved_pr,0xffffffffffff0000 |
196 | mov ar.pfs=saved_pfs // because of ar.ec, restore no matter what |
197 | br.ret.sptk.many rp |
198 | END(__strlen_user) |
199 |
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