1 /* SPDX-License-Identifier: GPL-2.0 */
2 /*
3  *
4  * Optimized version of the standard strlen() function
5  *
6  *
7  * Inputs:
8  *	in0	address of string
9  *
10  * Outputs:
11  *	ret0	the number of characters in the string (0 if empty string)
12  *	does not count the \0
13  *
14  * Copyright (C) 1999, 2001 Hewlett-Packard Co
15  *	Stephane Eranian <eranian@hpl.hp.com>
16  *
17  * 09/24/99 S.Eranian add speculation recovery code
18  */
19 
20 #include <asm/asmmacro.h>
21 #include <asm/export.h>
22 
23 //
24 //
25 // This is an enhanced version of the basic strlen. it includes a combination
26 // of compute zero index (czx), parallel comparisons, speculative loads and
27 // loop unroll using rotating registers.
28 //
29 // General Ideas about the algorithm:
30 //	  The goal is to look at the string in chunks of 8 bytes.
31 //	  so we need to do a few extra checks at the beginning because the
32 //	  string may not be 8-byte aligned. In this case we load the 8byte
33 //	  quantity which includes the start of the string and mask the unused
34 //	  bytes with 0xff to avoid confusing czx.
35 //	  We use speculative loads and software pipelining to hide memory
36 //	  latency and do read ahead safely. This way we defer any exception.
37 //
38 //	  Because we don't want the kernel to be relying on particular
39 //	  settings of the DCR register, we provide recovery code in case
40 //	  speculation fails. The recovery code is going to "redo" the work using
41 //	  only normal loads. If we still get a fault then we generate a
42 //	  kernel panic. Otherwise we return the strlen as usual.
43 //
44 //	  The fact that speculation may fail can be caused, for instance, by
45 //	  the DCR.dm bit being set. In this case TLB misses are deferred, i.e.,
46 //	  a NaT bit will be set if the translation is not present. The normal
47 //	  load, on the other hand, will cause the translation to be inserted
48 //	  if the mapping exists.
49 //
50 //	  It should be noted that we execute recovery code only when we need
51 //	  to use the data that has been speculatively loaded: we don't execute
52 //	  recovery code on pure read ahead data.
53 //
54 // Remarks:
55 //	- the cmp r0,r0 is used as a fast way to initialize a predicate
56 //	  register to 1. This is required to make sure that we get the parallel
57 //	  compare correct.
58 //
59 //	- we don't use the epilogue counter to exit the loop but we need to set
60 //	  it to zero beforehand.
61 //
62 //	- after the loop we must test for Nat values because neither the
63 //	  czx nor cmp instruction raise a NaT consumption fault. We must be
64 //	  careful not to look too far for a Nat for which we don't care.
65 //	  For instance we don't need to look at a NaT in val2 if the zero byte
66 //	  was in val1.
67 //
68 //	- Clearly performance tuning is required.
69 //
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)
84 	.prologue
85 	.save ar.pfs, saved_pfs
86 	alloc saved_pfs=ar.pfs,11,0,0,8 // rotating must be multiple of 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 v[1]=[src],8	// must not speculate: can fail here
100 	shl tmp=tmp,3		// multiply by 8bits/byte
101 	mov mask=-1		// our mask
102 	;;
103 	ld8.s w[1]=[src],8	// speculatively load next
104 	cmp.eq p6,p0=r0,r0	// sets p6 to true for 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 	or v[1]=v[1],mask	// now we have a safe initial byte pattern
112 	;;
113 1:
114 	ld8.s v[0]=[src],8	// speculatively load next
115 	czx1.r val1=v[1]	// search 0 byte from right
116 	czx1.r val2=w[1]	// search 0 byte from right following 8bytes
117 	;;
118 	ld8.s w[0]=[src],8	// speculatively load next to next
119 	cmp.eq.and p6,p0=8,val1	// p6 = p6 and val1==8
120 	cmp.eq.and p6,p0=8,val2	// p6 = p6 and mask==8
121 (p6)	br.wtop.dptk 1b		// loop until p6 == 0
122 	;;
123 	//
124 	// We must return try the recovery code iff
125 	// val1_is_nat || (val1==8 && val2_is_nat)
126 	//
127 	// XXX Fixme
128 	//	- there must be a better way of doing the test
129 	//
130 	cmp.eq  p8,p9=8,val1	// p6 = val1 had zero (disambiguate)
131 	tnat.nz p6,p7=val1	// test NaT on val1
132 (p6)	br.cond.spnt .recover	// jump to recovery if val1 is NaT
133 	;;
134 	//
135 	// if we come here p7 is true, i.e., initialized for // cmp
136 	//
137 	cmp.eq.and  p7,p0=8,val1// val1==8?
138 	tnat.nz.and p7,p0=val2	// test NaT if val2
139 (p7)	br.cond.spnt .recover	// jump to recovery if val2 is NaT
140 	;;
141 (p8)	mov val1=val2		// the other test got us out of the loop
142 (p8)	adds src=-16,src	// correct position when 3 ahead
143 (p9)	adds src=-24,src	// correct position when 4 ahead
144 	;;
145 	sub ret0=src,orig	// distance from base
146 	sub tmp=8,val1		// which byte in word
147 	mov pr=saved_pr,0xffffffffffff0000
148 	;;
149 	sub ret0=ret0,tmp	// adjust
150 	mov ar.pfs=saved_pfs	// because of ar.ec, restore no matter what
151 	br.ret.sptk.many rp	// end of normal execution
152 
153 	//
154 	// Outlined recovery code when speculation failed
155 	//
156 	// This time we don't use speculation and rely on the normal exception
157 	// mechanism. that's why the loop is not as good as the previous one
158 	// because read ahead is not possible
159 	//
160 	// IMPORTANT:
161 	// Please note that in the case of strlen() as opposed to strlen_user()
162 	// we don't use the exception mechanism, as this function is not
163 	// supposed to fail. If that happens it means we have a bug and the
164 	// code will cause of kernel fault.
165 	//
166 	// XXX Fixme
167 	//	- today we restart from the beginning of the string instead
168 	//	  of trying to continue where we left off.
169 	//
170 .recover:
171 	ld8 val=[base],8	// will fail if unrecoverable fault
172 	;;
173 	or val=val,mask		// remask first bytes
174 	cmp.eq p0,p6=r0,r0	// nullify first ld8 in loop
175 	;;
176 	//
177 	// ar.ec is still zero here
178 	//
179 2:
180 (p6)	ld8 val=[base],8	// will fail if unrecoverable fault
181 	;;
182 	czx1.r val1=val		// search 0 byte from right
183 	;;
184 	cmp.eq p6,p0=8,val1	// val1==8 ?
185 (p6)	br.wtop.dptk 2b		// loop until p6 == 0
186 	;;			// (avoid WAW on p63)
187 	sub ret0=base,orig	// distance from base
188 	sub tmp=8,val1
189 	mov pr=saved_pr,0xffffffffffff0000
190 	;;
191 	sub ret0=ret0,tmp	// length=now - back -1
192 	mov ar.pfs=saved_pfs	// because of ar.ec, restore no matter what
193 	br.ret.sptk.many rp	// end of successful recovery code
194 END(strlen)
195 EXPORT_SYMBOL(strlen)
196