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
81GLOBAL_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	;;
1111:
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	//
1772:
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
192END(strlen)
193