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
 83GLOBAL_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	;;
1131:
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	//
1792:
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
194END(strlen)
195EXPORT_SYMBOL(strlen)