196 lines
6.3 KiB
ArmAsm
196 lines
6.3 KiB
ArmAsm
/* SPDX-License-Identifier: GPL-2.0 */
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/*
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*
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* Optimized version of the standard strlen() function
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*
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*
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* Inputs:
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* in0 address of string
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*
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* Outputs:
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* ret0 the number of characters in the string (0 if empty string)
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* does not count the \0
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*
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* Copyright (C) 1999, 2001 Hewlett-Packard Co
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* Stephane Eranian <eranian@hpl.hp.com>
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*
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* 09/24/99 S.Eranian add speculation recovery code
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*/
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#include <asm/asmmacro.h>
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#include <asm/export.h>
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//
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//
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// This is an enhanced version of the basic strlen. it includes a combination
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// of compute zero index (czx), parallel comparisons, speculative loads and
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// loop unroll using rotating registers.
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//
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// General Ideas about the algorithm:
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// The goal is to look at the string in chunks of 8 bytes.
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// so we need to do a few extra checks at the beginning because the
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// string may not be 8-byte aligned. In this case we load the 8byte
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// quantity which includes the start of the string and mask the unused
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// bytes with 0xff to avoid confusing czx.
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// We use speculative loads and software pipelining to hide memory
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// latency and do read ahead safely. This way we defer any exception.
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//
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// Because we don't want the kernel to be relying on particular
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// settings of the DCR register, we provide recovery code in case
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// speculation fails. The recovery code is going to "redo" the work using
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// only normal loads. If we still get a fault then we generate a
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// kernel panic. Otherwise we return the strlen as usual.
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//
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// The fact that speculation may fail can be caused, for instance, by
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// the DCR.dm bit being set. In this case TLB misses are deferred, i.e.,
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// a NaT bit will be set if the translation is not present. The normal
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// load, on the other hand, will cause the translation to be inserted
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// if the mapping exists.
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//
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// It should be noted that we execute recovery code only when we need
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// to use the data that has been speculatively loaded: we don't execute
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// recovery code on pure read ahead data.
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//
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// Remarks:
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// - the cmp r0,r0 is used as a fast way to initialize a predicate
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// register to 1. This is required to make sure that we get the parallel
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// compare correct.
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//
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// - we don't use the epilogue counter to exit the loop but we need to set
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// it to zero beforehand.
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//
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// - after the loop we must test for Nat values because neither the
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// czx nor cmp instruction raise a NaT consumption fault. We must be
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// careful not to look too far for a Nat for which we don't care.
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// For instance we don't need to look at a NaT in val2 if the zero byte
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// was in val1.
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//
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// - Clearly performance tuning is required.
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//
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//
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//
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#define saved_pfs r11
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#define tmp r10
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#define base r16
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#define orig r17
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#define saved_pr r18
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#define src r19
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#define mask r20
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#define val r21
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#define val1 r22
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#define val2 r23
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GLOBAL_ENTRY(strlen)
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.prologue
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.save ar.pfs, saved_pfs
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alloc saved_pfs=ar.pfs,11,0,0,8 // rotating must be multiple of 8
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.rotr v[2], w[2] // declares our 4 aliases
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extr.u tmp=in0,0,3 // tmp=least significant 3 bits
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mov orig=in0 // keep trackof initial byte address
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dep src=0,in0,0,3 // src=8byte-aligned in0 address
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.save pr, saved_pr
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mov saved_pr=pr // preserve predicates (rotation)
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;;
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.body
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ld8 v[1]=[src],8 // must not speculate: can fail here
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shl tmp=tmp,3 // multiply by 8bits/byte
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mov mask=-1 // our mask
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;;
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ld8.s w[1]=[src],8 // speculatively load next
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cmp.eq p6,p0=r0,r0 // sets p6 to true for cmp.and
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sub tmp=64,tmp // how many bits to shift our mask on the right
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;;
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shr.u mask=mask,tmp // zero enough bits to hold v[1] valuable part
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mov ar.ec=r0 // clear epilogue counter (saved in ar.pfs)
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;;
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add base=-16,src // keep track of aligned base
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or v[1]=v[1],mask // now we have a safe initial byte pattern
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;;
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1:
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ld8.s v[0]=[src],8 // speculatively load next
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czx1.r val1=v[1] // search 0 byte from right
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czx1.r val2=w[1] // search 0 byte from right following 8bytes
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;;
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ld8.s w[0]=[src],8 // speculatively load next to next
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cmp.eq.and p6,p0=8,val1 // p6 = p6 and val1==8
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cmp.eq.and p6,p0=8,val2 // p6 = p6 and mask==8
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(p6) br.wtop.dptk 1b // loop until p6 == 0
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;;
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//
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// We must return try the recovery code iff
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// val1_is_nat || (val1==8 && val2_is_nat)
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//
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// XXX Fixme
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// - there must be a better way of doing the test
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//
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cmp.eq p8,p9=8,val1 // p6 = val1 had zero (disambiguate)
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tnat.nz p6,p7=val1 // test NaT on val1
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(p6) br.cond.spnt .recover // jump to recovery if val1 is NaT
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;;
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//
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// if we come here p7 is true, i.e., initialized for // cmp
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//
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cmp.eq.and p7,p0=8,val1// val1==8?
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tnat.nz.and p7,p0=val2 // test NaT if val2
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(p7) br.cond.spnt .recover // jump to recovery if val2 is NaT
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;;
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(p8) mov val1=val2 // the other test got us out of the loop
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(p8) adds src=-16,src // correct position when 3 ahead
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(p9) adds src=-24,src // correct position when 4 ahead
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;;
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sub ret0=src,orig // distance from base
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sub tmp=8,val1 // which byte in word
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mov pr=saved_pr,0xffffffffffff0000
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;;
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sub ret0=ret0,tmp // adjust
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mov ar.pfs=saved_pfs // because of ar.ec, restore no matter what
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br.ret.sptk.many rp // end of normal execution
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//
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// Outlined recovery code when speculation failed
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//
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// This time we don't use speculation and rely on the normal exception
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// mechanism. that's why the loop is not as good as the previous one
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// because read ahead is not possible
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//
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// IMPORTANT:
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// Please note that in the case of strlen() as opposed to strlen_user()
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// we don't use the exception mechanism, as this function is not
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// supposed to fail. If that happens it means we have a bug and the
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// code will cause of kernel fault.
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//
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// XXX Fixme
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// - today we restart from the beginning of the string instead
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// of trying to continue where we left off.
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//
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.recover:
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ld8 val=[base],8 // will fail if unrecoverable fault
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;;
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or val=val,mask // remask first bytes
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cmp.eq p0,p6=r0,r0 // nullify first ld8 in loop
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;;
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//
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// ar.ec is still zero here
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//
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2:
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(p6) ld8 val=[base],8 // will fail if unrecoverable fault
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;;
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czx1.r val1=val // search 0 byte from right
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;;
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cmp.eq p6,p0=8,val1 // val1==8 ?
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(p6) br.wtop.dptk 2b // loop until p6 == 0
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;; // (avoid WAW on p63)
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sub ret0=base,orig // distance from base
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sub tmp=8,val1
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mov pr=saved_pr,0xffffffffffff0000
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;;
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sub ret0=ret0,tmp // length=now - back -1
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mov ar.pfs=saved_pfs // because of ar.ec, restore no matter what
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br.ret.sptk.many rp // end of successful recovery code
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END(strlen)
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EXPORT_SYMBOL(strlen)
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